Active Directory and error 'COMException was Unhandled by user code'

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C#

Hi I have a Problem when I try give value using the function

public string AttributeValuesSingleString
    (string attributeName, string objectDn)
{
    string strValue;
    DirectoryEntry ent = new DirectoryEntry(objectDn);
    strValue = ent.Properties[attributeName].Value.ToString();
    ent.Close();
    ent.Dispose();
    return strValue;
}
}

The error occurs at line with code:

strValue = ent.Properties[attributeName].Value.ToString();

Below is the content error:

COMException was Unhandled by user code
Unspecified error

I was looking for problems like me, but I didn't found good solution.

I try using code in DirectoryEntry:

DirectoryEntry ent = new DirectoryEntry(objectDn,"Administrator","Pass",AuthenticationTypes.Secure);

and also tried disabling firewall and nothing has helped.