How to check if a user belongs to an AD group?

Since you’re on .NET 3.5 and up, you should check out the System.DirectoryServices.AccountManagement (S.DS.AM) namespace. Read all about it here:

• Managing Directory Security Principals in the .NET Framework 3.5
• MSDN docs on System.DirectoryServices.AccountManagement

Basically, you can define a domain context and easily find users and/or groups in AD:

// set up domain context
PrincipalContext ctx = new PrincipalContext(ContextType.Domain, "DOMAINNAME");

// find a user
UserPrincipal user = UserPrincipal.FindByIdentity(ctx, "SomeUserName");

// find the group in question
GroupPrincipal group = GroupPrincipal.FindByIdentity(ctx, "YourGroupNameHere");

if(user != null)
{
   // check if user is member of that group
   if (user.IsMemberOf(group))
   {
     // do something.....
   } 
}

The new S.DS.AM makes it really easy to play around with users and groups in AD!