TypeLoadException says 'no implementation'

Question

I've got a very weird bug on our test machine. The error is:

System.TypeLoadException: Method 'SetShort' in type 'DummyItem' from assembly 'ActiveViewers (...)' does not have an implementation.

I just can't understand why. SetShort is there in the DummyItem class, and I've even recompiled a version with writes to the event log just to make sure that it's not a deployment/versioning issue. The weird thing is that the calling code doesn't even call the SetShort method.

Answer 1

In addition to what the asker's own answer already stated, it may be worth noting the following. The reason this happens is because it is possible for a class to have a method with the same signature as an interface method without implementing that method. The following code illustrates that:

public interface IFoo
{
    void DoFoo();
}

public class Foo : IFoo
{
    public void DoFoo() { Console.WriteLine("This is _not_ the interface method."); }
    void IFoo.DoFoo() { Console.WriteLine("This _is_ the interface method."); }
}

Foo foo = new Foo();
foo.DoFoo();               // This calls the non-interface method
IFoo foo2 = foo;
foo2.DoFoo();              // This calls the interface method

Answer 2

I encountered this when I renamed a project (and the assembly name), which was depended upon by an ASP.NET project. Types in the web project implemented interfaces in the dependent assembly. Despite executing Clean Solution from the Build menu, the assembly with the previous name remained in the bin folder, and when my web project executed

var types = AppDomain.CurrentDomain.
   GetAssemblies().
   ToList().
   SelectMany( s => s.GetTypes() /* exception thrown in this call */ )
;

the above exception was thrown, complaining that interface methods in the implementing web types were not actually implemented. Manually deleting the assembly in the web project's bin folder resolved the problem.

Answer 3

I saw this in Visual Studio Pro 2008 when two projects built assemblies with the same name, one a class lib SDF.dll, and one that referenced the lib with assembly name sdf.exe. When I changed the name of the referencing assembly, the exception went away

Answer 4

I also got this error when I had previously enabled Code Coverage during unit testing for one of the assemblies. For some reason Visual Studio "buffered" the old version of this particular DLL even though I had updated it to implement a new version of the interface. Disabling Code Coverage got rid of the error.

Answer 5

Here's my take on this error.

Added an extern method, but my paste was faulty. The DllImportAttribute got put on a commented out line.

/// <returns>(removed for brevity lol)</returns>[DllImport("user32.dll")] 
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool IsWindowVisible(IntPtr hWnd);

Ensuring the attribute was actually included in source fixed the issue.

Answer 6

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

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.