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

You could exclude at source?

var sum = GroupProduct.Where(a => a.Product.ProductID==1 && a.Amount != null)
            .Sum(a => (decimal)a.Amount);