Question

I am implementing IListSource that requires a method GetList() with the following signature:

IList GetList()

I am using .NET framework 2 and I’m wanting to return an object that implements IList as follows:

public System.Collections.IList GetList()
{
    return this._mydata; // Implements IList<MyDataRow>            
}

But I get a compile error saying: Cannot implicitly convert type MyData to System.Collections.IList.

If I create a new list of type List<MyDataRow>, populate it and return this list object, then it works. So in other words, this works:

public System.Collections.IList GetList()
{
   List<MyDataRow> list = new List<MyDataRow>();
   foreach (MyDataRow row in this._mydata)
   {
       list.Add(row);
   }
   return list;
}

But it seems very inefficient to have to recreate the list just to get it from type IList<T> to IList. Why is it that I can return a List<MyDataRow>' from 'GetList(), but not an IList<MyDataRow>? Does anyone know of a way for me to return the IList<MyDataRow> without repopulating a new list?

UPDATE:

The _mydata member variable is declared:

private MyData _mydata;

And MyData is declared:

public class MyData : IList<MyDataRow>
{
   ....
}

Answer 1

Why is it that I can return a List<MyDataRow> from GetList(), but not an IList<MyDataRow>

This is because List<T> implements IList, IList<T> cannot be cast to IList they are 2 separate interfaces. So to answer your question:

Does anyone know of a way for me to return the IList<MyDataRow> without repopulating a new list?

If the concrete type implements IList (which List<T> does) then you can explicitly cast it e.g.

return (IList)this.mydata;

Update

Based on your update, you will have to update MyData to implement IList otherwise you have no choice but to return a new collection which does implement it.

Alternatively, if MyData is indeed a generic list then I would suggest you have it inherit from List<T>, that way you get a lot more flexibility & compatibility out of the box e.g.

class MyData : List<MyDataRow>
{
}