Question

I have a Data<TData, TKey> class that basically wraps around a dictionary. I want to have a constructor that can take another Data class, copy the values, and take a new key.

C# generics seem to prevent me from doing this however, because the Data class does not have to have the same type of key. All I care about is copying the values and then using a new key for a dictionary.

public class Data<TData, TKey>
{
    private Dictionary<TKey, List<TData>> keyedData;
    public delegate TKey Key(TData row);

    public Data(Data<T,K> data, Key keyDelegate, string keyName)
            : this(data.Values, keyDelegate, keyName)
    {   
    }
}

The Data<T,K> will not work, of course. If the constructor takes Data<TData, TKey>, though, it forces both the passed in class and the new class to have the same type key. They likely will not have the same type key. There should be a way to pass in Data<TData,?> as one would in Java.

Answer 1

Perhaps something like…

public interface IData<TData>
{
    IEnumerable<TData> Values { get; }
}

public class Data<TData, TKey> : IData<TData>
{
    private Dictionary<TKey, List<TData>> keyedData;
    public delegate TKey Key(TData row);

    public Data(IData<TData> data, Key keyDelegate, string keyName)
            : this(data.Values, keyDelegate, keyName)
    {
        //...
    }
}

Alternatively, you could make a true generic method to deal with it.

public class Data<TData, TKey>
{
    //...

    public void Populate<TOtherKey>(Data<TData, TOtherKey> otherData)
    {
        // copy otherData.Values into my values
    }
}

Then the consumer would do something like:

Data<DataType, Key1> data1 = new Data<DataType, Key2>(/*blah blah*/);
Data<DataType, Key2> data2 = new Data<DataType, Key2>(/*blah blah*/).Populate(data1);