I have this function below:

public string GetADDisplayName(string strLogin)
{
    int length = strLogin.IndexOf('\\');
    if (length == -1)
        length = strLogin.IndexOf('@');
    string str1;
    string str2;
    if (length != -1)
    {
        str1 = strLogin.Substring(0, length);
        str2 = strLogin.Substring(length + 1);
    }
    else
    {
        str1 = Environment.MachineName;
        str2 = strLogin;
    }
    string str3;
    try
    {
        str3 = new DirectoryEntry("WinNT://" + str1 + "/" + str2).Properties["FullName"].Value.ToString();
    }
    catch (Exception ex)
    {
        Page.ClientScript.RegisterStartupScript(Page.GetType(), "", "alert('GetADDisplayName ERROR: " + ex.ToString() + "');", true);
        str3 = "";
    }
    return str3;
}

The value I pass into the function is HttpContext.Current.User.Identity.Name.Trim().

When I debug/F5 in my visual studio, it show me the FullName.

When I deploy to web server and access it from within the web server itself, it show me the FullName.

But when I access the web server from client, it failed to show me the FullName, why is that?