I have the method shown below which is generating a CA1822 Code Analysis warning. CA1822 says this:
"The 'this parameter (or 'Me' in Visual Basic) of 'ImportForm.ProcessFile(StreamReader)' is never used. Mark the member as static (or Shared in Visual Basic) or use 'this/Me' in the method body or at least one property accessor, if appropriate."
Can anyone tell me why I am getting this warning, since the 'reader' parameter is in fact being used?
private void ProcessFile(StreamReader reader)
{
string[] lines;
lines = reader.ReadToEnd().Split(new string[] { Environment.NewLine }, StringSplitOptions.None);
ParseFile.IVAFile(lines);
}
It means you use no members of the object. All the items in the method come from the parameters.
Therefore the method can safely be made static.
"reader" is being used, but you're not using "this" anywhere, so you can make the method static.
The only reason not to make it static would be if you want to use polymorphism later - e.g. making it virtual and overriding it elsewhere.
Maybe I have found malicious behavior of this message.
In a Situation like
void Print()
{
Console.Writeline(GetType().Name);
}
I get this CA1822 reported, although GetType() is an instance method.
However, I found some Explanation, why GetType() is actually not a virtual method, not an instead method, and technicly behaving like a static method.
It's just that the code Analysis does not consider this special behavior.
The warning occurs because you don't use any member variables of that class in that method. E.g.
this.m_anyVariable = anyValue;
Therefore you can/should mark that method as static.
I think it is trying to tell you that this method can be made static.
The only thing this method needs to access is "reader", but nothing from the class instance to which it belongs ("this"). In which case, you can safely make it static.
Related
Can someone explain to me why in the following the 3rd invocation of DoSomething is invalid?
( Error message is "The name 'DoSomething' does not exist in the current context" )
public class A { }
public class B : A
{
public void WhyNotDirect()
{
var a = new A();
a.DoSomething(); // OK
this.DoSomething(); // OK
DoSomething(); // ?? Why Not
}
}
public static class A_Ext
{
public static void DoSomething(this A a)
{
Console.WriteLine("OK");
}
}
Extension methods can be invoked like other static methods.
Change it to A_Ext.DoSomething(this).
If you're asking why it isn't implicitly invoked on this, the answer is that that's the way the spec was written. I would assume that the reason is that calling it without a qualifier would be too misleading.
Because DoSomething takes a parameter.
DoSomething(a) would be legal.
Edit
I read the question a bit wrong here.
Since your calling it a a normal static method, and not a extension method, you need to prefic with the class name.
So A_Ext.DoSomething(a); will work.
If you call it like a normal static method, all the same rules apply.
Your second variant works because B inhetits A, and therefore you still end up calling it as an extension method, but the third does not.
sorry about the first version above that does not work. I'll leave it to keep the comment relevant.
Extension methods are still static methods, not true instance calls. In order for this to work you would need specific context using instance method syntax (from Extension Methods (C# Programming Guide))
In your code you invoke the extension
method with instance method syntax.
However, the intermediate language
(IL) generated by the compiler
translates your code into a call on
the static method. Therefore, the
principle of encapsulation is not
really being violated. In fact,
extension methods cannot access
private variables in the type they are
extending.
So while normally, both syntaxes would work, the second is without explicit context, and it would seem that the IL generated can't obtain the context implicitly.
DoSomething requires an instance of A to do anything, and without a qualifier, the compiler can't see which DoSomething you need to invoke. It doesn't know to check in A_Ext for your method unless you qualify it with this.
I am a vb.net programmer switching to C#.
I have the following code for a console application (targeting NET20)
using System;
using System.IO;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace eScoreSwapper
{
class Program
{
private string _dbName = ConfigurationManager.AppSettings["dbName"];
static void Main(string[] args) {}
static void InitVars()
{
if (string.IsNullOrEmpty(_dbName)) _dbName = "";
}
}
}
This is giving a compile error in the if clause of InitVars for the _dbName variable:
Error 1 An object reference is required for the non-static field, method, or property 'eScoreSwapper.Program._dbName' C:\Users\SethS\Documents\eScore\Versions\Trunk\dotNet\eScoreSwapper\eScoreSwapper\Program.cs 26 38 eScoreSwapper
Is it because it is true. C# does not allow you to reference private class fields unless they are declared static? I am sure I am doing something wrong.
While I am at it can I ask another C# question. Why is the if statement valid? Why are braces not required? Is it valid syntax as long as if condition is followed by a single expression (as in t-sql IF ).
Thanks for your help.
Seth
No, but private fields used by static method do. What I think you really want to do is remove the static from the declaration of InitVars().
I'm gonna assume that you previously tried:
static void Main(string[] args)
{
InitVars();
}
and that failed, so you added the static to InitVars(). Wrong answer. Basically, you should pretend Main() is not part of the class.
static void Main(string[] args)
{
Program prog = new Program();
prog.InitVars();
}
As far the If() statement
if (string.IsNullOrEmpty(_dbName)) _dbName = "";
is just a short version of:
if (string.IsNullOrEmpty(_dbName))
_dbName = "";
Now, in C-ish language, officially, an if() that is true, will execute the (one) next statement. But, you can make several statement into one statement, by wrapping them in curly-braces. But, if you really only have one statement, they aren't needed.
You can't use an instance member variable in a static method. Either make your variable static or put the method and variable in an instance class and new it up in main on program.
The reason you can't access an instance variable from a static method becomes obvious when you think about the fact that the method exists "on the class" while the variable exists "on the instance". Hope that helps.
As a final comment, I'd just add that the fact that the compiler is giving you the error has nothing to do with the fact that your field is private; if your field was public, internal, etc. the compiler would still give you the same error: ie. static methods can't access instance variables.
You can't access a non-static member in a static method.
Braces are not required unless you want it to apply to more than one statement. But you should put them anyways.
You can't access a non-static class member from within a static method without using a method parameter to bring a reference of the class member in.
The if syntax is normal c# (c++, c). Braces are only required for complex, multi-line blocks. The following else can have a singular line as well.
To answer the question about braces, they are only truly mandatory in C# if there is more than one statement in the if block. Otherwise, you can put your one-liner on the same line as the if, or on the next line. I believe that it is generally advised in coding standards that you always use the brackets. It is always often advised not to put the entire series of code on all one line, but that kind of went out the window when we got tertiary and null coalescing operators.
You're trying to access an instance field from a static context. Since _dbName isn't marked static, it could possibly be different for each instance of the Program class, so the static method InitVars() has no way of knowing what the right value is.
If every single instance of Program will have the same value for _dbName, then _dbName should be marked static. If different instances may have different values, then InitVars() should be an instance method (remove the static keyword and only invoke it on actual Program objects rather than from the static Main).
"static" in VB parlance would be "shared".
I've already read the MSDN article about it. It seems internally it is the way c# sets which is the function that is going to work as indexer(am I right?). Now, I've seen the following example:
[DefaultMemberAttribute("Main")]
public class Program {
public static void Main() {
...
}
}
Now, I don't get it what it means.
Thanks all. But I still can't get its usefulness, apart from the indexer thing. When are we going to call InvokeMember?
No, the DefaultMemberAttribute is used by languages such as VB.NET to find out the member that is acted on by default if no member is referenced from an object, i.e. the member invoked by InvokeMember. This is often used in conjunction with indexers, as you noted, but it is not used by C# directly (unless you use InvokeMember explicitly).
However, for the benefit of other .NET languages, C# does emit the DefaultMemberAttribute for the indexer of a class (if it has one), as indicated by MSDN:
The C# compiler emits the
DefaultMemberAttribute on any type
containing an indexer. In C# it is an
error to manually attribute a type
with the DefaultMemberAttribute if the
type also declares an indexer.
I think MSDN confuses things by referring to indexers a lot in the remarks but then giving an example that does not use an indexer. To clarify, the default member can be anything, but C# gives special behavior for indexers by emitting the attribute for you (if an indexer exists) to the exception of all other use cases.
I personally have never used it, but as far as I can tell you are defining the default method to be invoked when calling InvokeMember. So, using the code snippet you provided if I was to say:
Program prog = new Program();
typeof(Program).InvokeMember("", null, null, prog, null);
Because I left the first argument empty of the InvokeMember call it would use the attribute to determine what the default member is of your class, in your case it is Main.
The DefaultMemberAttribute attribute defines the default member to be called on a when InvokeMember is called with an empty string as the first argument.
If you read the MSDN docs for InvokeMember, it explicitly says:
Parameters
name
Type: System.String
The String containing the name of the constructor, method, property, or field member to invoke.
-or-
An empty string ("") to invoke the default member.
The default member will be the one declared by the DefaultMemberAttribute attribute.
How does the system know what to use when 'this' keyword is used?
Recently, I was asked this question in an interview. Having never thought about this, I replied back saying that the system will know the current context in which the flow of control is and decide the object to be used instead of this. The interviewer didn't look pleased and he moved on to the next question.
Can anyone tell me what did the interviewer may have wanted to ask and what would be the answer?
(I think this can interpreted in different ways and hence keeping this as wiki unless someone point out not to..)
Though the answers which point out that the "this" reference is essentially passed as a magic "hidden parameter" to the call are essentially correct, the full story is actually quite a bit more complicated in C# than one might think at first glance.
Reference types are straightforward; the referenced object is checked for null and then conceptually passed as an unnamed, non-variable parameter called "this". The story is complicated by value types.
Remember, value types are, by definition, passed by value -- that is, passed by making a copy of the data. Hence their name. But clearly mutable value types -- which are pure evil and should be avoided -- cannot be passed by value as the "this", because if you called a mutator, the "this" in the mutator method would be mutating the copy, not the original!
Therefore, in a value type's method call, "this" is not the value of the receiver, it is an alias to the variable representing the storage location of the receiver. We implement this by passing "this" as the managed address of the receiver, not the value of the receiver.
Now we can raise another difficulty. What if the variable storing the value being mutated is a read-only variable? Now what do we do? If you're curious, read my article on the subject and see if you can correctly answer the puzzle presented:
http://blogs.msdn.com/ericlippert/archive/2008/05/14/mutating-readonly-structs.aspx
The this keyword is a pointer to the current object. All non-static member functions of a class have access to a this pointer.
The pointer to the current object is normally made available by the compiler in a non-static member function by using a register, usually ECX. So when you write this in a non-static member function the compiler will translate that call into loading the address from ECX.
Check this simple example:
A t;
t.Test();
004114DE lea ecx,[t]
004114E1 call std::operator > (41125Dh)
Before calling the non-static member function Test() the compiler loads the register ECX with [t] (the address of variable t - will be this inside Test method).
004114DE lea ecx,[t]
And inside the function it can use ecx to obtain the address for the current object instance.
this is a hidden parameter in all methods of an object and contains a copy of the instance pointer.
Consider this class
class A {
private:
int data;
public:
void SetData(int arg) {
this->data = arg;
}
}
and this code that calls SetData():
A objA;
objA.SetData(1);
When the above code is compiled, the compiler emits something equivalent to this for the member function:
void SetData(A* this, int arg) {
this->data = arg;
}
And the calling code is converted to something like this:
A objA;
SetData(&objA, 1);
What this means is that upon compilation:
Member functions are converted to simple, global functions.
The class instance to which member functions "belong" is simply passed as the first argument to them (or rather its address is passed).
So in conclusion, what you refer to as this pointer in your code simply ends being the the first argument to the function. This is how the "system knows" which object to access through the "this" pointer.
The above example is of C++. If you for a moment forget about the CLR and the JITting and all that, what happens in C# is conceptually the same thing.
At run time "this" will resolve to the pointer to the current object, therefore "the system" will be able to call a respective method on that object.
I would have answered "It is a reference to the instance of the current class."
Dan
The compiler takes care of correctly resolving that reference at compile time. If you want to know more information about some of the techniques they use to do this, this book will tell you everything you need to know:
http://en.wikipedia.org/wiki/Compilers:_Principles,_Techniques,_and_Tools
in c the this pointer is an invisible pointer argument to your class.
So in essence the first argument to a class method is the pointer to the class itself. You reference it by using this.
My answer would have been "Who cares? It knows. If I ever need to find out in more detail I'll Google it."
You obviously know what the effect of using "this" would be, which surely is the important thing. For 99% of programming tasks I would have thought that the detail of how it is resolved internally is trivia.
Shematicaly compiler converts such code:
pObject->someMethod(A,B,C)
Into such code if 'someMethod' is not virtual:
someMethod(pObject,A,B,C)
Or into such code if 'someMethod' is virtual:
(*pObject->vtable[someMethodIndex]) (pObject, A,B,C)
And everywhere you place 'this' keyword first parameter is used instead of it;
Of course compiler may optimize/simplify by removing first argument and use some CPU register (normally esx) to store object's address.
The "this" operator will point to the current object. An example where the presence of a "this" operator will make a difference is this:
public class MyClass
{
int value;
public void Test(int value)
{
MessageBox.Show(value); // Will show the parameter to the function
MessageBox.Show(this.value); // Will show the field in the object
}
}
Note that the "this" operator will not change what virtual function will be called if it is overridden in a child class
public class MyClass
{
public virtual void Test() {}
public void CallTest()
{
this.Test();
}
}
public class MyClass2 : MyClass
{
public override void Test() {}
}
If you execute the following code
MyClass c = new MyClass2();
c.CallTest();
will still call the MyClass2.Test() and not MyClass.Test()
So the "this" operator simply tells you, that you are accessing something declared at class level.
I have a question that's not really a problem, but something that made me a little curious.
I have a class with two methods in it. One is a static method and the other one is an instance method. The methods have the same name.
public class BlockHeader
{
public static BlockHeader Peek(BinaryReader reader)
{
// Create a block header and peek at it.
BlockHeader blockHeader = new BlockHeader();
blockHeader.Peek(reader);
return blockHeader;
}
public virtual void Peek(BinaryReader reader)
{
// Do magic.
}
}
When I try to build my project I get an error saying:
The call is ambiguous between the
following methods or properties:
'MyApp.BlockHeader.Peek(System.IO.BinaryReader)'
and
'MyApp.BlockHeader.Peek(System.IO.BinaryReader)'
I know that the method signatures are virtually the same, but I can't see how I possibly could call a static method directly from an instance member.
I assume that there is a very good reason for this, but does anyone know what that reason is?
The general policy of the C# design is to force you to specify wherever there is potential ambiguity. In the face of refactoring tools that allow one to rejig whether things are static or not at the drop of a hat, this stance is great - especially for cases like this. You'll see many other cases like this (override vs virtual, new for shadowing etc.).
In general, removing this type of room for confusion will make the code clearer and forces you to keep your house in order.
EDIT: A good post from Eric Lippert discusses another reason for this ambiguity leading to the error you saw
Here's a excerpt from the C# 3.0 language specification.
The signature of a method must be unique in the class in which the method is declared. The signature of a method consists of the name of the method, the number of type parameters and the number, modifiers, and types of its parameters. The signature of a method does not include the return type.
The 'static' modifier is not part of the signature so your example violates this rule of unique signatures.
I don't know the reason behind the rule, though.
I think there's no technical reason to disallow it, but it is done more so to protect the programmer from himself. Consider the following example:
public static void Main()
{
BlockHeader BlockHeader = new BlockHeader();
BlockHeader.Peek();
}
The example above is perfectly valid, but if the situation you describe were allowed, would it be readable? Could you see, in the blink of an eye, whether the instance method or the static method was called?