LEFT JOIN 与 LINQ 产生 NULL 引用异常

Question

我正在尝试将使用 JOIN 的当前代码转换为使用 LINQ 的 LEFT JOIN。这是当前代码

globalFunctions = _rolesAdapter.GetSecurityRightsForRole(SysManagerConstants.GLOBAL_DEFAULTS_ROLE_NUMBER, "SecFunc").
                Select(secRight => new FuncViewModel()
                {
                    Descrip = secRight.SecFunc.Descrip,
                    FuncCode = secRight.SecFunc.FuncCode,
                    Grouping = secRight.SecFunc.Grouping,
                    Progmodule = secRight.SecFunc.Progmodule,
                    SubCat = secRight.SecFunc.SubCat,
                    GrantPerm = secRight.GrantPerm
                }).OrderBy(x => x.Descrip);

            //Unassigned
            editSecRoleViewModel.UnassignedFunctions = editSecRoleViewModel.UnassignedFunctions.Join(globalFunctions, uf => uf.FuncCode, gf => gf.FuncCode,
                    (uf, gf) => new FuncViewModel()
                    {
                        Descrip = uf.Descrip,
                        FuncCode = uf.FuncCode,
                        Grouping = uf.Grouping,
                        Progmodule = uf.Progmodule,
                        SubCat = uf.SubCat,
                        GrantPerm = (byte)(gf.GrantPerm | uf.GrantPerm),
                        DefaultGrantPerm = gf.GrantPerm
                    }).OrderBy(x => x.Descrip);

我尝试根据 How do you perform a left outer join using linq extension methods

//Unassigned
            editSecRoleViewModel.UnassignedFunctions = editSecRoleViewModel.UnassignedFunctions.GroupJoin(globalFunctions, 
                uf => uf.FuncCode, gf => gf.FuncCode,
                    (uf, gf) => new FuncViewModel()
                    {
                        Descrip = uf.Descrip,
                        FuncCode = uf.FuncCode,
                        Grouping = uf.Grouping,
                        Progmodule = uf.Progmodule,
                        SubCat = uf.SubCat,
                        GrantPerm = (byte)(((int?)gf.FirstOrDefault().GrantPerm??1) | uf.GrantPerm),
                        DefaultGrantPerm = (byte) ((int?)gf.FirstOrDefault().GrantPerm ?? 1)
                    }).OrderBy(x => x.Descrip);

我尝试了 SingleOrDefault 和 FirstOrDefault 并且我在运行时间内一直得到 NULL 对象引用。示例和我的案例之间的区别在于我正在创建一个新的 FuncViewModel 对象，而不是仅仅创建一个新对象。应该怎样做才能解决这个问题？

Answer 1

我用这段代码解决了

 //Unassigned
            editSecRoleViewModel.UnassignedFunctions = editSecRoleViewModel.UnassignedFunctions.SelectMany( 
                uf => globalFunctions.Where(gf => uf.FuncCode== gf.FuncCode).DefaultIfEmpty(),
                    (uf, gf) => new FuncViewModel()
                    {
                        Descrip = uf.Descrip,
                        FuncCode = uf.FuncCode,
                        Grouping = uf.Grouping,
                        Progmodule = uf.Progmodule,
                        SubCat = uf.SubCat,
                        GrantPerm = (byte)( ((gf==null)?1:gf.GrantPerm) | uf.GrantPerm) ,
                        DefaultGrantPerm = ((gf == null) ? (byte)1 : gf.GrantPerm)
                    }).OrderBy(x => x.Descrip);

LEFT JOIN 与 LINQ 产生 NULL 引用异常

LEFT JOIN with LINQ produces NULL reference exception

c#

linq

join

left-join