✅ Return Proper Dictionary

public class Data
{
}

public class MyData : Data
{
}

public interface IHelper
{
    public Dictionary<string, T> GetDictionary<T>();
}

public class MyClass : IHelper
{
    public Dictionary<string, MyData> myData = new Dictionary<string, MyData>();

    public Dictionary<string, Data> GetDictionary<Data>()
    {
        return myData as Dictionary<string, Data>;
    }
}