C# Issues and Solutions -- Method get_XXX from assembly YYYY does not have an implementation

This can occur if you have an interface in one assembly and its implementation in another, and the implementation assembly was built against a different version of the interface.

If you want to try reproducing this, try the following:
   1. Create a class library project: InterfaceDef, add just one class, and build:
public interface IInterface { string GetString(string key); //short GetShort(string key); }       
   2. Create a second class library project: Implementation (with separate solution), copy InterfaceDef.dll into project directory and add as file reference, add just one class, and build:

public class ImplementingClass : IInterface { #region IInterface Members public string GetString(string key) { return "hello world"; } //public short GetShort(string key) //{ // return 1; //} #endregion }
   3. Create a third, console project: ClientCode, copy the two dlls into the project directory, add file references, and add the following code into the Main method:

IInterface test = new ImplementingClass(); string s = test.GetString("dummykey"); Console.WriteLine(s); Console.ReadKey();
   4. Run the code once, the console says "hello world"
   5. Uncomment the code in the two dll projects and rebuild - copy the two dlls back into the ClientCode project, rebuild and try running again. TypeLoadException occurs when trying to instantiate the ImplementingClass.

http://stackoverflow.com/questions/948785/typeloadexception-says-no-implementation-but-it-is-implemented