Cast to Anonymous Type

Question

I had the following problem today, and I was wondering if there is a solution for my problem.

My idea was to build anonymous classes and use it as a datasource for a WinForm BindingSource:

public void Init()
{
    var option1 = new
                  {
                      Id = TemplateAction.Update,
                      Option = "Update the Templates",
                      Description = "Bla bla 1."
                  };

    var option2 = new
                  {
                      Id = TemplateAction.Download,
                      Option = "Download the Templates",
                      Description = "Bla bla 2."
                  };

    var list = new[] {option1, option2}.ToList();

    bsOptions.DataSource = list; // my BindingSource

    // cboTemplates is a ComboBox
    cboTemplates.DataSource = bsOptions; 
    cboTemplates.ValueMember = "Id";
    cboTemplates.DisplayMember = "Option";

    lblInfoTemplates.DataBindings.Add("Text", bsOptions, "Description");
}

That works fine so far.

The problem I had is to get Id out of the "Current" property of the BindingSource, because I can't cast it back to the Anonymous Type:

private void cmdOK_Click(object sender, EventArgs e)
{
    var option = (???)bsOptions.Current;
}

I guess there is no way to find out the type of "Current" and access the "Id" Property? Maybe someone has a good solution...

I know there are other (and also better) ways to get the Id (Reflection, reading the value from the ComboBox, not using anonymous tpyes,...) I'm just courious if it's possible to get the Type out of bsOptions.Current in an elegant way.

Answer 1

Note, as per the comment, I'd just like to point out that I too recommend using a real type when you need to pass it around the program like this. Anonymous types should only really be used locally in a single method at a time (in my opinion), but anyway, here's the rest of my answer.

---

You can do it using a trick, by tricking the compiler into inferring the right type for you:

using System;

namespace ConsoleApplication4
{
    class Program
    {
        static void Main(string[] args)
        {
            var a = new { Id = 1, Name = "Bob" };
            TestMethod(a);

            Console.Out.WriteLine("Press enter to exit...");
            Console.In.ReadLine();
        }

        private static void TestMethod(Object x)
        {
            // This is a dummy value, just to get 'a' to be of the right type
            var a = new { Id = 0, Name = "" };
            a = Cast(a, x);
            Console.Out.WriteLine(a.Id + ": " + a.Name);
        }

        private static T Cast<T>(T typeHolder, Object x)
        {
            // typeHolder above is just for compiler magic
            // to infer the type to cast x to
            return (T)x;
        }
    }
}

The trick is that inside the assembly, the same anonymous type (same properties, same order) resolves to the same type, which makes the trick above work.

private static T CastTo<T>(this Object value, T targetType)
{
    // targetType above is just for compiler magic
    // to infer the type to cast value to
    return (T)value;
}

usage:

var value = x.CastTo(a);

But we're really pushing the limits here. Use a real type, it'll look and feel cleaner as well.

Answer 2

Instead of casting to your custom type try using dynamic type.

Your event handler would look something like this:

private void cmdOK_Click(object sender, EventArgs e)
{
    dynamic option = bsOptions.Current;
    if (option.Id == 1) { doSomething(); }
      else { doSomethingElse(); }
}

Answer 3

To quote MSDN:

An anonymous type cannot be cast to any interface or type except for object.

Answer 4

In C# 3.0, this is not possible. You'll have to wait for C# 4.0, which allows accessing properties at runtime using "dynamic" variables.

Answer 5

public class MyExtensMethods{

    public static T GetPropertyValue<T>(this Object obj, string property)
    {
        return (T)obj.GetType().GetProperty(property).GetValue(obj, null);
    }
}

class SomeClass
{
    public int ID{get;set;}
    public int FullName{get;set;}
}


// casts obj to type SomeClass
public SomeClass CastToSomeClass(object obj)
{
     return new SomeClass()
     {
         ID = obj.GetPropertyValue<int>("Id"),
         FullName = obj.GetPropertyValue<string>("LastName") + ", " + obj.GetPropertyValue<string>("FirstName")
     };
}

.... then to cast you will do:

var a = new { Id = 1, FirstName = "Bob", LastName="Nam" };
SomeClass myNewVar = CastToSomeClass(a);

Answer 6

you can try this:

private void cmdOK_Click(object sender, EventArgs e)
{
    var option = Cast(bsOptions.Current, new { Id = 0, Option = "", Description = "" });
}

see: Can't return anonymous type from method? Really?

Answer 7

You can also declare an array of anonymous Types directly with that syntax:

var data = new [] {
  new {Id = 0, Name = "Foo"},
  new {Id = 42, Name = "Bar"},
};