Fix mod assembly loading on Windows.

OpenRA.Game/ObjectCreator.cs

@@ -10,23 +10,74 @@
 
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Reflection;
+using System.Security.Cryptography;
+using OpenRA.FileSystem;
 using OpenRA.Primitives;
 
 namespace OpenRA
 {
 	public class ObjectCreator
 	{
+		// .NET does not support unloading assemblies, so mod libraries will leak across mod changes.
+		// This tracks the assemblies that have been loaded since game start so that we don't load multiple copies
+		static readonly Dictionary<string, Assembly> ResolvedAssemblies = new Dictionary<string, Assembly>();
+
 		readonly Cache<string, Type> typeCache;
 		readonly Cache<Type, ConstructorInfo> ctorCache;
 		readonly Pair<Assembly, string>[] assemblies;
 
-		public ObjectCreator(IEnumerable<Assembly> sourceAssemblies)
+		public ObjectCreator(Assembly a)
 		{
 			typeCache = new Cache<string, Type>(FindType);
 			ctorCache = new Cache<Type, ConstructorInfo>(GetCtor);
-			assemblies = sourceAssemblies.SelectMany(asm => asm.GetNamespaces().Select(ns => Pair.New(asm, ns))).ToArray();
+			assemblies = a.GetNamespaces().Select(ns => Pair.New(a, ns)).ToArray();
+		}
+
+		public ObjectCreator(Manifest manifest, FileSystem.FileSystem modFiles)
+		{
+			typeCache = new Cache<string, Type>(FindType);
+			ctorCache = new Cache<Type, ConstructorInfo>(GetCtor);
+
+			// Allow mods to load types from the core Game assembly, and any additional assemblies they specify.
+			var assemblyList = new List<Assembly>() { typeof(Game).Assembly };
+			foreach (var path in manifest.Assemblies)
+			{
+				var data = modFiles.Open(path).ReadAllBytes();
+
+				// .NET doesn't provide any way of querying the metadata of an assembly without either:
+				//   (a) loading duplicate data into the application domain, breaking the world.
+				//   (b) crashing if the assembly has already been loaded.
+				// We can't check the internal name of the assembly, so we'll work off the data instead
+				string hash;
+				using (var ms = new MemoryStream(data))
+					using (var csp = SHA1.Create())
+						hash = new string(csp.ComputeHash(data).SelectMany(a => a.ToString("x2")).ToArray());
+
+				Assembly assembly;
+				if (!ResolvedAssemblies.TryGetValue(hash, out assembly))
+				{
+					assembly = Assembly.Load(data);
+					ResolvedAssemblies.Add(hash, assembly);
+				}
+
+				assemblyList.Add(assembly);
+			}
+
+			AppDomain.CurrentDomain.AssemblyResolve += ResolveAssembly;
+			assemblies = assemblyList.SelectMany(asm => asm.GetNamespaces().Select(ns => Pair.New(asm, ns))).ToArray();
+			AppDomain.CurrentDomain.AssemblyResolve -= ResolveAssembly;
+		}
+
+		Assembly ResolveAssembly(object sender, ResolveEventArgs e)
+		{
+			foreach (var a in AppDomain.CurrentDomain.GetAssemblies())
+				if (a.FullName == e.Name)
+					return a;
+
+			return assemblies.Select(a => a.First).FirstOrDefault(a => a.FullName == e.Name);
 		}
 
 		public static Action<string> MissingTypeAction =