Creating a property setter delegate

Question

I have created methods for converting a property lambda to a delegate:

public static Delegate MakeGetter<T>(Expression<Func<T>> propertyLambda)
{
    var result = Expression.Lambda(propertyLambda.Body).Compile();
    return result;
}

public static Delegate MakeSetter<T>(Expression<Action<T>> propertyLambda)
{
    var result = Expression.Lambda(propertyLambda.Body).Compile();
    return result;
}

These work:

Delegate getter = MakeGetter(() => SomeClass.SomeProperty);
object o = getter.DynamicInvoke();

Delegate getter = MakeGetter(() => someObject.SomeProperty);
object o = getter.DynamicInvoke();

but these won't compile:

Delegate setter = MakeSetter(() => SomeClass.SomeProperty);
setter.DynamicInvoke(new object[]{propValue});

Delegate setter = MakeSetter(() => someObject.SomeProperty);
setter.DynamicInvoke(new object[]{propValue});

The MakeSetter lines fail with "The type arguments cannot be inferred from the usage. Try specifying the type arguments explicitly."

Is what I'm trying to do possible? Thanks in advance.

Answer 1

The Expression API supports this in .NET 4.0, but sadly the C# compiler doesn't add any extra candy to support. But the good news is that you can trivially take a "get" expression (which the C# compiler can write) and re-write it as a "set" expression.

And even better; if you don't have .NET 4.0, there are still at least two other ways of performing a "set" via an expression written as a "get".

Here they all are, for info:

using System;
using System.Linq.Expressions;
using System.Reflection;
class Foo {
    public string Bar { get; set; }
    static void Main() {
        // take a "get" from C#
        Expression<Func<Foo, string>> get = foo => foo.Bar;

        // re-write in .NET 4.0 as a "set"
        var member = (MemberExpression)get.Body;
        var param = Expression.Parameter(typeof(string), "value");
        var set = Expression.Lambda<Action<Foo, string>>(
            Expression.Assign(member, param), get.Parameters[0], param);

        // compile it
        var action = set.Compile();
        var inst = new Foo();
        action(inst, "abc");
        Console.WriteLine(inst.Bar); // show it working

        //==== reflection
        MethodInfo setMethod = ((PropertyInfo)member.Member).GetSetMethod();
        setMethod.Invoke(inst, new object[] { "def" });
        Console.WriteLine(inst.Bar); // show it working

        //==== Delegate.CreateDelegate
        action = (Action<Foo, string>)
            Delegate.CreateDelegate(typeof(Action<Foo, string>), setMethod);
        action(inst, "ghi");
        Console.WriteLine(inst.Bar); // show it working
    }
}

Answer 2

As per my comments - because links go dead - I have posted the full code as an answer to the question. YES it is possible to do what the OP is requesting. and here is a nice little gem from Nick demonstrating it. Nick credits this page and another page for his complete solution along with performance metrics. I provide that below instead of just a link.

// returns property getter
public static Func<TObject, TProperty> GetPropGetter<TObject, TProperty>(string propertyName)
{
    ParameterExpression paramExpression = Expression.Parameter(typeof(TObject), "value");

    Expression propertyGetterExpression = Expression.Property(paramExpression, propertyName);

    Func<TObject, TProperty> result =
        Expression.Lambda<Func<TObject, TProperty>>(propertyGetterExpression, paramExpression).Compile();

    return result;
}

// returns property setter:
public static Action<TObject, TProperty> GetPropSetter<TObject, TProperty>(string propertyName)
{            
    ParameterExpression paramExpression = Expression.Parameter(typeof(TObject));

    ParameterExpression paramExpression2 = Expression.Parameter(typeof(TProperty), propertyName);

    MemberExpression propertyGetterExpression = Expression.Property(paramExpression, propertyName);

    Action<TObject, TProperty> result = Expression.Lambda<Action<TObject, TProperty>>
    (
        Expression.Assign(propertyGetterExpression, paramExpression2), paramExpression, paramExpression2
    ).Compile();

    return result;
}