Question

I have IPagedList interface which implements IList like this

public interface IPagedList<T> : IList<T>
{
    int PageCount { get; }
    int TotalItemCount { get; }
    int PageIndex { get; }
    ...
}

Now I want to use this IPagedList to represent data with pagination.
I’m trying to convert existing code which was using IList to IPagedList

public IList<MyViewModel> Data
{
  get
  {         
    List<MyViewModel> myVal = new List<MyViewModel>();
    foreach (MyModel m in data)
    {
       myVal.Add(new MyViewModel(m));
    }
    return myVal;
  }
}

So, I’m wondering why this cannot be converted to

public IPagedList<MyViewModel> Data
{
   get
    .... 

cause IPagedList implements IList and how can I fix this.

Answer 1

IPagedList<T> implements IList<T> and List<T> also implements IList<T>, that’s correct.

But that doesn’t mean that a List<T> can be converted into an IPagedList<T>. That are still two different types. Think about it: A Porsche is a car, a Peugeot is a car, still they aren’t interchangable.

You need to have a concrete implementation of the IPagedList<T> interface. If you have one, you can use this code:

public IList<MyViewModel> Data 
{ 
    get 
    { 
        var myVal = new PagedList<MyViewModel>(); 
        foreach (MyModel m in data) 
        { 
            myVal.Add(new MyViewModel(m)); 
        } 
        return myVal; 
    } 
}