I have come across this several times in the last week, and am curious to know the reason - I had a google, but couldn't find anything directly relevant.
I have a class with a dynamic method, and I can add a static method with the same interface:
public class MyClass
{
public int MyMethod()
{
//do something #1;
}
public static int MyMethod()
{
//do something
}
}
This is fine, but if I try to call the static method from the dynamic method, replacing #1 with return MyClass.MyMethod(), I get an error "The call is ambiguous between the following methods or properties: MyClass.MyMethod() and MyClass.MyMethod().
If the static method is deleted, the error changes to "An object reference is required..", which makes sense.
So why is this ambiguous? It has been prefaced with the class name to specify the static method, which works from anywhere else in code.
Why not here?
EDIT: I hadn't actually tried to compile it without the dynamic method calling the static one, I had just gone by VS not underlining it.
But still a similar question I suppose but with an added "Why can't there be both, as one is static, and one not"
Besides here is a similar question on SO, giving some explanation, why you can't have two methods with the same signature.
public class MyClass
{
public int MyMethod()
{
return 0;
}
public static int MyMethod() //Here compiler says, that you've already got method MyMethod with same parameter list
{
return 0;
}
}
So, you can't have those methods at all
Have a look at this
At first:
The signature of a method consists of the name of the method and the
type and kind (value, reference, or output) of each of its formal
parameters, considered in the order left to right. The signature of a
method specifically does not include the return type, nor does it
include the params modifier that may be specified for the right-most
parameter.
Secondly:
Overloading of methods permits a class, struct, or interface to
declare multiple methods with the same name, provided their signatures
are unique within that class, struct, or interface.
EDIT
As for the reason why you get that error: you probably didn't compile yet and see an error underlined with red. If you do compile, you'll see the error underlined with blue not in the line where you call your static method, but on the line where the static method is defined.
So why is this ambiguous? It has been prefaced with the class name to
specify the static method, which works from anywhere else in code. Why
not here?
The reason it is ambiguous is because you could have an instance property on MyClass called MyClass. I believe Eric Lippert describes the reasoning in more detail. I can't quite find that specific post, but here is a similar series from him.
Related
The following program does not compile, because in the line with the error, the compiler chooses the method with a single T parameter as the resolution, which fails because the List<T> does not fit the generic constraints of a single T. The compiler does not recognize that there is another method that could be used. If I remove the single-T method, the compiler will correctly find the method for many objects.
I've read two blog posts about generic method resolution, one from JonSkeet here and another from Eric Lippert here, but I could not find an explanation or a way to solve my problem.
Obviously, having two methods with different names would work, but I like the fact that you have a single method for those cases.
namespace Test
{
using System.Collections.Generic;
public interface SomeInterface { }
public class SomeImplementation : SomeInterface { }
public static class ExtensionMethods
{
// comment out this line, to make the compiler chose the right method on the line that throws an error below
public static void Method<T>(this T parameter) where T : SomeInterface { }
public static void Method<T>(this IEnumerable<T> parameter) where T : SomeInterface { }
}
class Program
{
static void Main()
{
var instance = new SomeImplementation();
var instances = new List<SomeImplementation>();
// works
instance.Method();
// Error 1 The type 'System.Collections.Generic.List<Test.SomeImplementation>'
// cannot be used as type parameter 'T' in the generic type or method
// 'Test.ExtensionMethods.Method<T>(T)'. There is no implicit reference conversion
// from 'System.Collections.Generic.List<Test.SomeImplementation>' to 'Test.SomeInterface'.
instances.Method();
// works
(instances as IEnumerable<SomeImplementation>).Method();
}
}
}
Method resolution says that closer is better. See the blog post for exact rules.
What does the closer mean? Compiler will see if it can find exact match, if it can't find for some reason it will find next possible compatible methods and so forth.
Let's first make that method compile by removing the SomeInterface constraint.
public static class ExtensionMethods
{
public static void Method<T>(this T parameter) //where T : SomeInterface
{ }
public static void Method<T>(this IEnumerable<T> parameter) //where T : SomeInterface
{ }
}
Now compiler is happy to compile, and do note that both method calls Goes to Method(T) rather than Method(IEnumerable<T>). Why is that?
Because Method(T) is closer in the sense that can take any type as the parameter and also it doesn't require any conversion.
Why is Method(IEnumerable<T>) not closer?
It is because you have the compile time type of the variable as List<T>, so it needs a reference conversion from List<T> to IEnumerable<T>. Which is closer but far from doing no conversions at all.
Back to your question.
Why instances.Method(); doesn't compile?
Again, as said earlier to use Method(IEnumerable<T>) we need some reference conversion, so obviously that's not closer. Now we're left with only one method which is very closer is Method<T>. But the problem is you have constrained it with SomeInterface and clearly List<SomeImplementation>() is not convertible to SomeInterface.
The problem is (am guessing) checking for generic constraints happens after the compiler chooses the closer overload. That invalidates the chosen best overload in this case.
You could easily fix it by changing the static type of the variable to IEnumerable<SomeImplementation> that will work and now you know why.
IEnumerable<SomeImplementation> instances = new List<SomeImplementation>();
Have you tried implementing the first one without generics, as it should behave the same:
public static void Method(this SomeInterface parameter) { /*...*/ }
Or, as Dmitry suggested, by calling the second one the following way:
instances.Method<SomeImplementation>();
But here you need to add the <SomeImplementation> to every call...
While I know you dont want it, I think you should really re-think if method names should be the same. I cannot see how the same name can act on an instance, and collection of such instances. For eg, if your method name is Shoot for T, then the other method should sound like ShootThemAll or something similar.
Or else you should make your assignment slightly different:
IEnumerable<SomeImplementation> instances = new List<SomeImplementation>();
instances.Method(); //now this should work
As a last option, as Dimitry says in comments you have to explicitly specify the type argument.
instances.Method<SomeImplementation>();
For some reason I have a feeling only Jon Skeet would know the answer to this but it's worth a shot.
I have this method stub which is used as a proxy to generate commands.
public static void SetCommand<T>(string commandName, Action<T> execution)
where T : new()
{
//Omitted unimportant
}
I have a function which is calling this code, but the behavior differs depending on how it's called.
If I call the code by explictly declaring an Action all of the parameter can resolve fine
Action<ClassDeclarationOptions> test = (t) => { };
SetCommand(GENERATE_CLASS_COMMAND, test);
However if I declare a function to represent my action
public static void GenerateClass(ClassDeclarationOptions classOptions)
{
}
Then I need to explictly declare the parameter when passing it to the function like so:
SetCommand<ClassDeclarationOptions>(GENERATE_CLASS_COMMAND, Commands.GenerateClass);
Can someone explain why the compiler cannot resolve my Generic Parameter from Method Definition
Let's say there's an overload on Commands.GenerateClass that takes a different parameter type, for example:
public static void GenerateClass(SomeOtherClass stuff)
{
}
Now there's no way for the compiler to figure out which GenerateClass method to use. However, as soon as you specify the type parameter the ambiguity goes away.
So even if you had a single method and the compiler inferred the type arguments, you could add that second method later on and break existing code without even realising it. Much safer just to force code to be explicit.
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.
Let we have two members equal by signature, but one is static and another - is not:
class Foo
{
public void Test() { Console.WriteLine("instance"); }
public static void Test() { Console.WriteLine("static"); }
}
but such code generate brings a compiler error:
Type 'Foo' already defines a member called 'Test' with the same parameter types
But why?
Let we compiled that successfully, then:
Foo.Test() should output "static"
new Foo().Test();should output "instance"
Can't call the static member instead of instance one because in this case another, more reasonable compiler error will occur:
Member 'Foo.Test()' cannot be accessed with an instance reference; qualify it with a type name instead
What about, from an instance method:
Test();
What would that call? You'd probably want to give the instance method "priority" over the static method, but both would be applicable.
I would say that even if it were allowed, it would be a fundamentally bad idea to do this from a readability point of view... for example, if you changed a method which called Test from being static to instance, it would change the meaning in a subtle way.
In other words, I have no problem with this being prohibited :)
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?