null - Linq error generic parameter or the query must use a nullable type

Question

I got this error when i use sum function in LINQ:

The cast to value type 'Decimal' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

GroupProduct.Where(a => a.Product.ProductID==1).Sum(Content => Content.Amount==null?0:Content.Amount),

Answer 1

This is what I usually use. This will cover the possibility of Amount being null and also cover the possibility of an empty set.

GroupProduct.Where(a => a.Product.ProductID == 1)
    .Select(c => c.Amount ?? 0) // select only the amount field
    .DefaultIfEmpty()  // if selection result is empty, return the default value
    .Sum(c => c)

DefaultIfEmpty() returns the default value associated with Amount's type, which is int, in which case the default value is 0.