You can get a list of loaded assemblies by using this:
Assembly assembly = System.Reflection.AppDomain.CurrentDomain.GetAssemblies()
From there you can get a list of types in the assembly (assuming public types):
Type types = assembly.GetExportedTypes();
Then you can ask each type whether it supports that interface by finding that interface on the object:
Type interfaceType = type.GetInterface("ISomething");
Not sure if there is a more efficient way of doing this with reflection off the top if my head.
You could use something like the following and tailor it to your needs.
var _interfaceType = typeof(ISomething);
var currentAssembly = System.Reflection.Assembly.GetExecutingAssembly();
var types = GetType().GetNestedTypes();
foreach (var type in types)
if (_interfaceType.IsAssignableFrom(type) && type.IsPublic && !type.IsInterface)
ISomething something = (ISomething)currentAssembly.CreateInstance(type.FullName, false);
This code could use some performance enhancements but it's a start.