Question

Why is there no dynamic dictionary object for TempData as there is for ViewData?

Answer 1

There isn’t because no-one ever bothered to implement it. But that would be trivially easy to do. For example as an extension method (unfortunately extension properties are not yet supported in .NET so you cannot quite get the syntax you might have hoped for):

public class DynamicTempDataDictionary : DynamicObject
{
    public DynamicTempDataDictionary(TempDataDictionary tempData)
    {
        _tempData = tempData;
    }

    private readonly TempDataDictionary _tempData;

    public override IEnumerable<string> GetDynamicMemberNames()
    {
        return _tempData.Keys;
    }

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        result = _tempData[binder.Name];
        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        _tempData[binder.Name] = value;
        return true;
    }
}

public static class ControllerExtensions
{
    public static dynamic TempBag(this ControllerBase controller)
    {
        return new DynamicTempDataDictionary(controller.TempData);
    }
}

and then:

public ActionResult Index()
{
    this.TempBag().Hello = "abc";
    return RedirectToAction("Foo");
}

The question is: why would you need that and how is it better/safer than:

public ActionResult Index()
{
    TempData["Hello"] = "abc";
    return RedirectToAction("Foo");
}