How to set a property of a C# 4 dynamic object when you have the name in another variable

Question

I'm looking for a way to modify properties on a dynamic C# 4.0 object with the name of the property known only at runtime.

Is there a way to do something like (ExpandoObject is just used as an example, this could be any class that implements IDynamicMetaObjectProvider):

string key = "TestKey"; dynamic e = new ExpandoObject(); e[key] = "value"; 

Which would be equivalent to:

dynamic e = new ExpandoObject(); e.TestKey = "value"; 

Or is the only way forward reflection?

Answer 1

Not very easily, no. Reflection doesn't work, since it assumes a regular type model, which is not the full range of dynamic. If you are actually just talking to regular objects, then just use reflection here. Otherwise, I expect you may want to reverse-engineer the code that the compiler emits for a basic assignment, and tweak it to have a flexibly member-name. I'll be honest, though: this isn't an attractive option; a simple:

dynamic foo = ... foo.Bar = "abc"; 

translates to:

if (<Main>o__SiteContainer0.<>p__Site1 == null) {     <Main>o__SiteContainer0.<>p__Site1 = CallSite<Func<CallSite, object, string, object>>.Create(Binder.SetMember(CSharpBinderFlags.None, "Bar", typeof(Program), new CSharpArgumentInfo[] { CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null), CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.Constant | CSharpArgumentInfoFlags.UseCompileTimeType, null) })); } <Main>o__SiteContainer0.<>p__Site1.Target(<Main>o__SiteContainer0.<>p__Site1, foo, "abc"); 

---

If you want an approach that works for both dynamic and non-dynamic objects: FastMember is handy for this, and works at either the type or object level:

// could be static or DLR  var wrapped = ObjectAccessor.Create(obj);  string propName = // something known only at runtime  Console.WriteLine(wrapped[propName]); 

available on Nuget, and heavily optimised for both dynamic and non-dynamic scenarios.