The underlying connection was closed: The connection was closed unexpectedly

If you’re attempting to return a LINQ to SQL object using a WCF service and are getting this error, what’s really happening is that .NET can’t serialize your object. This occurs when your LINQ to SQL objects contain circular references (example: customer points to an order, order points back to customer). To get around this, open your data context and in the properties window of the visual designer, set the “Serialzation Mode” option to “Unidirectional”. Thanks to Rick Stahl for his post on the subject.