Why it is not possible to use IList<T> in interface definition and List<T> in inplementation?

Question

Why it is not possible to use IList in interface definition and then implement this property using List? Am I missing something here or just C# compiler doesn't allow this?

public interface ICategory
{
    IList<Product> Products { get; }
}

public class Category : ICategory
{
    public List<Product> Products { get { new List<Product>(); } }        
}

compiler says Error 82 'Category' does not implement interface member 'ICategory.Products'. 'Category.Products' cannot implement 'ICategory.Products' because it does not have the matching return type of 'System.Collections.Generic.IList'

Answer 1

Change your code to:

public interface ICategory
{
    IList<Product> Products { get; }
}

public class Category : ICategory
{
    // Return IList<Product>, not List<Product>
    public IList<Product> Products { get { new List<Product>(); } }        
}

You cannot change the signature of an interface method when you implement it.