CODEWITHSUNDEEP
c#c#-4.0c#-3.0
Alan B
Alan B

Reputation: 2299

Adding different type of generic objects into generic list

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?

Upvotes: 8

Views: 13590

Answers (4)

Jon Skeet
Jon Skeet

Reputation: 1503469

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>)

Upvotes: 13

Davide Piras
Davide Piras

Reputation: 44605

it's not possible as far as I know.

the line:

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

you wrote in your sample is not providing a concrete type for T and this is the issue, once you pass it, you can only add object of that specific type.

Upvotes: 1

Lorenzo Dematt&#233;
Lorenzo Dematt&#233;

Reputation: 7849

No, it is not possible. You could create, in your case, a base class ValuePair from which ValuePair<T> derives. Depends on your purposes.

Upvotes: 1

Marc Gravell
Marc Gravell

Reputation: 1064114

Not unless you have a non-generic base-type ValuePair with ValuePair<T> : ValuePair (it would work for an interface too), or use List<object>. Actually, though, this works reasonably:

public abstract class ValuePair
{
    public string Name { get; set; }
    public object Value
    {
        get { return GetValue(); }
        set { SetValue(value); }
    }
    protected abstract object GetValue();
    protected abstract void SetValue(object value);
}
public class ValuePair<T> : ValuePair
{
    protected override object GetValue() { return Value; }
    protected override void SetValue(object value) { Value = (T)value; }
    public new T Value { get; set; }
}

Upvotes: 5

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