Using Sum() with Linq-2-Entities navigation properties.

Linq2EntitySum.cs

// Fixes error: The null value cannot be assigned to a member with type System.Decimal which is a non-nullable value type.

// In the case where none of the navigation property's StringPropA properties match a null will returned.
// .Sum() will throw an error if you try to sum null so we replace the null with a default value using .DefaultIfEmpty()

var total = Parent
  .Where(x => x.NavigationProperty.StringPropA == "Foo")
  .Select(x => x.DecimalPropB)
  .DefaultIfEmpty(0)
  .Sum()