How to get list of Users from Active Directory by attributes such as Department

Question

I need to provide a search on Active Directory using filters such as Display name, Telephone and Department. Display name and Telephone are easy but I'm stuck on department. This is the bit that works:

using (PrincipalContext context = new PrincipalContext(ContextType.Domain))
{
    UserPrincipal userPrincipal = new UserPrincipal(context);

    if (txtDisplayName.Text != "")
        userPrincipal.DisplayName = "*" + txtDisplayName.Text + "*";

    using (PrincipalSearcher searcher = new PrincipalSearcher(userPrincipal))
    {
        foreach (Principal result in searcher.FindAll())
        {
           DirectoryEntry directoryEntry = result.GetUnderlyingObject() as DirectoryEntry;
           DataRow drName = dtProfile.NewRow();
           drName["displayName"] = directoryEntry.Properties["displayName"].Value;
           drName["department"] = directoryEntry.Properties["department"].Value;
           dtProfile.Rows.Add(drName);
        }
    }
} 

I was hoping that I could just add something like:

DirectoryEntry userDirectoryEntry = userPrincipal.GetUnderlyingObject() as DirectoryEntry;
if (ddlDepartment.SelectedValue != "")
    userDirectoryEntry.Properties["title"].Value = ddlDepartment.SelectedValue;

But that doesn't work. Anyone know how I can do this?

Edit: I'm an idiot, changed the search term and found the answer. The extra fields are called attibutes. Thanks Raymund Macaalay for your blog article on extending Principals.

My extended UserPrincipal:

[DirectoryObjectClass("user")]
[DirectoryRdnPrefix("CN")]

public class UserPrincipalExtended : UserPrincipal
{    
    public UserPrincipalExtended(PrincipalContext context) : base(context) 
    {
    }
    [DirectoryProperty("department")]
    public string department
    {
        get
        {
            if (ExtensionGet("department").Length != 1)
                return null;
            return (string)ExtensionGet("department")[0];
        }
        set { this.ExtensionSet("department", value); }
    }
} 

Answer 1

Since you've already extended the UserPrincipal to include the Department attribute, you'll need to use that extended version of the user principal when you want to search.

Try this:

using (PrincipalContext context = new PrincipalContext(ContextType.Domain))
{
    UserPrincipalExtended userPrincipal = new UserPrincipalExtended(context);

    if (txtDisplayName.Text != "")
    {
        userPrincipal.DisplayName = "*" + txtDisplayName.Text + "*";
    }

    if (!string.IsNullOrEmpty(txtDepartment.Text.Trim())
    {
        userPrincipal.department = txtDepartment.Text.Trim();
    }

    using (PrincipalSearcher searcher = new PrincipalSearcher(userPrincipal))
    {
        foreach (Principal result in searcher.FindAll())
        {
           UserPrincipalExtended upe = result as UserPrincipalExtended;

           if (upe != null)
           {
               DataRow drName = dtProfile.NewRow();
               drName["displayName"] = upe.DisplayName;
               drName["department"] = upe.department;
               dtProfile.Rows.Add(drName);
           }
        }
    }
}