Flash message implementation for ASP.NET MVC

Flash.cs

public class Flash : DynamicObject
{
    private readonly TempDataDictionary _store;
    private const string KeyPrefix = "Flash";

    public Flash(TempDataDictionary store)
    {
        _store = store;
    }

    // Implementing this function improves the debugging experience as it provides the debugger with the list of all
    // the properties currently defined on the object
    public override IEnumerable<string> GetDynamicMemberNames()
    {
        return _store.Keys.Where(x => x.StartsWith(KeyPrefix)).Select(x => x.Substring(5));
    }

    private string GetKey(string messageName)
    {
        return string.Format("{0}{1}", KeyPrefix, messageName);
    }

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        if (binder.Name.StartsWith("Has"))
        {
            var requestedKey = GetKey(binder.Name.Substring(3));
            result = _store.ContainsKey(requestedKey);
        }
        else
        {
            var requestedKey = GetKey(binder.Name);
            result = _store[requestedKey];
        }
        // since ViewDataDictionary always returns a result even if the key does not exist, always return true
        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        var key = GetKey(binder.Name);
        _store[key] = value;
        // you can always set a key in the dictionary so return true 
        return true;
    }
}

FlashExtensions.cs

public static class FlashExtensions
{
    public static dynamic Flash(this TempDataDictionary @this)
    {
        return new Flash(@this);
    }
}

FlashMessage.cshtml

@if (TempData.Flash().HasSuccess)
{
    <div class="message success" role="alert">
        <span class="icon"> </span>

        <p>
            @TempData.Flash().Success
        </p>
    </div>
}

FlashSpecs.cs

public class FlashSpecs
{
    [Subject(typeof(Flash), "setting message")]
    public class when_setting_message
    {
        private static dynamic flash;
        private static TempDataDictionary tempData;

        Establish context = () =>
                            {
                                tempData = new TempDataDictionary();
                                flash = new Flash(tempData);
                            };

        Because of = () => flash.Success = "succes";

        It should_populate_temp_data = () => tempData["FlashSuccess"].ShouldNotBeNull();
    }

    [Subject(typeof(Flash), "testing for message presence")]
    public class when_message_is_set
    {
        private static dynamic flash;
        private static bool hasMessage;

        Establish context = () =>
        {
            var tempData = new TempDataDictionary();
            tempData["FlashSuccess"] = "success";
            flash = new Flash(tempData);
        };

        Because of = () => hasMessage = flash.HasSuccess;

        It should_report_has_message = () => hasMessage.ShouldBeTrue();
    }

    [Subject(typeof(Flash), "testing for message presence")]
    public class when_message_is_not_set
    {
        private static dynamic flash;
        private static bool hasMessage;

        Establish context = () =>
        {
            var tempData = new TempDataDictionary();
            flash = new Flash(tempData);
        };

        Because of = () => hasMessage = flash.HasSuccess;

        It should_report_does_not_have_message = () => hasMessage.ShouldBeFalse();
    }
}