Adding different type of generic objects into generic list

Question

Is it possible to add different type of generic objects to a list?. As below.

public class ValuePair<T>
{        
    public string Name { get; set;}
    public T Value { get; set;                     
}

and let say I have all these objects...

 ValuePair<string> data1 =  new ValuePair<string>();
 ValuePair<double> data2 =  new ValuePair<double>();
 ValuePair<int> data3 =  new ValuePair<int>();

I would like to hold these objects in a generic list.such as

List<ValuePair> list = new List<ValuePair>();

list.Add(data1);
list.Add(data2);
list.Add(data3);

Is it possible?

Answer 1

In general, you'd have to either use a List<object> or create a non-generic base class, e.g.

public abstract class ValuePair
{
    public string Name { get; set;}
    public abstract object RawValue { get; }
}

public class ValuePair<T> : ValuePair
{
    public T Value { get; set; }              
    public object RawValue { get { return Value; } }
}

Then you can have a List<ValuePair>.

Now, there is one exception to this: covariant/contravariant types in C# 4. For example, you can write:

var streamSequenceList = new List<IEnumerable<Stream>>();

IEnumerable<MemoryStream> memoryStreams = null; // For simplicity
IEnumerable<NetworkStream> networkStreams = null; // For simplicity
IEnumerable<Stream> streams = null; // For simplicity

streamSequenceList.Add(memoryStreams);
streamSequenceList.Add(networkStreams);
streamSequenceList.Add(streams);

This isn't applicable in your case because:

• You're using a generic class, not an interface
• You couldn't change it into a generic covariant interface because you've got T going "in" and "out" of the API
• You're using value types as type arguments, and those don't work with generic variable (so an IEnumerable<int> isn't an IEnumerable<object>)