How to recognize extension method from assembly?

Question

As far as I know, the C# extension method has the attribute of 'System.Runtime.CompilerServices.ExtensionAttribute'

so i wrote simple test and I check with ilspy and this does not exist there.
I also on creating an object instance and investigating the instance method - I can't recognize if some method is an extension method or not.

I also look on the il viewer but on the viewer i see the 'System.Runtime.CompilerServices.ExtensionAttribute'

I looking for another way to know if some object/assembly method are extension method from runtime code.

(.net core 6.0)

Answer 1

Pretty easily, just check whether a given MethodInfo has the ExtensionAttribute applied to it using the GetCustomAttributes(Type, bool) method

So:

public static bool IsExtensionMethod(this MethodInfo method)
{
    // The 'false' is because static classes must derive from 'object'
    // So checking for inherited attributes is unnecessary
    return method.GetCustomAttributes(typeof(ExtensionAttribute), false).Length > 0;
}

Demo

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FWIW I recommend not looking at IL directly (unless you really need to), instead I recommend you use something like Sharplab to decompile the C# code to actually see what gets compiled, as "code lowering" (sometimes also called desugaring/ stripping syntactic sugar away) is one of the first steps the compiler does