如果id为null，可以使用null-coalescing运算符提供默认值：

from c in DepartmentMaster
join b in DepartmentDetail
    on (a.ID ?? 0) equals (b.ID ?? 0)
select new {
    a.Code, b.Name
}

这将生成查询

SELECT [t0].[Code], [t1].[Name] AS [ID1]
FROM [DepartmentMaster] AS [t0]
INNER JOIN [DepartmentDetail] AS [t1] 
    ON (COALESCE([t0].[ID],@p0)) = (COALESCE([t1].[ID],@p1))

这与ISNULL不完全相同，但结果应该相同。

这可能对你有帮助

Var r=from a in DepartmentMaster
Join b in DepartmentDetail  on a.(x => (int?)a.ID) ?? 0 equals b.(y => (int?)b.ID) ?? 0
Select a.Code,b.Name

您可以使用DefaultIfEmpty，它将空集合替换为一个默认值的集合。因为int的默认值是0

var r=from a in DepartmentMaster
join b in DepartmentDetail  on a.ID.DefaultIfEmpty() equals b.ID.DefaultIfEmpty()
Select a.Code,b.Name

你应该能够做这样的事情：

Join b in DepartmentDetail  on (a.ID == null ? 0 : a.ID) equals (b.ID == null ? 0 : b.ID)

我做了一些测试，发现在.NET 6.0与Microsoft.EntityFrameworkCore 6上，当我们比较可空字符串时，两者之间有很大的区别：

var queriableResult = from valueCount in queriableValueCount
join valueIds in queriableValueIds
on valueCount.Value equals valueIds.Value
select new { ... }

这将给予以下ON子句：

ON [t].[Value] = [t0].[Value]

和

var queriableResult = from valueCount in queriableValueCount
join valueIds in queriableValueIds
on new { valueCount.Value } equals new { valueIds.Value }
select new { ... }

这将给予所需的ON子句：

ON ([t].[Value] = [t0].[Value]) OR (([t].[Value] IS NULL) AND ([t0].[Value] IS NULL))