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Difference in conversion of anonymous method and lambda expression

  • Artikkeli

Consider the following class

 delegate void Moo(int a, int b, int c);

class A
{
    public event Moo moo;
}

In case I want to create a handler for the moo event where I don't care about the 3 parameters needed by the event handler I can easily write it as

 A a = new A();
a.moo += delegate { count++; };

However, in case I want to create the same handler with a lambda expression. I have to do it as

 A a = new A();
a.moo += (a1,a2,a3) => count++;

There is no shortcut to this. I can't use say a.moo += () => count++;

The reason lies in the fact that both support a different set of conversions.

From C# 2.0 spec (§21.3 - Anonymous methods conversions)

An implicit conversion (§6.1) exists from an anonymous-method-expression to any compatible delegate type. If D is a delegate type, and A is an anonymous-method-expression, then D is compatible with A if and only if the following two conditions are met.

• First, the parameter types of D must be compatible with A:

o If A does not contain an anonymous-method-signature, then D may have zero or more parameters of any type, as long as no parameter of D has the out parameter modifier.

In C# 3.0 spec for Lambda expression conversion it sez

Specifically, a delegate type D is compatible with a lambda-expression L provided:
• D and L have the same number of parameters.
• If L has an explicitly typed parameter list, each parameter in D has the same type and modifiers as the
corresponding parameter in L.
• If L has an implicitly typed parameter list, D has no ref or out parameters.
...

In both the cases the lines marked in red indicate why one is different from the other.

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