Is there an equivalent of this in C# for static members?
I like to use this to make my code more readable, but wondered if there was an equivalent for static members.
I suppose if you use this. to reinforce that you are referring to instance members, the equivalent in a static member would be to use ClassName.
But stylistically, why add code that doesn't change meaning?
edit to add various clarifications:
My last sentence above can be illustrated with these examples:
class Example1
{
public int J { get; set; }
public Example1()
{
J = 0;
}
// These two methods have *exactly* the same CIL
public int InstanceMethodLong()
{
return this.J;
}
public int InstanceMethodShort()
{
return J;
}
}
The this. in InstanceMethodLong does not change the meaning as compared with InstanceMethodShort.
Statically:
class Example2
{
public static int K { get; set; }
static Example2()
{
K = 0;
}
// These two methods have *exactly* the same CIL
public int StaticMethodLong()
{
return Example2.K;
}
public int StaticMethodShort()
{
return K;
}
The Example2. in StaticMethodLong does not change the meaning as compared with StaticMethodShort.
In both these cases, adding the qualifier results in the same CIL, the same behaviour, and is more source to write, read, and understand. Stylistically - and I will happily accept that this is a question of code style - I see no reason for it to be there.
With underscore prefixes the situation is slightly different:
class Example3
{
int _j;
public int J
{
get { return _j; }
set
{
_j = value;
// and do something else,
// to justify not using an auto-property
}
}
public Example3()
{
J = 0;
}
public int MethodWithParameter(int j)
{
// Now there is a *difference* between
return j;
// and
return _j;
}
}
Here, in MethodWithParameter, there is a difference between referring to _j and j - so we are deliberately and explicitly expressing different meaning. It's true that the compiler doesn't care what we call our variable names, but it does care what variables we are referring to! So in the body of MethodWithParameter, using or not using an underscore isn't just stylistic, it's semantic. Which isn't the particular issue we're addressing in this question.
As a static member is not meant to belong to any particular instance (as this refers to an instance of an object, with different settings possible per instance), what you would instead want to do is use ClassName.Member instead of this.Member.
public class Orange
{
public static string Tastes = "sweet";
public static string FoodType(){
return "fruit";
}
}
Would be called by:
Console.WriteLine(Orange.Tastes);
Same goes for static methods, as well:
Console.WriteLine(Orange.FoodType()).
Please note this is a contrived example for demonstration only. :)
You may able to use the class name to reference other static properties.
Your code becomes a bit more resistant to copy/paste but that's not always a bad thing.
Unfortunately no, there is no this for static methods. To help differentiate static members from class members I prefix it with the class name.
class Test {
static Regex TextRegex = new Regex(...);
public static bool TestString(string input) {
return Test.TextRegex.IsMatch(input);
}
}
I like "this" as well to realsie from the first look where the state is changing. You may want to consider type's name for static members in this case.
Related
I've read the answers for Class with indexer and property named "Item", but they do not explain why can I have a class with multiple indexers, all of them creating Item property and get_Item/set_Item methods (of course working well, as they are different overloads), but I cannot have an explicit Item property.
Consider the code:
namespace Test
{
class Program
{
static void Main(string[] args)
{
}
public int this[string val]
{
get
{
return 0;
}
set
{
}
}
public string this[int val] //this is valid
{
get
{
return string.Empty;
}
set
{
}
}
public int Item { get; set; } //this is not valid
}
}
For two indexers there are four methods created:
Int32 get_Item(String val)
Void set_Item(String val, Int32 value)
String get_Item(Int32 val)
Void set_Item(Int32 val, String value)
I'd expect my property to create
Int32 get_Item()
Void set_Item(Int32 value)
These overloads are generally acceptable, but somehow the compiler won't let me create such a property.
Please note that I don't need a way to rename the indexer etc, this is known - I need an explanation. This answer: https://stackoverflow.com/a/5110449/882200 doesn't explain why I can have multiple indexers.
For the same reason that the following won't compile:
public class Test
{
public int Foo
{
get;
set;
}
public void Foo()
{
return;
}
}
The above results in "The type 'Test' already contains a definition for 'Foo'". Although these could be implemented as a Foo() method and a get_Foo() method, the naming is an implementation detail - at the language level, it's .Foo() and .Foo and since not all languages would support that, the compiler considers it an error.
Similarly, other languages may not support having an indexer and a property with the same name. So, although as you point out this could be compiled as get_Item() and get_Item(Int32), the CLR designers nevertheless chose not to allow it. Although the CLR could have been designed to allow this, it may not be supported at the language level, so they chose to avoid any such issues.
Is there a "correct" way to name a variable when it is being passed in a constructor to a property? Example (in c#):
public class MyClass
{
private int index;
public MyClass(int i)
{
index = i;
}
}
When the two variables are the "same", is it common to name one more concisely, etc.? I want to be consistent at least, but I'm as to whether there is an accepted way to do this.
public MyClass(int index)
{
this.index = index;
}
This is at least the common way of doing in Java, and I guess it's the same in C#. It makes the name more meaningful than i, and uses the same name as the property to designate the same thing.
Refactor recommends private fields of a class to be camelCase prefixed them with _ (_index in your case). They recommend parameters to be camelCase without the underscore.
As mentioned in comments, there is no MSDN-recommended standard for private fields.
You can use as you wrote
or
below can be another option if the variable names are same
public class MyClass
{
private int index;
public MyClass(int index)
{
this.index = index;
}
}
You can name them the same. Use this to clarify what you're doing.
this.index = index;
If you make a distinction (which I do no recommend) it's better to be most explicit about the parameter name passed to the method instead of your internal name. If other programmers use your class then at least they know what the parameter means. So if you want to distinct:
public MyClass(int index)
{
i = index;
}
I think you should focus on making the "visible" variable more readable (in this case, that's the constructor's parameter). After all, this is what the users of your class will see when typing out.
Yes, there is a correct way to avoid such dilemmas, which is to follow the naming conventions established for your language. For example, in C# it's a convention that private fields begin with an underscore. There's probably something equivalent to Java.
That being said, it's definitely possible to differentiate between fields and local variables by explicitly using this to reference fields (a practice that's appropriate when there's a naming ambiguity).
There are set of coding standards in .NET defined in MSDN.
http://msdn.microsoft.com/en-us/library/ms229002.aspx
but for the total list of guidelines:
http://msdn.microsoft.com/en-us/library/ms229042.aspx
The C# standard would write your code like this:
public class MyClass {
public MyClass(int index) {
Index = index;
}
public int Index {
get { return _index; }
private set { _idex = value; /* Perform other actions triggered by the set */ }
}
private int _index;
}
In cases where no backing field is required the property (and backing field) can and should be simplified to:
public Index { get; private set; }
My problem, narrowed down to a simple explaination, is the following:
I have a class which needs to work with a number (without changing it) which is subject to change. This number doesn't necessarily come from another class, and it can be anything.
But I'd like to only "give" it to the class once, instead of constantly having to call update methods or having to create a wrapper (since again, as I said, this should work with any kind of number and having to wrap up everything is kind of unpratical).
Here's some code, hoping it helps:
public class SimpleExample
{
int value;
public SimpleExample(int variableOfWhichINeedAReference)
{
//Of course this won't work, but I'll keep it simple.
value = variableOfWhichINeedAReference;
}
public void DisplayValue()
{
print(value);
}
}
public class RandomClass
{
int myValue = 10;
SimpleExample s = new SimpleExample(myValue);
public void WorkWithValue()
{
myValue++;
}
public void Display()
{
print(foo);
print(bar);
s.DisplayValue();
}
}
Now, the problem seems pretty obvious: If I instantiate a SimpleExample and give it a variable as a parameter, it will get its value rather than a reference to it.
Is there a simple enough way that can avoid me the creation of a wrapper? Thanks.
Make a really simple class:
class Ref<T>
{
public T Value;
public Ref<T>()
{
}
public Ref<T>(T value)
{
this.Value = value;
}
}
Then use it like this:
class A
{
Ref<int> x;
public A(Ref<int> x)
{
this.x = x;
}
public void Increment()
{
x.Value++;
}
}
...
Ref<int> x = new Ref<int>(7);
A a = new A(x);
a.Increment();
Debug.Assert(x.Value == 8);
Note that the Ref<T> class here is a reference to a value - not a reference to a variable. If you want a reference to a variable, use Eric Lippert's solution (as pointed out by Filip).
So what you want is not an int, but rather a way of getting an int at some point in time. There are several ways of doing this, one of which is to have your object accept a Func<int>. Then the code can pass in a method that returns the current value of...whatever, rather than the value at the time SimpleExample is created. Using a lambda to close over a variable makes doing this much easier as well.
public class SimpleExample
{
Func<int> func;
public SimpleExample(Func<int> func)
{
this.func = func;
}
public void DisplayValue()
{
print(func());
}
}
public class RandomClass
{
int myValue = 10;
SimpleExample s;
public RandomClass()
{
s = new SimpleExample(() => myValue);
}
public void WorkWithValue()
{
myValue++;
}
public void Display()
{
print(foo);
print(bar);
s.DisplayValue();
}
}
There is no standard wrapper for the purpose you seek, though a single-element array could be used for that purpose. Alternatively, one could define a simple wrapper type:
public class ExposedValueHolder<T> { public T Value; } // Really simple class, eh?
and then use an ExposedValueHolder<YourStructType> to wrap your object. It's not possible in general to capture something passed as an arbitrary ref parameter, since objects may live indefinitely but byrefs (the things which are actually passed when using ref parameters) may die any time after function they're passed to goes out of scope.
I'm looking to use "phantom types" to implement type-safe identifiers. There's a question here about doing this in F#.
I'd like to do this in C#. How?
I've got a solution (which has problems), so I'll post it as a possible answer to see if anyone can improve it.
Why not make it a sealed class with its constructor private?
public sealed class Id<TDiscriminator>
{
private Id() { }
//some static methods
}
I've come up with the following:
struct Id<TDiscriminator>
{
private readonly Guid _id;
private Id(Guid id)
{
_id = id;
}
public Guid Value
{
get { return _id; }
}
public static Id<TDiscriminator> NewId()
{
return From(Guid.NewGuid());
}
public static Id<TDiscriminator> From(Guid id)
{
return new Id<TDiscriminator>(id);
}
public static readonly Id<TDiscriminator> Empty = From(Guid.Empty);
// Equality operators ellided...
}
...which I can use as follows:
class Order { /* empty */ }
class Customer { /* empty */ }
void Foo()
{
var orderId = Id<Order>.NewId();
var customerId = Id<Customer>.NewId();
// This doesn't compile. GOOD.
bool same = (orderId == customerId);
}
I don't particularly want concrete classes for the discriminator, because I don't want anyone instantiating them.
I could get around that by using an interface or an abstract class. Unfortunately, these can still be derived from and instantiated.
C# won't let you use a static class as a type argument. I can't say that I'm totally happy with the answers to that question, because the answers basically say "just because".
How about?
public sealed class Order
{
private Order() {}
}
public static sealed class Id<T>
{
// ...
}
I think that's exactly what you say. No one (except some special cases) can construct it and no one can inherit from it.
Well, as far as I could understand, you would like to provide a mechanism for distinguishing different types by a custom identifier object. I think you are almost near a working solution. In .NET when having a generic class, each substitution of the generic argument (or each unique combination of the generic arguments, if more than one) creates a unique type in the runtime. In your code Id<Order> and Id<Customer> are two distinct types. The NewId() method returns an instance of Id<Order> for the orderId and Id<Customer> for the customerId variables. The two types do not implement the == operator and therefore cannot be compared. Moreover, such comparison would be difficult to implement, since you cannot determine all possible uses of the Id<TDsicriminator> - you cannot guess what type will the TDsicriminator be substituted with.
1
A fast and simple solution will be to do this:
class Order { /* skipped */ }
class Customer { /* skipped */ }
void Foo()
{
var orderId = Id<Order>.NewId();
var customerId = Id<Customer>.NewId();
bool sameIds = (orderId.Value == customerId.Value); // true
bool sameObjects = orderId.Equals(customerId); // false
}
Since the Value properties are both of the Guid type, comparison is possible.
2
If you need however, to implement the == operator, or some sort of equality comparisons for instances of Id<TDisciminator>, the approach will be different. What comes up to my mind is the following:
public abstract class IdBase
{
public abstract Guid Value { get; protected set; }
public static bool operator == (IdBase left, IdBase right)
{
return left.Value == right.Value;
}
}
public sealed class Id<TDiscriminator> : IdBase
{
// your implementation here, just remember the override keyword for the Value property
}
Many people would not recommend the second approach though, since different implementations of IdBase may happen to have the same Value property (if you used the constructor that passes an existing ID). For instance:
var guid = Guid.NewGuid();
var customerID = Id<Customer>.From(guid);
var orderID = Id<Order>.From(guid);
Here (customerID == orderID) will then return true which is probably not what you want.
Shortly, in such a case, two different types will count as equal, which is a big logical mistake, so I'd stick to the first approach.
If you need Id<Customer>.Value to always be different than Id<Order>.Value, because of the different generic arguments (Customer is different than Order), then the following approach will work:
public sealed class Id<in TDiscriminator>
{
private static readonly Guid _idStatic = Guid.NewGuid();
private Id()
{
}
public Guid Value
{
get { return _idStatic; }
}
}
Notice the in keyword used here. This is applicable for .NET 4.0 where generics can be covariant and ensures that your class uses contravariant generics. (see http://msdn.microsoft.com/en-us/library/dd469487.aspx). In the above code, the _idStatic field will have a unique value for every different type supplied as a generic argument.
I hope this info is helpful.
In C#, you can use properties to make a data field publicly accessible (allowing the user to directly access it), and yet retain the ability to perform data validation on those directly-accessed fields. Does Java have something similar? For Instance, suppose there exists a C# class with the following implementation(see below):
public class newInt{
public newInt(){...}
public int x{
get{ return this.x }
set{ this.x = isValid(value) }
}
}
private static int isValid(int value){...}
This definition in the class allows the user to "naturally" use the data field 'x' when retrieving values from it and assigning values to it. Below is how it would be used in main.
public class Test{
public static void main(String[] args){
newInt a = new newInt();
a.x = 50;
int b = a.x;
}
}
The question is... can java do this as well? if so, what is it called?
No.
That's why Java has getters/setters.
In C# you typically have something like:
public class SomeObject
{
private string _title = "";
public string Title { get { return _title; } set { _title = value; } }
}
// Or with Auto-Properties
public class SomeObjectAutoProperties
{
public string Title { get; set; }
}
The Java getter/setter equivalent would be:
public class SomeObject
{
private String _title = "";
public string getTitle() { return _title; }
public void setTitle(String value) { _title = value; }
}
There's the Java platform, and there's the Java language.
The Java language does not support properties (and probably never will), but you are not forced to use the Java language to work with the Java platform (just as you don't need to stick to C# in order to work with .NET platform).
Check:
http://groovy.codehaus.org/
http://www.jython.org/
And many others.
Nope, you would use getter and setter methods instead. This is a Java convention.
public class newInt {
public newInt() {...}
private int _x = 0;
public int getX() {
return this._x;
}
public void setX(int x) {
this._x = isValid(x);
}
}
No. Java doesn't have properties. The Java idiom is to use mutator/accessor (getter/setter). Even though a lot of people are in favor of adding them, it is unlikely they will be included in the next version (Java 7).
Curiously, JavaFX has properties.
Keep in mind that when Java was born it borrowed a lot of ideas from C++. Thus some of the syntax and idioms are very similar to that language.
No, it hasn't.
I really have a bit of a problem to understand this C# properties, because, I think one of the rules is to perform as less code as possible in them and since they are already public, why don't use public attributes instead?
So, your Java equivalent ( and probably ugly ) would be:
public class NewInt { // In Java class names start with uppercase by convention
public int x;
}
And you use it like:
NewInt ni = new NewInt();
ni.x = 50;
int b = ni.x;
There is something I'm missing that's for sure, but, most of the times this would do ( BTW I never code like this :P )
BTW
I don't really like getters and setters, but I accept them as part of the Java conventions.
I just would like they have used this instead:
public class NewInt {
private int x;
public int x(){
return this.x;
}
public void x(int value ) {
this.x=value;
}
}
So the usage would've been:
NewInt a = new NewInt();
a.x(10);
int b = a.x();
Probably in the next Java life.