How to create a lambda expression from a type object with LINQ?

Question

I'm trying to speed up reflection -> SetValue with a LINQ expression.

My problem is this method:

public void SetValue<T>(T obj)
{
    FieldInfo field = typeof(T).GetField("Title", BindingFlags.Instance |
                                                  BindingFlags.Public |
                                                  BindingFlags.IgnoreCase);

    ParameterExpression targetExp = Expression.Parameter(typeof(T), "target");
    ParameterExpression valueExp = Expression.Parameter(field.FieldType, "value");

    // Expression.Property can be used here as well
    MemberExpression fieldExp = Expression.Field(targetExp, field);
    BinaryExpression assignExp = Expression.Assign(fieldExp, valueExp);

    var setter = Expression.Lambda<Action<T, string>>(assignExp, targetExp, valueExp).Compile();

    setter(obj, "Hello World");

    //Console.WriteLine(obj.title);
}

which I call like this:

var ii = new Controllers.SearchController.InstantItem();

SetValue<Controllers.SearchController.InstantItem>(ii);

The problem is this line:

var setter = Expression.Lambda<Action<T, string>>(assignExp, targetExp, valueExp).Compile();

Because Action uses generics, I cannot replace string with field.FieldType...

Is there any possibility I can do this without having to make a switch(field.FieldType) statement, and put a generic method for each possible type, which would suck big time?

Compiling a dynamic method is slower by far than using reflection if you do it every time. I think you need to cache the setter. — usr
– usr, Commented Jun 18, 2012 at 13:37
@usr: True, but I can worry about caching it later. For now I need to create it first. — Stefan Steiger
– Stefan Steiger, Commented Jun 18, 2012 at 13:51

L.B · Accepted Answer · 2012-06-18 14:00:29Z

1

Maybe something like this

action = FormAction(fieldInfo);
action(obj,valueToSet);

Of course caching the Actions in a dictionary will be needed.

static Action<object, object> FormAction(FieldInfo fieldInfo)
{
    ParameterExpression obj = Expression.Parameter(typeof(object), fieldInfo.Name);
    ParameterExpression value = Expression.Parameter(typeof(object));

    MemberExpression fieldExp = Expression.Field(Expression.Convert(obj, fieldInfo.DeclaringType), fieldInfo.Name);
    BinaryExpression assignExp = Expression.Assign(fieldExp, Expression.Convert(value, fieldInfo.FieldType));

    return Expression.Lambda<Action<object, object>>(assignExp, obj, value).Compile();
}

answered Jun 18, 2012 at 14:00

L.B

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1 Comment

Stefan Steiger Over a year ago

I tried that, but missed Expression.Convert. Nice, works - thanks !

usr · Accepted Answer · 2012-06-18 13:57:51Z

0

Make the parameter ob type object and make the lambda cast to the correct field-type internally.

Or, construct the delegate type (Action) dynamically at runtime.

Or, you could define SetValue as follows:

SetValue<TObject, TProperty>

That would make this method generic over the property type.

Edit: It sounds like option 1 is best for you. You need to change the parameter type to typeof(object) and add a cast:

valueExp = Expression.Convert(valueExp, field.FieldType)

And you need to use Action<T, object> as the delegate type.

edited Jun 18, 2012 at 13:57

answered Jun 18, 2012 at 13:36

usr

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2 Comments

Stefan Steiger Over a year ago

I know. But HOW !!! As for SetValue: Then I would need the field/property type at compile time again, which I can't have, since the type is different for each field. Same problem really, just without Lambda expression.

usr Over a year ago

Ok I added some instructions. That is really all there is to it.

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