C# adding AD object to group it's already member of doesn't throw an exception

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C#

I've a c# console application that is meant to be used to add/remove computer accounts from certain groups and the application works properly, except for a small "but". It adds/removes the relevant computer to/from the relevant AD group, but even if the computer is already member of the group or if it already has been removed no exception is thrown.

This is my code:

try
{
    string strDC = GetDomainController(strOldDAGroupDN.Substring(strOldDAGroupDN.IndexOf("DC")).Replace("DC=", "").Replace(",", ".").Replace("}", ""));
    DirectoryEntry dirEntry = new DirectoryEntry("LDAP://" + strDC + "/" + strOldDAGroupDN);
    dirEntry.Username = strEntAdminUser;
    dirEntry.Password = strEntAdminPW;
    dirEntry.Properties["member"].Remove(strHostDN);
    dirEntry.CommitChanges();
    dirEntry.Close();
}
catch (Exception e)
{
    TextWriter errorWriter = Console.Error;
    errorWriter.WriteLine("Error while removing computer account to AD group!");
    errorWriter.WriteLine(e.Message);
}

When I start debugging and go through the code line, by line, the CommitChanges() call should throw an exception but it doesn't.

What am I doing wrong?

Thanks in advance!

Best,
Markus

What I have tried:

Initially the add/remove computer to group parts were not directly in the main code itself but in functions, but this didn't work either.