Linq Join子句类型错误排查：类型推断失败问题求助

问题代码 #

var model = await (from a in _context.Mfemployees
                    join b in _context.Mfclients on a.MfemployeeClientCode equals b.MfclientCode into ab
                    from x in ab.DefaultIfEmpty()
                    join c in _context.Mujls on new { a.MfemployeeGroup, a.MfemployeeEmploymentLevel } equals new { MujlGroup = c.MujlGroup, MujlLevel = c.MujlLevel }
                    select new ExportPartiesDTO
                    {
                        MFEMPLOYEEGroup = a.MfemployeeGroup,
                        MFEMPLOYEEEmployeeCode = a.MfemployeeEmployeeCode ?? "",
                        MFEMPLOYEEEmploymentDate = a.MfemployeeEmploymentDate,
                        MFCLIENTName = x.MfclientName ?? "",
                        MFCLIENTSurname = x.MfclientSurname ?? "",
                        MFCLIENTPhone = x.MfclientPhone ?? "",
                        MFCLIENTHP = x.MfclientHp ?? "",
                        MFCLIENTEmail = x.MfclientEmail ?? ""
                    }).ToListAsync(cancellationToken);

报错信息 #

The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to join.

已确认关联字段均为String类型，但报错依旧。

问题根源 #

LINQ的join操作要求用于匹配的匿名类型属性名称、顺序、类型完全一致。你在第二条join的右侧匿名类型中给属性设置了别名（MujlGroup、MujlLevel），但左侧匿名类型的属性名是MfemployeeGroup、MfemployeeEmploymentLevel，两边属性名不匹配，导致编译器无法推断出这两个匿名类型是同一类型，进而报错。

修复方案 #

只需让两边匿名类型的属性名称一致即可，两种方式任选：

方式1：右侧使用左侧的属性名 #

join c in _context.Mujls on new { a.MfemployeeGroup, a.MfemployeeEmploymentLevel } equals new { MfemployeeGroup = c.MujlGroup, MfemployeeEmploymentLevel = c.MujlLevel }

方式2：左侧使用右侧的属性名 #

join c in _context.Mujls on new { MujlGroup = a.MfemployeeGroup, MujlLevel = a.MfemployeeEmploymentLevel } equals new { c.MujlGroup, c.MujlLevel }

修改后编译器就能正确识别匿名类型，join操作可以正常执行。

内容的提问来源于stack exchange，提问作者Haminteu