How do I set a field value in an C# Expression tree?

Question

Given:

FieldInfo field = ;
ParameterExpression targetExp = Expression.Parameter(typeof(T), \"target\");
ParameterExp         


        

Answer 1

.Net 4.0 : now that there's Expression.Assign, this is easy to do:

FieldInfo field = typeof(T).GetField("fieldName");
ParameterExpression targetExp = Expression.Parameter(typeof(T), "target");
ParameterExpression valueExp = Expression.Parameter(typeof(string), "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>
    (assignExp, targetExp, valueExp).Compile();

setter(subject, "new value");

.Net 3.5 : you can't, you'll have to use System.Reflection.Emit instead:

class Program
{
    class MyObject
    {
        public int MyField;
    }

    static Action MakeSetter(FieldInfo field)
    {
        DynamicMethod m = new DynamicMethod(
            "setter", typeof(void), new Type[] { typeof(T), typeof(TValue) }, typeof(Program));
        ILGenerator cg = m.GetILGenerator();

        // arg0. = arg1
        cg.Emit(OpCodes.Ldarg_0);
        cg.Emit(OpCodes.Ldarg_1);
        cg.Emit(OpCodes.Stfld, field);
        cg.Emit(OpCodes.Ret);

        return (Action) m.CreateDelegate(typeof(Action));
    }

    static void Main()
    {
        FieldInfo f = typeof(MyObject).GetField("MyField");

        Action setter = MakeSetter(f);

        var obj = new MyObject();
        obj.MyField = 10;

        setter(obj, 42);

        Console.WriteLine(obj.MyField);
        Console.ReadLine();
    }
}