[英]Default value if null in lambda C#
我正在嘗試使用lambda表達式加入三個表。 如果兩個表必須相互匹配,則下面的代碼有效,但是如果存在與其他表不對應的記錄,則返回錯誤。 Object reference not set to an instance of an object.
using (WeeklyCreditEntities WCE = new WeeklyCreditEntities())
{
var xx = Raw.GroupJoin(WCE.tblCHRIS,
a => a.CHRISID,
b => b.CHRISID,
(a, b) => new
{
raw = a,
chris = b.DefaultIfEmpty().FirstOrDefault()
}).Select(x => new
{
RawEMpid = x.raw.EmployeeID,
CHRISEmpID = x.chris.EmployeeID,
RawCHRISID = x.raw.CHRISID,
RawFullname = x.raw.Fullname,
RawAmount = x.raw.Amount,
chrisBank = x.chris.BankAccount
}).ToList();
您可以將安全導航運算符與null-coalescing運算符一起使用:
using (WeeklyCreditEntities WCE = new WeeklyCreditEntities())
{
var xx = Raw.GroupJoin(WCE.tblCHRIS,
a => a.CHRISID,
b => b.CHRISID,
(a, b) => new
{
raw = a,
chris = b.DefaultIfEmpty().FirstOrDefault()
}).Select(x => new
{
RawEMpid = x.raw?.EmployeeID ?? defaultValue,
CHRISEmpID = x.chris?.EmployeeID ?? defaultValue,
RawCHRISID = x.raw?.CHRISID ?? defaultValue,
RawFullname = x.raw?.Fullname ?? defaultValue,
RawAmount = x.raw?.Amount ?? defaultValue,
chrisBank = x.chris?.BankAccount ?? defaultValue
}).ToList();
或者,如果您使用的是C#/ Visual Studio的舊版本,則可以使用三元運算符:
using (WeeklyCreditEntities WCE = new WeeklyCreditEntities())
{
var xx = Raw.GroupJoin(WCE.tblCHRIS,
a => a.CHRISID,
b => b.CHRISID,
(a, b) => new
{
raw = a,
chris = b.DefaultIfEmpty().FirstOrDefault()
}).Select(x => new
{
RawEMpid = x.raw != null? x.raw.EmployeeID : defaultValue,
CHRISEmpID = x.chris != null? x.chris.EmployeeID : defaultValue,
RawCHRISID = x.raw != null? x.raw.CHRISID : defaultValue,
RawFullname = x.raw != null? x.raw.Fullname : defaultValue,
RawAmount = x.raw != null? x.raw.Amount : defaultValue,
chrisBank = x.chris != null?x.chris.BankAccount : defaultValue
}).ToList();
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