Question

The following method I created seem does not work. An error always happens on foreach loop.

NotSupportedException was unhandled…The provider does not support
searching and cannot search WinNT://WIN7,computer.

I’m querying the local machine

 private static void listUser(string computer)
 {
        using (DirectoryEntry d= new DirectoryEntry("WinNT://" + 
                     Environment.MachineName + ",computer"))
        {
           DirectorySearcher ds = new DirectorySearcher(d);
            ds.Filter = ("objectClass=user");
            foreach (SearchResult s in ds.FindAll())
            {

              //display name of each user

            }
        }
    }

Answer 1

You cannot use a DirectorySearcher with the WinNT provider. From the documentation:

Use a DirectorySearcher object to search and perform queries against an Active Directory Domain Services hierarchy using Lightweight Directory Access Protocol (LDAP). LDAP is the only system-supplied Active Directory Service Interfaces (ADSI) provider that supports directory searching.

Instead, use the DirectoryEntry.Children property to access all child objects of your Computer object, then use the SchemaClassName property to find the children that are User objects.

With LINQ:

string path = string.Format("WinNT://{0},computer", Environment.MachineName);

using (DirectoryEntry computerEntry = new DirectoryEntry(path))
{
    IEnumerable<string> userNames = computerEntry.Children
        .Cast<DirectoryEntry>()
        .Where(childEntry => childEntry.SchemaClassName == "User")
        .Select(userEntry => userEntry.Name);

    foreach (string name in userNames)
        Console.WriteLine(name);
}       

Without LINQ:

string path = string.Format("WinNT://{0},computer", Environment.MachineName);

using (DirectoryEntry computerEntry = new DirectoryEntry(path))
    foreach (DirectoryEntry childEntry in computerEntry.Children)
        if (childEntry.SchemaClassName == "User")
            Console.WriteLine(childEntry.Name);