Extension functions does not work for dynamic objects [duplicate]

Question

I have a Extension Function named ParseLong for string.

public static long ParseLong(this string x, long Default = 0) 
{ 
if (!string.IsNullOrEmpty(x)) 
     long.TryParse(x, out Default);
 return Default; 
}

And works fine:

long x = "9".ParseLong();

However for dynamic objects like:

dynamic x = GetValues();
x.StartValue.ToString().ParseLong();

generates the error:

'string' does not contain a definition for 'ParseLong'

Answer 1

Correct, extension functions do not work for dynamic objects. That is because the dynamic object, when being told to execute ParseLong, has no clue what using directives were in your C# code, so cannot guess what you want to do.

Extension methods are 100% a compiler feature (only); dynamic is primarily a runtime feature (although the compiler has to help it in places).

You could just cast, though, if you know the type:

long x = ((string)x.StartValue).ParseLong();

(which swaps back from dynamic to regular C#, so extension methods work)