C# LINQ Multiple GroupJoin 使用 Lambda 语法

Question

我有 3 个表，我正在尝试使用左连接将它们组合在一起。在我的应用程序中，所有 LINQ 表达式都是查询链格式(lambda 表达式)。

我正在尝试找出如何使用 LINQ 完成此查询；

这是我的 SQL 查询；

select i.INVOICE_NUMBER, ic.CustomerName, o.BusinessUnit
from Invoices_Daily as i
left join intercompany as ic on i.customer_number = ic.customernumber
left join ordertypes as o on i.LINE_ORDERTYPE = o.OrderType

我尝试按如下方式组合 GroupJoin；

var commissions = data
    .GroupJoin(genericNameList,
        d => d.CUSTOMER_NUMBER,
        g => g.CustomerId,
        (d, g) => new { d, generic = g.FirstOrDefault() })
    .GroupJoin(intercompanies,
        dd => dd.d.CUSTOMER_NAME,
        i => i.CustomerId,
        (d, i) => new { data = d.d, intercompanies = i.FirstOrDefault() })
    .Select(_ => new MainGridViewModel
    {
        INVOICE_DATE = _.d.INVOICE_DATE,
        EndCustomer = _.generic == null ? _.d.CUSTOMER_NAME : _.generic.EndCustomer ?? _.d.CUSTOMER_NAME,
        LINE_ORDERTYPE = _.i.OrderType
    })
    .ToList();

但是我有一个语法错误，写在下面；

Error CS0411 The type arguments for method 'Enumerable.GroupJoin(IEnumerable, IEnumerable, Func, Func, Func, TResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Answer 1

我根据请求编写了一个测试示例，显示了正确的语法，这应该一切正常。

public class Data
{
    public string CUSTOMER_NUMBER { get; set; }
    public string CUSTOMER_NAME { get; set; }
}

public class NameList
{
    public string CustomerId { get; set; }

}

public class InterCompanies
{
    public string CustomerId { get; set; }
}

public class Test
{
    public void TMP()
    {
        var data = new List<Data>();
        var genericNameList = new List<NameList>();
        var intercompanies = new List<InterCompanies>();

        var commissions = data
            .GroupJoin(genericNameList,
                d => d.CUSTOMER_NUMBER,
                g => g.CustomerId,
                (d, g) => new { d, generic = g.FirstOrDefault() })
            .GroupJoin(intercompanies,
                dd => dd.d.CUSTOMER_NAME,
                i => i.CustomerId,
                (d, i) => new { data = d.d, intercompanies = i.FirstOrDefault() })
            .ToList();
    }
}

我最初的想法是您的 CUSTOMER_NUMBER 和 CUSTOMER_NAME 是错误的，因为为什么您在同一个类中有 2 个具有相同值的字段。但既然你向我保证它们是正确的，也许你正在交换连接中数据预期的字段顺序，如果没有看到你的数据模型就很难判断。

也许您的第二个连接应该如下所示:

.GroupJoin(intercompanies,
            dd => dd.d.CUSTOMER_NUMBER,
            i => i.CUSTOMER_NAME,