This is a common trap in the database world. In SQL the sum of an empty resultset is NULL and not zero. This goes for AVG, MAX, MIN etc. For example you can run the following query against NorthwindIB and you'll see it returns NULL.

LINQ infers the type of the scalar result from the source, so if you sum a property of type double, the result is double, but if you sum an empty result set as established above, the result value is NULL which cannot be assigned to type double. The workaround as Kim already mentioned is to make sure that the property you sum is nullable by casting it yourself if you expect that the sum might be done on an empty resultset.

 

The exception is coming from EF, and according to MS this behavior is by design.  You can work around this by casting the value to be summed to a nullable, eg:

  var sum = await em.OrderSummaries.Select(o => (decimal?) o.Freight).AsScalarAsync().Sum();

or

  var sum = await em.OrderSummaries.AsScalarAsync().Sum(o => (decimal?) o.Freight);

 

We'll see if there's anything we can add to DevForce so the workaround isn't required.