Linq默认如果空不起作用

Question

我正在加入两个表供应商和productCharges这样的

var result = (from vendor in cont.VendorCont
              join ProdCharge in cont.AdminSalesProductServiceCharge on 
              vendor.VendorId equals ProdCharge.VendorId  into a
              from b in a.Where(x=>x.VendorId==null).DefaultIfEmpty()

               select b.ProductServiceCharge).FirstOrDefault();

enter image description here

我的问题是它总是返回空值，但我想第一行值即-5 如何解决此问题。请帮助