There is collection of Func<...> classes - Func that is probably what you are looking for:
void MyMethod(Func<int> param1 = null) This defines method that have parameter param1 with default value null (similar to AS), and a function that returns int. Unlike AS in C# you need to specify type of the function's arguments.
So if you AS usage was
MyMethod(function(intArg, stringArg) { return true; }) Than in C# it would require param1 to be of type Func<int, siring, bool> and usage like
MyMethod( (intArg, stringArg) => { return true;} );