How to find an implementation of a C# interface in the current assembly with a specific name?

Question

I have an Interface called IStep that can do some computation (See "Execution in the Kingdom of Nouns"). At runtime, I want to select the appropriate implementation by class name.

// use like this:
IStep step = GetStep(sName);

Answer 1

Your question is very confusing...

If you want to find types that implement IStep, then do this:

foreach (Type t in Assembly.GetCallingAssembly().GetTypes())
{
  if (!typeof(IStep).IsAssignableFrom(t)) continue;
  Console.WriteLine(t.FullName + " implements " + typeof(IStep).FullName);
}

If you know already the name of the required type, just do this

IStep step = (IStep)Activator.CreateInstance(Type.GetType("MyNamespace.MyType"));

Answer 2

If the implementation has a parameterless constructor, you can do this using the System.Activator class. You will need to specify the assembly name in addition to the class name:

IStep step = System.Activator.CreateInstance(sAssemblyName, sClassName).Unwrap() as IStep;

http://msdn.microsoft.com/en-us/library/system.activator.createinstance.aspx

Answer 3

Based on what others have pointed out, this is what I ended up writing:

/// 
/// Some magic happens here: Find the correct action to take, by reflecting on types 
/// subclassed from IStep with that name.
/// 
private IStep GetStep(string sName)
{
    Assembly assembly = Assembly.GetAssembly(typeof (IStep));

    try
    {
        return (IStep) (from t in assembly.GetTypes()
                        where t.Name == sName && t.GetInterface("IStep") != null
                        select t
                        ).First().GetConstructor(new Type[] {}
                        ).Invoke(new object[] {});
    }
    catch (InvalidOperationException e)
    {
        throw new ArgumentException("Action not supported: " + sName, e);
    }
}

Answer 4

Well Assembly.CreateInstance would seem to be the way to go - the only problem with this is that it needs the fully qualified name of the type, i.e. including the namespace.