Operate on Marketo-Only custom objects through API

[Anonymous]

You may want to open a support case. Note that support can help provide you with a working REQUEST/RESPONSE XML but they can't debug or support your Code Specific (C#) API Client Code.

Let them know your account, the custom object name (CustomTest) and the records you believe should be returned from the GetCustomObject API call and they should be able to create a REQUEST/RESPONSE XML for you that works.