使用C＃中的表达式树的lambda表达式排序

Question

我的要求是在运行时根据列来命令表。所以我经历了参数表达的路径。当所有列都存在于单个表中时，我能够实现它。这是我的代码：

我的要求是在运行时根据buildStatusOrder订购表格：

 List<BuildStatusEnum> buildStatusOrder = new List<BuildStatusEnum>
            {
                BuildStatusEnum.h,
                BuildStatusEnum.f,
                BuildStatusEnum.a
            };

            ParameterExpression parameterExpression = Expression.Parameter(typeof(companyQueue));
            MemberExpression memberExpression = Expression.PropertyOrField(parameterExpression, "buildStatusID");
            MemberExpression valueExpression = Expression.Property(memberExpression, "Value");

            Expression orderByExpression = Expression.Constant(buildStatusOrder.Count);
            for (int statusIndex = buildStatusOrder.Count - 1; statusIndex >= 0; statusIndex--)
            {
                ConstantExpression constantExpression = Expression.Constant((int)buildStatusOrder[statusIndex]);
                ConstantExpression indexConstantExpression = Expression.Constant(statusIndex);
                BinaryExpression equalExpression = Expression.Equal(valueExpression, constantExpression);

                orderByExpression = Expression.Condition(equalExpression, indexConstantExpression, orderByExpression);
            }

            MemberExpression hasValueExpression = Expression.Property(memberExpression, "HasValue");
            ConditionalExpression nullCheckExpression = Expression.Condition(hasValueExpression, orderByExpression, Expression.Constant(buildStatusOrder.Count));

            var orderByLambda = Expression.Lambda<Func<companyQueue, int>>(nullCheckExpression, parameterExpression);

            using (KapowContext context = new KapowContext())
            {
                var queue = context.companyQueues

                        .Where(cq => cq.u.s.cq.userID == Utilities.Authentication.UserId)
                         .Where(cq => buildStatusOrder.Contains((BuildStatusEnum)cq.u.s.cq.buildStatusID))
                         .OrderBy(o => o.u.se.segmentId)
                         .ThenBy(orderByLambda)

这很好用。我必须显示一个额外的列。它来自另一张桌子。所以我加入了桌子。我的lambda表达式现在看起来像这样：

 var queue = context.companyQueues
                         .Join(context.abcd, cq => cq.robotID, r => r.robotId, (cq, r) => new { cq, r })
                         .Join(context.efgh, s => s.r.segmentId, se => se.segmentId, (s, se) => new { s, se })
                         .Join(context.hijk, u => u.s.cq.userID, us => us.userID, (u, us) => new { u, us })

                        .Where(cq => cq.u.s.cq.userID == Utilities.Authentication.UserId)
                         .Where(cq => buildStatusOrder.Contains((BuildStatusEnum)cq.u.s.cq.buildStatusID))
                        .OrderBy(orderByLambda)

现在我的orderbyLambda无效了。我收到以下错误：

  

＆＃39; System.Linq.Queryable.ThenBy<TSource,TKey>(System.Linq.IOrderedQueryable<TSource>, System.Linq.Expressions.Expression<System.Func<TSource,TKey>>)＆＃39;无法从使用中推断出来。尝试显式指定类型参数。