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 I haven't been able to finger what I am doing wrong on this one.  After a PUT to update a contact, I always get the response 409 Conflict.
 This is what i am PUTing (slightly modified site names)
 

<entry xmlns="http://www.w3.org/2005/Atom">
<id>https://api.constantcontact.com/ws/customers/accountname/contacts/828</id>
<title type="text">Contact: ddddddddd@asdf.com</title>
<updated>2008-07-23T14:21:06.407Z</updated>
<author>asdf</author>
<content type="application/vnd.ctct+xml">
<Contact xmlns="http://ws.constantcontact.com/ns/1.0/" id="https://api.constantcontact.com/ws/customers/accounbtname/contacts/828">
<EmailAddress>ddddddddd@asdf.com</EmailAddress>
<OptInSource>ACTION_BY_CUSTOMER</OptInSource>
<ContactLists></ContactLists>
</Contact>
</content>
</entry>
 
 
And that is built by this code:
 
 

StringBuilder reqString = new StringBuilder();

reqString.Append( "<entry xmlns=\"http://www.w3.org/2005/Atom\">" );

reqString.Append( "<id>https://api.constantcontact.com/ws/customers/" + cci.APILogin + "/contacts/" + link + "</id>" );

reqString.Append( "<title type=\"text\">Contact: " + cci.Email + "</title>" );

reqString.Append( "<updated>2008-07-23T14:21:06.407Z</updated>" );

reqString.Append( "<author>asdf</author>" );

reqString.Append( "<content type=\"application/vnd.ctct+xml\">" );

reqString.Append( "<Contact xmlns=\"http://ws.constantcontact.com/ns/1.0/\" id=\"https://api.constantcontact.com/ws/customers/" + cci.APILogin + "/contacts/" + link + "\">" );

reqString.Append( "<EmailAddress>" + cci.Email.ToLower() + "</EmailAddress>" );

reqString.Append( "<OptInSource>ACTION_BY_CUSTOMER</OptInSource>" );

reqString.Append( "<ContactLists>" );

reqString.Append( "</ContactLists>" );

reqString.Append( "</Contact>" );

reqString.Append( "</content>" );

reqString.Append( "</entry>" );

byte[] buffer = Encoding.UTF8.GetBytes( reqString.ToString() );

HttpWebRequest WebReq = (HttpWebRequest)WebRequest.Create( "https://api.constantcontact.com/ws/customers/" + cci.APILogin + "/contacts/" + link );

WebReq.Credentials = new NetworkCredential( cci.APIKey + "%" + cci.APILogin, cci.APIPassword );

WebReq.Method = "PUT";

WebReq.ContentType = "application/atom+xml";

WebReq.ContentLength = buffer.Length;

Stream PostData = WebReq.GetRequestStream();

PostData.Write( buffer, 0, buffer.Length );

PostData.Close();