Change C# reflection to avoid using expression trees

This should work on Unity, Mono and .NET 3.5 as far as I'm aware.
It won't work on platforms where reflection itself is prohibited,
but that's a non-starter basically.

csharp/src/Google.Protobuf/Reflection/OneofAccessor.cs

@@ -52,7 +52,7 @@ namespace Google.Protobuf.Reflection
                 throw new ArgumentException("Cannot read from property");
             }
             this.descriptor = descriptor;
-            caseDelegate = ReflectionUtil.CreateFuncIMessageT<int>(caseProperty.GetGetMethod());
+            caseDelegate = ReflectionUtil.CreateFuncIMessageInt32(caseProperty.GetGetMethod());
 
             this.descriptor = descriptor;
             clearDelegate = ReflectionUtil.CreateActionIMessage(clearMethod);

csharp/src/Google.Protobuf/Reflection/ReflectionUtil.cs

@@ -31,8 +31,6 @@
 #endregion
 
 using System;
-using System.Collections.Generic;
-using System.Linq.Expressions;
 using System.Reflection;
 
 namespace Google.Protobuf.Reflection
@@ -56,52 +54,74 @@ namespace Google.Protobuf.Reflection
         /// Creates a delegate which will cast the argument to the appropriate method target type,
         /// call the method on it, then convert the result to object.
         /// </summary>
-        internal static Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method)
-        {
-            ParameterExpression parameter = Expression.Parameter(typeof(IMessage), "p");
-            Expression downcast = Expression.Convert(parameter, method.DeclaringType);
-            Expression call = Expression.Call(downcast, method);
-            Expression upcast = Expression.Convert(call, typeof(object));
-            return Expression.Lambda<Func<IMessage, object>>(upcast, parameter).Compile();
-        }
+        internal static Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method) =>
+            GetReflectionHelper(method.DeclaringType, method.ReturnType).CreateFuncIMessageObject(method);
 
         /// <summary>
         /// Creates a delegate which will cast the argument to the appropriate method target type,
         /// call the method on it, then convert the result to the specified type.
         /// </summary>
-        internal static Func<IMessage, T> CreateFuncIMessageT<T>(MethodInfo method)
-        {
-            ParameterExpression parameter = Expression.Parameter(typeof(IMessage), "p");
-            Expression downcast = Expression.Convert(parameter, method.DeclaringType);
-            Expression call = Expression.Call(downcast, method);
-            Expression upcast = Expression.Convert(call, typeof(T));
-            return Expression.Lambda<Func<IMessage, T>>(upcast, parameter).Compile();
-        }
+        internal static Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method) =>
+            GetReflectionHelper(method.DeclaringType, typeof(object)).CreateFuncIMessageInt32(method);
 
         /// <summary>
         /// Creates a delegate which will execute the given method after casting the first argument to
         /// the target type of the method, and the second argument to the first parameter type of the method.
         /// </summary>
-        internal static Action<IMessage, object> CreateActionIMessageObject(MethodInfo method)
-        {
-            ParameterExpression targetParameter = Expression.Parameter(typeof(IMessage), "target");
-            ParameterExpression argParameter = Expression.Parameter(typeof(object), "arg");
-            Expression castTarget = Expression.Convert(targetParameter, method.DeclaringType);
-            Expression castArgument = Expression.Convert(argParameter, method.GetParameters()[0].ParameterType);
-            Expression call = Expression.Call(castTarget, method, castArgument);
-            return Expression.Lambda<Action<IMessage, object>>(call, targetParameter, argParameter).Compile();
-        }
+        internal static Action<IMessage, object> CreateActionIMessageObject(MethodInfo method) =>
+            GetReflectionHelper(method.DeclaringType, method.GetParameters()[0].ParameterType).CreateActionIMessageObject(method);
 
         /// <summary>
         /// Creates a delegate which will execute the given method after casting the first argument to
         /// the target type of the method.
         /// </summary>
-        internal static Action<IMessage> CreateActionIMessage(MethodInfo method)
+        internal static Action<IMessage> CreateActionIMessage(MethodInfo method) =>
+            GetReflectionHelper(method.DeclaringType, typeof(object)).CreateActionIMessage(method);        
+
+        /// <summary>
+        /// Creates a reflection helper for the given type arguments. Currently these are created on demand
+        /// rather than cached; this will be "busy" when initially loading a message's descriptor, but after that
+        /// they can be garbage collected. We could cache them by type if that proves to be important, but creating
+        /// an object is pretty cheap.
+        /// </summary>
+        private static IReflectionHelper GetReflectionHelper(Type t1, Type t2) =>
+            (IReflectionHelper) Activator.CreateInstance(typeof(ReflectionHelper<,>).MakeGenericType(t1, t2));
+
+        // Non-generic interface allowing us to use an instance of ReflectionHelper<T1, T2> without statically
+        // knowing the types involved.
+        private interface IReflectionHelper
+        {
+            Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method);
+            Action<IMessage> CreateActionIMessage(MethodInfo method);
+            Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method);
+            Action<IMessage, object> CreateActionIMessageObject(MethodInfo method);
+        }
+
+        private class ReflectionHelper<T1, T2> : IReflectionHelper
         {
-            ParameterExpression targetParameter = Expression.Parameter(typeof(IMessage), "target");
-            Expression castTarget = Expression.Convert(targetParameter, method.DeclaringType);
-            Expression call = Expression.Call(castTarget, method);
-            return Expression.Lambda<Action<IMessage>>(call, targetParameter).Compile();
-        }        
+            public Func<IMessage, int> CreateFuncIMessageInt32(MethodInfo method)
+            {
+                var del = (Func<T1, int>) method.CreateDelegate(typeof(Func<T1, int>));
+                return message => del((T1) message);
+            }
+
+            public Action<IMessage> CreateActionIMessage(MethodInfo method)
+            {
+                var del = (Action<T1>) method.CreateDelegate(typeof(Action<T1>));
+                return message => del((T1) message);
+            }
+
+            public Func<IMessage, object> CreateFuncIMessageObject(MethodInfo method)
+            {
+                var del = (Func<T1, T2>) method.CreateDelegate(typeof(Func<T1, T2>));
+                return message => del((T1) message);
+            }
+
+            public Action<IMessage, object> CreateActionIMessageObject(MethodInfo method)
+            {
+                var del = (Action<T1, T2>) method.CreateDelegate(typeof(Action<T1, T2>));
+                return (message, arg) => del((T1) message, (T2) arg);
+            }
+        }
     }
 }
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