Compiler Error CS0439
An extern alias declaration must precede all other elements defined in the namespace
This error occurs when an extern declaration comes after something else, such as a using declaration, in the same namespace. The extern declarations must come before all other namespace elements.
Example
The following example generates CS0439:
// CS0439.cs
using System;
extern alias MyType; // CS0439
// To resolve the error, make the extern alias the first line in the file.
public class Test
{
public static void Main()
{
}
}