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

c# expression-trees lambda

Question

Given:

FieldInfo field = <some valid string field on type T>;
ParameterExpression targetExp = Expression.Parameter(typeof(T), "target");
ParameterExpression valueExp = Expression.Parameter(typeof(string), "value");

How do I compile a lambda expression to set the field on the "target" parameter to "value"?

Accepted Answer

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

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

        return (Action<T,TValue>) m.CreateDelegate(typeof(Action<T,TValue>));
    }

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

        Action<MyObject,int> setter = MakeSetter<MyObject,int>(f);

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

        setter(obj, 42);

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

Expert Answer

Setting a field is, as already discussed, problematic. You can can (in 3.5) a single method, such as a property-setter - but only indirectly. This gets much easier in 4.0, as discussed here. However, if you actually have properties (not fields), you can do a lot simply with Delegate.CreateDelegate:

using System;
using System.Reflection;
public class Foo
{
    public int Bar { get; set; }
}
static class Program
{
    static void Main()
    {
        MethodInfo method = typeof(Foo).GetProperty("Bar").GetSetMethod();
        Action<Foo, int> setter = (Action<Foo, int>)
            Delegate.CreateDelegate(typeof(Action<Foo, int>), method);

        Foo foo = new Foo();
        setter(foo, 12);
        Console.WriteLine(foo.Bar);
    }
}



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