TypeRegistry.cs

using System;
using System.Collections.Generic;

namespace Utils
{
	public class UnknownTypeException : Exception
	{
	}
	
	public class TypeRegistry
	{
		private static Dictionary<string, Type> reverseLookupTable = new Dictionary<string, Type>(); // first level cache (caches GetInstance<T>(string))
		private static Dictionary<string, List<Type>> implementationCache = new Dictionary<string, List<Type>>(); // second level cache (caches GetAllImplementationsOfInterface(Type))
		
		public static List<Type> GetAllImplementationsOfInterface<T>()
		{
			Type interfaceType = typeof(T);
			var typeName = interfaceType.ToString();
			if (implementationCache.ContainsKey(typeName)) {
				return implementationCache[typeName];
			}
			
			var result = new List<Type>();
			System.Reflection.Assembly[] allAssemblies = AppDomain.CurrentDomain.GetAssemblies();
			foreach (var assembly in allAssemblies) {
				Type[] types = assembly.GetTypes();
				foreach (var inspectedType in types) {
					var interfaces = inspectedType.FindInterfaces((Type typeObj, Object criteriaObj) => {
						if (typeObj.ToString().Equals(typeName)) { // Type.IsEquivalentTo(Type) is unavailable in MonoTouch
							return true;
						}
						return false;
					}, null);
					if (interfaces.Length > 0) {
						result.Add(inspectedType);
					}
				}
			}
			implementationCache[typeName] = result;
			return result;
		}
		
		public static T GetInstance<T>(string implementationName)
		{
			Type desiredType = null;
			if (reverseLookupTable.ContainsKey(implementationName)) {
				desiredType = reverseLookupTable[implementationName];
			} else {
				List<Type> allImplementations = GetAllImplementationsOfInterface<T>();
				foreach (Type inspectedType in allImplementations) {
					if (inspectedType.ToString().Equals(implementationName)) {
						desiredType = inspectedType;
						reverseLookupTable[implementationName] = desiredType;
					}
				}
			}
			if (desiredType == null) {
				throw new UnknownTypeException();
			}
			T ret = (T)Activator.CreateInstance(desiredType);
			return ret;
		}
	}
}