C# generic method with one parameter of multiple types

Question

I'm having a problem writing a generic method to retrieve AD Groups or Users with a parameter that can be one of two types - either System.DirectoryServices.AccountManagement GroupPrincipal or UserPrincipal

The method is as follows:-

private static IEnumerable<string> GetGroupsOrUsers<T>(T GroupOrUserPrincipal)
{
   PrincipalSearcher ps = new PrincipalSearcher();
   ps.QueryFilter = GroupOrUserPrincipal;

   etc.........
}

The problem is the GroupOrUserPrincipal is showing the following error:-

Cannot implicitly convert type 'T' to System.DirectoryServices.AccountManagement.Principal

Am I able to do this or am I missing something ?

Answer 1

You should restrict T to types that your method makes sense for:

private static IENumerable<string> GetGroupsOrUsers<T>(T GroupOrUserPrincipal)
        where T : Principal
{
      // .....

That prevents calls of GetGroupsOrUsers<int>, and lets T be implicitly converted to Principal, fixing your error (or so I hope).

Answer 2

Possibly you want to look at Generic Contraints in particular a derivation contraint where your various T objects all impliment a given interterface. Eg

where T : Principle

Answer 3

You need to specify a type parameter constraint; for example:

private static IEnumerable<string> GetGroupsOrUsers<T>(T GroupOrUserPrincipal) where T: Principal

This limits the classes that can be used as T to only classes of type Principal or a subclass of Principal. The C# compiler then knows that everything passed to GetGroupsOrUsers(...) will be of a type compatible with Principal and will no longer error.

Answer 4

You have to write a cast in this line:

ps.QueryFilter = (Principal) GroupOrUserPrincipal;