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1018

answers:

5

Let's say I have the following code which update a field of a struct using reflection. Since the struct instance is copied into the DynamicUpdate method, it needs to be boxed to an object before being passed.

struct Person
{
    public int id;
}

class Test
{
    static void Main()
    {
        object person = RuntimeHelpers.GetObjectValue(new Person());
        DynamicUpdate(person);
        Console.WriteLine(((Person)person).id); // print 10
    }

    private static void DynamicUpdate(object o)
    {
        FieldInfo field = typeof(Person).GetField("id");
        field.SetValue(o, 10);
    }
}

The code works fine. Now, let's say I don't want to use reflection because it's slow. Instead, I want to generate some CIL directly modifying the id field and convert that CIL into a reusable delegate (say, using Dynamic Method feature). Specially, I want to replace the above code with s/t like this:

static void Main()
{
    var action = CreateSetIdDelegate(typeof(Person));
    object person = RuntimeHelpers.GetObjectValue(new Person());
    action(person, 10);
    Console.WriteLine(((Person)person).id); // print 10
}

private static Action<object, object> CreateSetIdDelegate(Type t)
{
    // build dynamic method and return delegate
}


My question: is there any way to implement CreateSetIdDelegate excepts from using one of the following techniques?

  1. Generate CIL that invoke the setter using reflection (as the 1st code segment in this post). This makes no sense, given the requirement is to get rid of reflection, but it's a possible implementation so I just mention.
  2. Instead of using Action<object, object>, use a custom delegate whose signature is public delegate void Setter(ref object target, object value).
  3. Instead of using Action<object, object>, use Action<object[], object> with the 1st element of the array being the target object.

The reason I don't like 2 & 3 is because I don't want to have different delegates for the setter of object and setter of struct (as well as not wanting to make the set-object-field delegate more complicated than necessary, e.g. Action<object, object>). I reckon that the implementation of CreateSetIdDelegate would generate different CIL depending whether the target type is struct or object, but I want it to return the same delegate offering the same API to user.

+1  A: 

You may want to have a look at dynamic methods (reflection does not have to be slow!)...

Gerhard has a nice post about that: http://jachman.wordpress.com/2006/08/22/2000-faster-using-dynamic-method-calls/

Buu Nguyen
+3  A: 

EDIT again: This works structs now.

There's a gorgeous way to do it in C# 4, but you'll have to write your own ILGenerator emit code for anything before that. They added an ExpressionType.Assign to the .NET Framework 4.

This works in C# 4 (tested):

public delegate void ByRefStructAction(ref SomeType instance, object value);

private static ByRefStructAction BuildSetter(FieldInfo field)
{
    ParameterExpression instance = Expression.Parameter(typeof(SomeType).MakeByRefType(), "instance");
    ParameterExpression value = Expression.Parameter(typeof(object), "value");

    Expression<ByRefStructAction> expr =
        Expression.Lambda<ByRefStructAction>(
            Expression.Assign(
                Expression.Field(instance, field),
                Expression.Convert(value, field.FieldType)),
            instance,
            value);

    return expr.Compile();
}

Edit: Here was my test code.

public struct SomeType
{
    public int member;
}

[TestMethod]
public void TestIL()
{
    FieldInfo field = typeof(SomeType).GetField("member");
    var setter = BuildSetter(field);
    SomeType instance = new SomeType();
    int value = 12;
    setter(ref instance, value);
    Assert.AreEqual(value, instance.member);
}
280Z28
Buu Nguyen
For a struct, you have to pass it by ref to make changes to the original. Since you can't add `ref` or `out` to the `Action<>` delegates, you have the following options: 1) Use a class instead of struct, 2) Use custom delegates, or 3) Engineer it so you don't have to make changes to the struct, and pass the struct by [boxed] value. :)
280Z28
+2  A: 

I ran into a similar issue, and it took me most of a weekend, but I finally figured it out after a lot of searching, reading, and disassembling C# test projects. And this version only requires .NET 2, not 4.

public delegate void SetterDelegate(ref object target, object value);
private static Type[] ParamTypes = new Type[]
{
    typeof(object).MakeByRefType(), typeof(object)
};
private static SetterDelegate CreateSetMethod(MemberInfo memberInfo)
{
    Type ParamType;
    if (memberInfo is PropertyInfo)
        ParamType = ((PropertyInfo)memberInfo).PropertyType;
    else if (memberInfo is FieldInfo)
        ParamType = ((FieldInfo)memberInfo).FieldType;
    else
        throw new Exception("Can only create set methods for properties and fields.");

    DynamicMethod setter = new DynamicMethod(
        "",
        typeof(void),
        ParamTypes,
        memberInfo.ReflectedType.Module,
        true);
    ILGenerator generator = setter.GetILGenerator();
    generator.Emit(OpCodes.Ldarg_0);
    generator.Emit(OpCodes.Ldind_Ref);

    if (memberInfo.DeclaringType.IsValueType)
    {
#if UNSAFE_IL
        generator.Emit(OpCodes.Unbox, memberInfo.DeclaringType);
#else
        generator.DeclareLocal(memberInfo.DeclaringType.MakeByRefType());
        generator.Emit(OpCodes.Unbox, memberInfo.DeclaringType);
        generator.Emit(OpCodes.Stloc_0);
        generator.Emit(OpCodes.Ldloc_0);
#endif // UNSAFE_IL
    }

    generator.Emit(OpCodes.Ldarg_1);
    if (ParamType.IsValueType)
        generator.Emit(OpCodes.Unbox_Any, ParamType);

    if (memberInfo is PropertyInfo)
        generator.Emit(OpCodes.Callvirt, ((PropertyInfo)memberInfo).GetSetMethod());
    else if (memberInfo is FieldInfo)
        generator.Emit(OpCodes.Stfld, (FieldInfo)memberInfo);

    if (memberInfo.DeclaringType.IsValueType)
    {
#if !UNSAFE_IL
        generator.Emit(OpCodes.Ldarg_0);
        generator.Emit(OpCodes.Ldloc_0);
        generator.Emit(OpCodes.Ldobj, memberInfo.DeclaringType);
        generator.Emit(OpCodes.Box, memberInfo.DeclaringType);
        generator.Emit(OpCodes.Stind_Ref);
#endif // UNSAFE_IL
    }
    generator.Emit(OpCodes.Ret);

    return (SetterDelegate)setter.CreateDelegate(typeof(SetterDelegate));
}

Note the "#if UNSAFE_IL" stuff in there. I actually came up with 2 ways to do it, but the first one is really... hackish. To quote from Ecma-335, the standards document for IL:

"Unlike box, which is required to make a copy of a value type for use in the object, unbox is not required to copy the value type from the object. Typically it simply computes the address of the value type that is already present inside of the boxed object."

So if you want to play dangerously, you can use OpCodes.Unbox to change your object handle into a pointer to your structure, which can then be used as the first parameter of a Stfld or Callvirt. Doing it this way actually ends up modifying the struct in place, and you don't even need to pass your target object by ref.

However, note that the standard doesn't guarantee that Unbox will give you a pointer to the boxed version. Particularly, it suggests that Nullable<> can cause Unbox to create a copy. Anyway, if that happens, you'll probably get a silent failure, where it sets the field or property value on a local copy which is then immediately discarded.

So the safe way to do it is pass your object by ref, store the address in a local variable, make the modification, and then rebox the result and put it back in your ByRef object parameter.

I did some rough timings, calling each version 10,000,000 times, with 2 different structures:

Structure with 1 field: .46 s "Unsafe" delegate .70 s "Safe" delegate 4.5 s FieldInfo.SetValue

Structure with 4 fields: .46 s "Unsafe" delegate .88 s "Safe" delegate 4.5 s FieldInfo.SetValue

Notice that the boxing makes the the "Safe" version speed decrease with structure size, whereas the other two methods are unaffected by structure size. I guess at some point the boxing cost would overrun the reflection cost. But I wouldn't trust the "Unsafe" version in any important capacity.

Bryce Wagner
Nice answer. For my particular problem (i.e. to implement my library http://fasterflect.codeplex.com), I didn't want to make use of 'ref' at all. Instead I require the calling code to pass in some struct wrapper which is a value type.
Buu Nguyen
It won't let me comment on Hugh's answer below, so I'll answer here. Basically to set a field on a struct, you have to box the struct first, like this: object x = new MyStruct(); Then you pass the boxed object in by ref. If you already know what type you're dealing with ahead of time, you don't need this code at all. And if you don't know what type it is, that means it's probably in an object reference already. Hope that makes sense.
Bryce Wagner
A: 

I was interested in Bryce's code but can't get it to run.

First off I can't pass a struct by ref to the delegate (as declared in his code) because it won't compile.

Next if I modify the code (so that it refers to my struct type explicitly rather than just 'object') it also fails at runtime (somewhere in the IL, null ref complaint).

It would help to see an example of his code being called with several different structs.

Hugh

Hugh
A: 

You could fairly easily modify this to work with structs. It's currently dictionary based but your situation is easier.

http://www.damonpayne.com/2009/09/07/TwoWayBindingToNameValuePairs.aspx

D.R. Payne