Apex Managed Sharing as customer portal user if class has "with sharing" modifier

Question

Schema.getGlobalDescribe().get() on a custom Share object is returning null when the user is a CP user and the class has the with sharing modifier even though the custom object has an OWD of private. If you remove with sharing or invoke the transaction as a standard SF user, it works as expected. I have other methods in the class that need sharing rule enforcement.

static sObject shareRecordWithUser(Id recordId, String objectName, Id userId) {
Schema.SObjectType shareType = Schema.getGlobalDescribe().get(objectName + '__Share');
SObject share = shareType.newSObject();    //  <--- NullPointerException
share.put('ParentId', recordId);
share.put('UserOrGroupId', userId);
share.put('AccessLevel', 'Edit');
return share;
}

Any ideas on what's causing this?

Answer 1

This is in the documentation for "share" objects:

Customer Portal users can't access this object.

Therefore, you need to use one of the other methods:

1. You can do this in a trigger, which always operate "without sharing" by default (preferred).
2. You can have an inner class "without sharing," which will allow the DML to succeed.
3. You can have a separate class, such as a utility class, that houses the function.