One problem with C# generics is the lack of member constraints. In other words, this is impossible:

public static T Add<T>(T left, T right)
{
    return left + right;
}

People have come up with a large number of solutions, but they all have their own problems, ranging from messy looking syntax to less-than-optimal performance (particularly due to virtual function calls). The solution I propose is a somewhat hacky one (yes that's a word) which maintains both good performance and clean looking code: write the function in IL.

The code for the add method boils down to something like this:

ldarg.0
ldarg.1
add
ret

For those not familiar with IL, the above code adds the first to arguments of the method, then returns the result.

I found that this actually works with generic types. No funny errors, no nothing. Of course, I didn't actually compile any IL source code - I wrote a separate program to generate the assembly. The source code for it is this:

namespace Generator
{
    using System;
    using System.Linq;
    using System.Reflection;
    using System.Reflection.Emit;

    public static class Program
    {
        public static void Main()
        {
            var assemblyBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("Operations"), AssemblyBuilderAccess.Save);
            var module = assemblyBuilder.DefineDynamicModule("Operations", "Operations.dll");
            var type = module.DefineType("Operator", TypeAttributes.Public | TypeAttributes.Sealed);

            type.DefineBinaryOperatorMethod("Add", OpCodes.Add);
            type.DefineBinaryOperatorMethod("Subtract", OpCodes.Sub);
            type.DefineBinaryOperatorMethod("Multiply", OpCodes.Mul);
            type.DefineBinaryOperatorMethod("Divide", OpCodes.Div);
            type.DefineBinaryOperatorMethod("Remainder", OpCodes.Rem);
            type.CreateType();

            assemblyBuilder.Save("Operations.dll");
        }

        private static MethodBuilder DefineBinaryOperatorMethod(this TypeBuilder type, string name, OpCode operation)
        {
            var method = type.DefineMethod(name, MethodAttributes.Public | MethodAttributes.Static);
            var genericParameter = method.DefineGenericParameters("T").First();
            genericParameter.SetBaseTypeConstraint(typeof(ValueType));
            
            method.SetReturnType(genericParameter);
            method.SetParameters(genericParameter, genericParameter);
            method.DefineParameter(1, ParameterAttributes.None, "left");
            method.DefineParameter(2, ParameterAttributes.None, "right");

            var ilGenerator = method.GetILGenerator();
            var continueLabel = ilGenerator.DefineLabel();

            ilGenerator.Emit(OpCodes.Ldtoken, genericParameter);
            ilGenerator.EmitCall(OpCodes.Call, typeof(Type).GetMethod("GetTypeFromHandle"), null);
            ilGenerator.EmitCall(OpCodes.Callvirt, typeof(Type).GetProperty("IsPrimitive").GetGetMethod(), null);
            ilGenerator.Emit(OpCodes.Brtrue_S, continueLabel);
            ilGenerator.Emit(OpCodes.Ldstr, "The specified type is not supported by this operation.");
            ilGenerator.Emit(OpCodes.Newobj, typeof(ArgumentException).GetConstructor(new[] { typeof(string) }));
            ilGenerator.Emit(OpCodes.Throw);
            ilGenerator.MarkLabel(continueLabel);
            ilGenerator.Emit(OpCodes.Ldarg_0);
            ilGenerator.Emit(OpCodes.Ldarg_1);
            ilGenerator.Emit(operation);
            ilGenerator.Emit(OpCodes.Ret);

            return method;
        }
    }
}

In case your wondering what the ldtoken/call/callvirt/brtrue.s/ldstr/newobj/throw stuff is doing there, that just checks to make sure the type argument is a primitive type (yes this only works on primitive types), and throws an ArgumentException otherwise. Before adding that, .NET threw some pretty nasty exceptions when I tried it on some non-primitive types.

As a final note, the methods generated with indeed work on all primitive types. Even booleans. I was surprised to get a DivideByZeroException upon attempting this:

Operator.Divide(true, false);

Again, this only works on primitive types. Remember, string and decimal are not primitive types. Perhaps this could be extended to calling the op_* functions...

After a couple small performance tests, I found that this method was nearly as fast as doing simple addition (x + y). Then again, I know nothing about microbenchmarks. Try it for yourself :D

Enjoy