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C#: After copying a class the "this" keyword throws an error [duplicate]

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Closed 11 years ago.
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I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}

I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.

There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.

I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)

Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.

I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.

I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.

I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.

Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}

I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.

I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.

Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}

Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).

[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.

You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))

I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.

In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.

I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.

I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...

I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.

I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}

[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}

this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life

'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.

I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.

When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.

It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.

I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.

1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);

There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.

You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.

Is there a need for this. and me. on properties [duplicate]

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}

I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.

There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.

I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)

Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.

I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.

I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.

I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.

Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}

I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.

I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.

Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}

Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).

[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.

You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))

I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.

In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.

I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.

I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...

I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.

I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}

[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}

this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life

'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.

I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.

When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.

It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.

I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.

1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);

There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.

You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.

What is the current convention on using the 'this' keyword in C#? [duplicate]

As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 11 years ago.
Locked. This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
I was curious about how other people use the this keyword. I tend to use it in constructors, but I may also use it throughout the class in other methods. Some examples:
In a constructor:
public Light(Vector v)
{
this.dir = new Vector(v);
}
Elsewhere
public void SomeMethod()
{
Vector vec = new Vector();
double d = (vec * vec) - (this.radius * this.radius);
}

I don't mean this to sound snarky, but it doesn't matter.
Seriously.
Look at the things that are important: your project, your code, your job, your personal life. None of them are going to have their success rest on whether or not you use the "this" keyword to qualify access to fields. The this keyword will not help you ship on time. It's not going to reduce bugs, it's not going to have any appreciable effect on code quality or maintainability. It's not going to get you a raise, or allow you to spend less time at the office.
It's really just a style issue. If you like "this", then use it. If you don't, then don't. If you need it to get correct semantics then use it. The truth is, every programmer has his own unique programing style. That style reflects that particular programmer's notions of what the "most aesthetically pleasing code" should look like. By definition, any other programmer who reads your code is going to have a different programing style. That means there is always going to be something you did that the other guy doesn't like, or would have done differently. At some point some guy is going to read your code and grumble about something.
I wouldn't fret over it. I would just make sure the code is as aesthetically pleasing as possible according to your own tastes. If you ask 10 programmers how to format code, you are going to get about 15 different opinions. A better thing to focus on is how the code is factored. Are things abstracted right? Did I pick meaningful names for things? Is there a lot of code duplication? Are there ways I can simplify stuff? Getting those things right, I think, will have the greatest positive impact on your project, your code, your job, and your life. Coincidentally, it will probably also cause the other guy to grumble the least. If your code works, is easy to read, and is well factored, the other guy isn't going to be scrutinizing how you initialize fields. He's just going to use your code, marvel at it's greatness, and then move on to something else.

There are several usages of this keyword in C#.
To qualify members hidden by similar name
To have an object pass itself as a parameter to other methods
To have an object return itself from a method
To declare indexers
To declare extension methods
To pass parameters between constructors
To internally reassign value type (struct) value.
To invoke an extension method on the current instance
To cast itself to another type
To chain constructors defined in the same class
You can avoid the first usage by not having member and local variables with the same name in scope, for example by following common naming conventions and using properties (Pascal case) instead of fields (camel case) to avoid colliding with local variables (also camel case). In C# 3.0 fields can be converted to properties easily by using auto-implemented properties.

I only use it when absolutely necessary, ie, when another variable is shadowing another. Such as here:
class Vector3
{
float x;
float y;
float z;
public Vector3(float x, float y, float z)
{
this.x = x;
this.y = y;
this.z = z;
}
}
Or as Ryan Fox points out, when you need to pass this as a parameter. (Local variables have precedence over member variables)

Personally, I try to always use this when referring to member variables. It helps clarify the code and make it more readable. Even if there is no ambiguity, someone reading through my code for the first time doesn't know that, but if they see this used consistently, they will know if they are looking at a member variable or not.

I use it every time I refer to an instance variable, even if I don't need to. I think it makes the code more clear.

I can't believe all of the people that say using it always is a "best practice" and such.
Use "this" when there is ambiguity, as in Corey's example or when you need to pass the object as a parameter, as in Ryan's example. There is no reason to use it otherwise because being able to resolve a variable based on the scope chain should be clear enough that qualifying variables with it should be unnecessary.
EDIT: The C# documentation on "this" indicates one more use, besides the two I mentioned, for the "this" keyword - for declaring indexers
EDIT: #Juan: Huh, I don't see any inconsistency in my statements - there are 3 instances when I would use the "this" keyword (as documented in the C# documentation), and those are times when you actually need it. Sticking "this" in front of variables in a constructor when there is no shadowing going on is simply a waste of keystrokes and a waste of my time when reading it, it provides no benefit.

I use it whenever StyleCop tells me to. StyleCop must be obeyed. Oh yes.

Any time you need a reference to the current object.
One particularly handy scenario is when your object is calling a function and wants to pass itself into it.
Example:
void onChange()
{
screen.draw(this);
}

I tend to use it everywhere as well, just to make sure that it is clear that it is instance members that we are dealing with.

I use it anywhere there might be ambiguity (obviously). Not just compiler ambiguity (it would be required in that case), but also ambiguity for someone looking at the code.

Another somewhat rare use for the this keyword is when you need to invoke an explicit interface implementation from within the implementing class. Here's a contrived example:
class Example : ICloneable
{
private void CallClone()
{
object clone = ((ICloneable)this).Clone();
}
object ICloneable.Clone()
{
throw new NotImplementedException();
}
}

Here's when I use it:
Accessing Private Methods from within the class (to differentiate)
Passing the current object to another method (or as a sender object, in case of an event)
When creating extension methods :D
I don't use this for Private fields because I prefix private field variable names with an underscore (_).

[C++]
I agree with the "use it when you have to" brigade. Decorating code unnecessarily with this isn't a great idea because the compiler won't warn you when you forget to do it. This introduces potential confusion for people expecting this to always be there, i.e. they'll have to think about it.
So, when would you use it? I've just had a look around some random code and found these examples (I'm not passing judgement on whether these are good things to do or otherwise):
Passing "yourself" to a function.
Assigning "yourself" to a pointer or something like that.
Casting, i.e. up/down casting (safe or otherwise), casting away constness, etc.
Compiler enforced disambiguation.

You should always use it, I use it to diferantiate private fields and parameters (because our naming conventions state that we don't use prefixes for member and parameter names (and they are based on information found on the internet, so I consider that a best practice))

I use it when, in a function that accepts a reference to an object of the same type, I want to make it perfectly clear which object I'm referring to, where.
For example
class AABB
{
// ... members
bool intersects( AABB other )
{
return other.left() < this->right() &&
this->left() < other.right() &&
// +y increases going down
other.top() < this->bottom() &&
this->top() < other.bottom() ;
}
} ;
(vs)
class AABB
{
bool intersects( AABB other )
{
return other.left() < right() &&
left() < other.right() &&
// +y increases going down
other.top() < bottom() &&
top() < other.bottom() ;
}
} ;
At a glance which AABB does right() refer to? The this adds a bit of a clarifier.

In Jakub Šturc's answer his #5 about passing data between contructors probably could use a little explanation. This is in overloading constructors and is the one case where use of this is mandatory. In the following example we can call the parameterized constructor from the parameterless constructor with a default parameter.
class MyClass {
private int _x
public MyClass() : this(5) {}
public MyClass(int v) { _x = v;}
}
I've found this to be a particularly useful feature on occasion.

I got in the habit of using it liberally in Visual C++ since doing so would trigger IntelliSense ones I hit the '>' key, and I'm lazy. (and prone to typos)
But I've continued to use it, since I find it handy to see that I'm calling a member function rather than a global function.

I tend to underscore fields with _ so don't really ever need to use this. Also R# tends to refactor them away anyway...

I pretty much only use this when referencing a type property from inside the same type. As another user mentioned, I also underscore local fields so they are noticeable without needing this.

I use it only when required, except for symmetric operations which due to single argument polymorphism have to be put into methods of one side:
boolean sameValue (SomeNum other) {
return this.importantValue == other.importantValue;
}

[C++]
this is used in the assignment operator where most of the time you have to check and prevent strange (unintentional, dangerous, or just a waste of time for the program) things like:
A a;
a = a;
Your assignment operator will be written:
A& A::operator=(const A& a) {
if (this == &a) return *this;
// we know both sides of the = operator are different, do something...
return *this;
}

this on a C++ compiler
The C++ compiler will silently lookup for a symbol if it does not find it immediately. Sometimes, most of the time, it is good:
using the mother class' method if you did not overloaded it in the child class.
promoting a value of a type into another type
But sometimes, You just don't want the compiler to guess. You want the compiler to pick-up the right symbol and not another.
For me, those times are when, within a method, I want to access to a member method or member variable. I just don't want some random symbol picked up just because I wrote printf instead of print. this->printf would not have compiled.
The point is that, with C legacy libraries (§), legacy code written years ago (§§), or whatever could happen in a language where copy/pasting is an obsolete but still active feature, sometimes, telling the compiler to not play wits is a great idea.
These are the reasons I use this.
(§) it's still a kind of mystery to me, but I now wonder if the fact you include the <windows.h> header in your source, is the reason all the legacy C libraries symbols will pollute your global namespace
(§§) realizing that "you need to include a header, but that including this header will break your code because it uses some dumb macro with a generic name" is one of those russian roulette moments of a coder's life

'this.' helps find members on 'this' class with a lot of members (usually due to a deep inheritance chain).
Hitting CTRL+Space doesn't help with this, because it also includes types; where-as 'this.' includes members ONLY.
I usually delete it once I have what I was after: but this is just my style breaking through.
In terms of style, if you are a lone-ranger -- you decide; if you work for a company stick to the company policy (look at the stuff in source control and see what other people are doing). In terms of using it to qualify members, neither is right or wrong. The only wrong thing is inconsistency -- that is the golden rule of style. Leave the nit-picking others. Spend your time pondering real coding problems -- and obviously coding -- instead.

I use it every time I can. I believe it makes the code more readable, and more readable code equals less bugs and more maintainability.

When you are many developers working on the same code base, you need some code guidelines/rules. Where I work we've desided to use 'this' on fields, properties and events.
To me it makes good sense to do it like this, it makes the code easier to read when you differentiate between class-variables and method-variables.

It depends on the coding standard I'm working under. If we are using _ to denote an instance variable then "this" becomes redundant. If we are not using _ then I tend to use this to denote instance variable.

I use it to invoke Intellisense just like JohnMcG, but I'll go back and erase "this->" when I'm done. I follow the Microsoft convention of prefixing member variables with "m_", so leaving it as documentation would just be redundant.

1 - Common Java setter idiom:
public void setFoo(int foo) {
this.foo = foo;
}
2 - When calling a function with this object as a parameter
notifier.addListener(this);

There is one use that has not already been mentioned in C++, and that is not to refer to the own object or disambiguate a member from a received variable.
You can use this to convert a non-dependent name into an argument dependent name inside template classes that inherit from other templates.
template <typename T>
struct base {
void f() {}
};
template <typename T>
struct derived : public base<T>
{
void test() {
//f(); // [1] error
base<T>::f(); // quite verbose if there is more than one argument, but valid
this->f(); // f is now an argument dependent symbol
}
}
Templates are compiled with a two pass mechanism. During the first pass, only non-argument dependent names are resolved and checked, while dependent names are checked only for coherence, without actually substituting the template arguments.
At that step, without actually substituting the type, the compiler has almost no information of what base<T> could be (note that specialization of the base template can turn it into completely different types, even undefined types), so it just assumes that it is a type. At this stage the non-dependent call f that seems just natural to the programmer is a symbol that the compiler must find as a member of derived or in enclosing namespaces --which does not happen in the example-- and it will complain.
The solution is turning the non-dependent name f into a dependent name. This can be done in a couple of ways, by explicitly stating the type where it is implemented (base<T>::f --adding the base<T> makes the symbol dependent on T and the compiler will just assume that it will exist and postpones the actual check for the second pass, after argument substitution.
The second way, much sorter if you inherit from templates that have more than one argument, or long names, is just adding a this-> before the symbol. As the template class you are implementing does depend on an argument (it inherits from base<T>) this-> is argument dependent, and we get the same result: this->f is checked in the second round, after template parameter substitution.

You should not use "this" unless you absolutely must.
There IS a penalty associated with unnecessary verbosity. You should strive for code that is exactly as long as it needs to be, and no longer.

When To Use this.method()? [duplicate]

This question already has answers here:
Closed 12 years ago.
Possible Duplicates:
Why does StyleCop recommend prefixing method or property calls with “this”?
When do you use the “this” keyword?
Hi SO, and happy Friday
I have a question regarding the use of this.method();. My code seems to work without using the this., but I include it because it seems like the right thing to do. When should .this be used, and because it's presence doesn't always make a difference, what's the best practice for .this?

My opinion: use this for disambiguation purposes, e.g., when a parameter name collides with a class property, otherwise leave it out for less noise.

For the pure sake of readbility and understandabilitly you should always use the this. whenever you call an instance member. It is considered best practice and StyleCop suggests it too:
(With the this prefix) all calls to class
members (are) instantly recognizable,
regardless of which editor is being
used to view the code. Another
advantage is that it creates a quick,
recognizable differentiation between
instance members and static members,
which are not be prefixed.
A final advantage of using the ‘this.’
prefix is that typing this. will cause
Visual Studio to show the IntelliSense
popup, making it quick and easy for
the developer to choose the class
member to call.
In other words, if you omit the this. prefix you can not quickly understand, wether soemthing is a static member, a local variable, a delegate ... With the prefix you'll see it at first glimpse.
But the most important thing is: Whatever you chose, you should keep it consistent across the whole file, and across all other files and propably also across your whole team !

You've got two disagreeing answers so far so I'll add my perspective as an ex-Java, ex-C# and now a mostly-Python programmer. In Python we have self that does pretty much the same job, just it's not optional.
I would say use this as often as you can. It doesn't change anything as it's implicit when you call it from within an object but it does help differentiate so you know you're calling a class member (vs a global function).
You'll appreciate it when you read over the code in a year's time.

You don't need it if the call is not ambiguous, but I prefer using it, too. In my opinion, it is simply consistent for Bar to access properties and methods of Foo by foo.Property and foo.Method(), and access its own members as this.Property and this.Method(). You're still dealing with properties and methods of objects, except in the case of this, you're dealing with members in the same class. But why should that matter regarding coding style?
I use this all the time. It's clean, it's clear, and it's consistent.

When referring to a variable, this. can distinguish between a local variable or a parameter, and a member variable. But when referring to a method, it doesn't offer any useful distinction, and is therefore, to my way of thinking, codejunk. Leave it off.

Effectively the this keyword used in the context you describe is most useful as a disambiguator if you have more that one item in scope with the same name (e.g. an instance variable and a local variable of the same name)

one way you really need it is when pass a parameter named as the member
in constructors
Myclass(int id)
{
this.id = id;
}

I normally don't leave 'this.' in code at all. But I do use it for intellisense purposes when I can't remember what 'this.' contains. Then I remove it.
But one instance you have to keep it, is to qualify members hidden by similar names. For example:
public Employee(string name, string alias)
{
this.name = name;
this.alias = alias;
}

Several people have mentioned the use of this with regard to member variables. I would recommend...
Either:
Prefix member variables with this.
Or:
Adopt a naming convention for member variables that requires you to name them with a _leadingUnderscore
Stylecop rules can be set to enforce whichever of these rules your team prefers.

This points to the current object, it's basically used to remove the ambiguity of
method/variable names for e.g.
public class Test
{
private string name;
public Test(string name)
{
this.name = name;//using this coz the parameter name and variable names are same
}
}
You don't need to use it always.
I hope it helps you.

What is the correct naming notation for classes, functions, variables etc in c#?

I'm a web developer with no formal computing background behind me, I've been writing code now some years now, but every time I need to create a new class / function / variable, I spend about two minutes just deciding on a name and then how to type it.
For instance, if I write a function to sum up a bunch of numbers. Should I call it
Sum()
GetSum()
getSum()
get_sum()
AddNumbersReturnTotal()
I know there is a right way to do this, and a link to a good definitive source is all I ask :D
Closed as a duplicate of c# Coding standard / Best practices

You're looking for StyleCop.

Classes should be in camel notation with the first letter capitalized
public class MyClass
Functions and Methods in C# should act in a similar fashion except for private methods
public void MyMethod()
private void myPrivateMethod()
Variables I tend to do a little differently:
Member Variables
private int _count;
Local variables
int count;

I agree on the calculate vs get distinction: get() should be used for values that are already calculated or otherwise trivial to retrieve.
Additionally, I would suggest in many cases adding a noun to the name, so that it's obvious exactly what sum you are calculating. Unless, of course, the noun you would add is the class name or type itself.

All of the above.
I believe the official C# guidelines would say call it calculateSum() as getSum() would be used if the sum was an instance variable. But it depends on the coding style used and how any existing code in the project is written.

Luckily enough I don't believe there is a standardized way this is done. I pick the one that I like, which consequently also seems to be the standard all other source code I've seen uses, and run with it.

Sum() if it's public and does the work itself.
GetSum() if it's public and it retrieves the information from somewhere else.
sum() / getSum() as above, but for internal/private methods.
(Hmm... That's a bit vague, since you shift the meaning of "Sum" there slightly. So, let try this again.
XXX if xxx is a process (summing values).
GetXXX if xxx is a thing. (the sum of the values)

Method names are verbs. Class, field and property names are nouns. In this case, Sum could pass as either a verb or a noun...
AddNumbersReturnTotal fits the above definition, but it's a little long. Out of kindness to the guy who gets to maintain my code (usually me!) I try and avoid including redundant words in identifiers, and I try to avoid words that are easy to make typos on.