Question

This seems correct :

IList<IList<string>> MyList = new List<IList<string>>();
IList<string> List_Temp = new List<string>();
MyList .Add(List_Temp );

This seems incorrect :

IList<List<string>> MyList = new List<List<string>>();
IList<string> List_Temp = new List<string>();
MyList .Add(List_Temp );

why the second is incorrect?

Answer 1

Because you’re trying to add some IList implementation istead of List class, which is requirement by definition – IList<List>. Look at this:

IList<List<string>> MyList = new List<List<string>>();
IList<string> List_Temp = new Collection<string>(); // ooops!
MyList .Add(List_Temp );

The second line of sample is correct, because Collection<T> implements IList<T>, but the third line is incorrect, because Collection<T> doesn’t inherit List<T>.