Extension method priority

Question

I read from https://msdn.microsoft.com/en-us/library/vstudio/bb383977.aspx that extension methods with same name and signature as existing ones on the base type are never ca

Answer 1

Here is a longer example with five possible Where<> methods:

using System;
using System.Collections.Generic;
using System.Linq; // **OUTSIDE**

namespace Me.MyName
{
  using System.MySuperLinq; // **INSIDE**

  static class Test
  {
    static void Main()
    {
      (new[] { "Hans", "Birgit" }).Where(x => x == "Hans");
    }
  }
}

namespace System.MySuperLinq
{
  static class Extensions
  {
    public static IEnumerable Where(this IEnumerable source, Func predicate)
    {
      Console.WriteLine("My own MySuperLinq method");
      return null; // will fix one day
    }
  }
}

namespace Me.MyName
{
  static class Extensions
  {
    public static IEnumerable Where(this IEnumerable source, Func predicate)
    {
      Console.WriteLine("My own MyName method");
      return null; // will fix one day
    }
  }
}

namespace Me
{
  static class Extensions
  {
    public static IEnumerable Where(this IEnumerable source, Func predicate)
    {
      Console.WriteLine("My own Me method");
      return null; // will fix one day
    }
  }
}

// this is not inside any namespace declaration
static class Extensions
{
  public static IEnumerable Where(this IEnumerable source, Func predicate)
  {
    Console.WriteLine("My own global-namespace method");
    return null; // will fix one day
  }
}

Try successively removing which ever method is preferred, to see the "priority" the C# compiler uses.

• The C# compiler will first search for static classes inside Me.MyName namespace since that is the "current" namespace.
• It will then search in System.MySuperLinq because that using is inside.
• It will then go out one level and search in Me namespace
• It will then search in the global (null) namespace.
• Finally, it will serach System, System.Collections.Generic, and System.Linq.

If two or more equally relevant methods are found at one "level" (bullets above), that is a compile-time ambiguity (will not compile).