System.TypeLoadException

NOTE - If this answer doesn't help you, please take the time to scroll down through the other answers that people have added since.

Really short answer

Delete all your bin and obj directories and rebuild everything. The versions of the dlls don't match.

Short answer

This can happen if you add a method to an interface in one assembly, and then to an implementing class in another assembly, but you rebuild the implementing assembly without referencing the new version of the interface assembly.

In this case, DummyItem implements an interface from another assembly. The SetShort method was recently added to both the interface and the DummyItem - but the assembly containing DummyItem was rebuilt referencing the previous version of the interface assembly. So the SetShort method is effectively there, but without the magic sauce linking it to the equivalent method in the interface.

Long answer

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.