Creating a Generic Dictionary From a Generic List

Question

I am trying to do the following but I think I must be missing something...(fairly new to generics)

(Need to target .NET 2.0 BTW)

interface IHasKey
{
    string LookupKey { get; set; }
}
...

public static Dictionary<string, T> ConvertToDictionary(IList<T> myList) where T : IHasKey
{
    Dictionary<string, T> dict = new Dictionary<string, T>();
    foreach(T item in myList)
    {
        dict.Add(item.LookupKey, item);
    }

    return dict;
}

Unfortunately, this gives a "Constraints are not allowed on non-generic declarations" error. Any ideas?

Answer 1

You have not declared the generic parameter.
Change your declaration to:

public static Dictionary<string, T> ConvertToDictionary<T> (IList<T> myList) where T : IHasKey{
}

Answer 2

Try something like this

public class MyObject : IHasKey
{
    public string LookupKey { get; set; }
}

public interface IHasKey
{
    string LookupKey { get; set; }
} 


public static Dictionary<string, T> ConvertToDictionary<T>(IList<T> myList) where T: IHasKey 
{ 
    Dictionary<string, T> dict = new Dictionary<string, T>(); 
    foreach(T item in myList) 
    { 
        dict.Add(item.LookupKey, item); 
    } 
    return dict; 
}

List<MyObject> list = new List<MyObject>();
MyObject o = new MyObject();
o.LookupKey = "TADA";
list.Add(o);
Dictionary<string, MyObject> dict = ConvertToDictionary(list);

You forgot the Generic Paramter in the method

public static Dictionary<string, T> ConvertToDictionary<T>(IList<T> myList) where T: IHasKey

Answer 3

Since the classes in the input list are different (as you say in your comment) you can either implement it like suggested by @orsogufo, or you could just as well implement your signature on the interface itself:

public static Dictionary<string, IHasKey> ConvertToDictionary(IList<IHasKey> myList) 
{
    var dict = new Dictionary<string, IHasKey>();
    foreach (IHasKey item in myList)
    {
        dict.Add(item.LookUpKey, item);
    }
    return dict;
}

Using the generic declaration is best if you have a list of one specific implementation of the interface as noted in the comments to the other answer.