Invoke a member of a dynamic object with a name defined at runtime in a String

Question

I want to access a property on an object while leveraging the DLR binding mechanism.

• I cannot use the native binding mechanism (dynamic keyword in C#) because I do not know the property name at compile-time;
• I cannot use reflection because it only retrieves static type information;
• casting to an IDictionary<string, object>, to my knowledge, only solves the case of dynamic classes that choose to implement that interface (such as ExpandoObject).

Here is the demonstration code:

    static void Main(string[] args)
    {
        dynamic obj = new System.Dynamic.ExpandoObject();
        obj.Prop = "Value";
        // C# dynamic binding.
        Console.Out.WriteLine(obj.Prop);
        // IDictionary<string, object>
        Console.Out.WriteLine((obj as IDictionary<string, object>)["Prop"]);
        // Attempt to use reflection.
        PropertyInfo prop = obj.GetType().GetProperty("Prop");
        Console.Out.WriteLine(prop.GetValue(obj, new object[] { }));
        Console.In.ReadLine();
    }

Answer 1

I was finally able to do it by using the C# runtime binder. (I believe it should be possible to do that with a simpler binder implementation, but I was unable to write a working one - probably writing a "simple" implementation is already quite tough).

using Microsoft.CSharp.RuntimeBinder;

    private class TestClass
    {
        public String Prop { get; set; }
    }

    private static Func<object, object> BuildDynamicGetter(Type targetType, String propertyName)
    {
        var rootParam = Expression.Parameter(typeof(object));
        var propBinder = Microsoft.CSharp.RuntimeBinder.Binder.GetMember(CSharpBinderFlags.None, propertyName, targetType, new[] { CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null) });
        DynamicExpression propGetExpression = Expression.Dynamic(propBinder, typeof(object), 
            Expression.Convert(rootParam, targetType));
        Expression<Func<object, object>> getPropExpression = Expression.Lambda<Func<object, object>>(propGetExpression, rootParam);
        return getPropExpression.Compile();
    }

    static void Main(string[] args)
    {
        dynamic root = new TestClass { Prop = "StatValue" };

        Console.Out.WriteLine(root.Prop);
        var dynGetter = BuildDynamicGetter(root.GetType(), "Prop");
        Console.Out.WriteLine(dynGetter(root));

        root = new System.Dynamic.ExpandoObject();
        root.Prop = "ExpandoValue";

        Console.WriteLine(BuildDynamicGetter(root.GetType(), "Prop").Invoke(root));
        Console.Out.WriteLine(root.Prop);
        Console.In.ReadLine();
    }

Output:

StatValue
StatValue
ExpandoValue
ExpandoValue

Answer 2

The opensource framework dynamitey will do this (available via nuget). It uses that same api calls as the c# compiler.

Console.Out.WriteLine(Dynamic.InvokeGet(obj,"Prop"));