Question

I always run in circles with this problem. i wrote code that uses the following interface:

public interface IStorage
{
    T GetElement<T>(String id) where T : class;
    void SetElement<T>(String id, T element) where T : class;
}

But i am just not able to write generic get and set functions. I always end up trying to access dictionaries but all my attempts never even compile. I am mising something huge here, what is it?

Answer 1

The way you defined the interface, method invocation on an interface implementation requires specification of the base type <T>. At that point, you might as well be casting.

[Edited to note] Also, the base class for a generic method is scoped to the individual method, not to the class/interface, which is probably what was causing your problems.

First off, I’d simplify your interface to something like this:

public interface IStorage<T> where T : class
{
  T this[ string id ] { get ; set ; }
}

Then I’d implement it something like:

public class Repository : IStorage<Widget>
{
  private IDictionary<string,Widget> backingStore = new Dictionary<string,Widget>() ;

  public Widget this[string id]
  {
  get
    {
      Widget instance ;
      bool   exists   = backingStore.TryGetValue(id, out instance ) ;
      return instance ; // null if the dictionary doesn't contain the key
    }
  set
    {
      if ( string.IsNullOrWhiteSpace(id) ) throw new ArgumentException("id") ;
      if ( value == null                 ) throw new ArgumentNullException("value") ;
      backingStore.Add( id , value ) ;
      return ;
    }
  }
}