How to create a new instance of an object by dynamic lambda and IL?

Question

I know that Activator.CreateInstance() can create a new instance of an object. But I'm searching for a way to create an instance by IL and Expression. I think I can create a dynamic lambda to create an instance of a type, and cache the lambda to speed up objects initialization. Am I right? Can you help me please?

Answer 1

You can represent creation of an object using Expression.New(). You can pass to it either a Type that has parameterless constructor, or a ConstructorInfo, along with Expressions representing the constructor parameters. If you want to return an object and you want it to work for value types too, you also need to add an Expression.Convert().

Putting it all together, the equivalent to Activator.CreateInstance() could look like this:

object CreateInstance(Type type)
{
    return Expression.Lambda<Func<object>>(
        Expression.Convert(Expression.New(type), typeof(object)))
        .Compile()();
}

If you want to do the same in IL, you need to use the newobj instruction for reference types. If you want to do the same for values types, you can create a local variable of that type, box it and return it:

object CreateInstance(Type type)
{
    var method = new DynamicMethod("", typeof(object), Type.EmptyTypes);
    var il = method.GetILGenerator();

    if (type.IsValueType)
    {
        var local = il.DeclareLocal(type);
        // method.InitLocals == true, so we don't have to use initobj here
        il.Emit(OpCodes.Ldloc, local);
        il.Emit(OpCodes.Box, type);
        il.Emit(OpCodes.Ret);
    }
    else
    {
        var ctor = type.GetConstructor(Type.EmptyTypes);
        il.Emit(OpCodes.Newobj, ctor);
        il.Emit(OpCodes.Ret);
    }

    return method.Invoke(null, null);
}