Why Does Method Group Overload Resolution Fail with Ambiguous Invocation Errors in C#?

C# Method Group Overload Resolution Ambiguity

The Issue:

Using method group syntax with overloaded delegate methods in C# can trigger an "Ambiguous invocation" compiler error, even if anonymous methods successfully call the same overloads.

Why it Happens:

The C# compiler lacks a definitive "better" rule when converting a method group to multiple compatible delegate types during overload resolution. For instance, if classWithSimpleMethods.GetString could be converted to both Func<string> and Action, the compiler can't choose between them, leading to the ambiguity.

Resolution (Explicit Type Casting):

The solution is to explicitly cast the method group to the desired delegate type:

<code class="language-csharp">classWithDelegateMethods.Method((Func<string>)classWithSimpleMethods.GetString);</code>

C# 7.3 Improvement:

C# 7.3 enhanced overload resolution for method groups. In many cases (like the example provided), classWithDelegateMethods.Method(classWithSimpleMethods.GetString) and similar calls now compile without explicit casting. However, ambiguity can still arise in complex scenarios.

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