Generic this index property in dictionary

How can I achieve something like this. I don't want method that return casted value, I want use this[] is this possible?

public class DynamicDictionary : IDictionary<string, object>
{
    public T this<T>[string key] where T : class
    {
        get
        {
            object value = null;
            TryGetValue(key, out value);
            return (T)value;
        }
    }

    // ...
}