Can't commit changes in Active Directory from C#

Question

In the following code I want to assign user to a group in active directory. The problem is when I commit it breaks. I think its permission related error. After searching on internet I found that I have to use

using (HostingEnvironment.Impersonate())
{ ... }

but again its not working. Does anyone know how to fix this issue. Here is the code I am using

using (HostingEnvironment.Impersonate())
{
    using (var pc = new PrincipalContext(ContextType.Domain, "192.168.1.100", "username", "password"))
    {
        using (var up = new UserPrincipal(pc))
        {
            //up.Name = txtFirstName.Text + " " + txtLastName.Text;
            //up.Save();
        }

        using (var searcher = new PrincipalSearcher(new UserPrincipal(pc)))
        {
            foreach (var result in searcher.FindAll())
            {
                DirectoryEntry de = result.GetUnderlyingObject() as DirectoryEntry;
                if (ASPxGridView1.GetRowValues(ASPxGridView1.FocusedRowIndex, "Корисничко име").ToString() == de.Properties["samAccountName"].Value.ToString())
                {
                    try { de.Properties["member"].Add(edGrupi.SelectedItem.Text.ToString()); }
                    catch { }
                    de.CommitChanges();
                }

            }
        }
    }
}

Thank you.