Linq中的右连接，未返回正确的值
Linq中的右连接，未返回正确的值
提问于 2017-05-09 04:34:28
我正在尝试构建这个linq查询，但遇到了一些问题
我有两个列表
房间，包含Id，Name，RoomStatus，OrderStatus，ConsructionType
STRINGMAP，包含AttributeName、AttributeValue、Value
var stringMap = from sm in _CRMcontext.StringMaps
where sm.ObjectTypeCode == 10001
&& sm.AttributeName == "new_status"
|| sm.AttributeName == "new_projecttype"
|| sm.AttributeName == "new_orderstatus"
|| sm.AttributeName == "new_construction_type"
|| sm.AttributeName == "new_stage"
select sm;
var result = from set in ROOMS
join roomStatus in stringMap on set.Status equals status.AttributeValue
where roomStatus.AttributeName == "new_status"
join orderStatus in stringMap on set.OrderStatus equals orderStatus.AttributeValue
where orderStatus.AttributeName == "new_orderstatus"
join consructionType in stringMap on set.ConstructionType equals consructionType.AttributeValue
where consructionType.AttributeName == "new_construction_type"
select new DTO.Shared.Project() {
Id = set.Id,
Name = set.Name,
RoomStatus = roomStatus.Value,
OrderStatus = orderStatus.Value,
ConstructionType = constructionType.Value,
};因此，我只返回连接中没有null的结果，我希望无论null如何，都要返回它们，这类似于sql中的右连接
谢谢，迈克尔
回答 1
Stack Overflow用户
发布于 2017-05-09 07:03:03
下面是一个右外连接的IQueryable实现：
private static Expression<Func<TP, TC, TResult>> CastSMBody<TP, TC, TResult>(LambdaExpression ex, TP unusedP, TC unusedC, TResult unusedRes) => (Expression<Func<TP, TC, TResult>>)ex;
public static IQueryable<TResult> RightOuterJoin<TLeft, TRight, TKey, TResult>(
this IQueryable<TLeft> leftItems,
IQueryable<TRight> rightItems,
Expression<Func<TLeft, TKey>> leftKeySelector,
Expression<Func<TRight, TKey>> rightKeySelector,
Expression<Func<TLeft, TRight, TResult>> resultSelector) where TLeft : class where TRight : class where TResult : class {
var sampleAnonLR = new { leftg = (IEnumerable<TLeft>)null, right = (TRight)null };
var parmP = Expression.Parameter(sampleAnonLR.GetType(), "p");
var parmC = Expression.Parameter(typeof(TLeft), "c");
var argRight = Expression.PropertyOrField(parmP, "right");
var newrightrs = CastSMBody(Expression.Lambda(Expression.Invoke(resultSelector, parmC, argRight), parmP, parmC), sampleAnonLR, (TLeft)null, (TResult)null);
return rightItems.GroupJoin(leftItems, rightKeySelector, leftKeySelector, (right, leftg) => new { leftg, right }).SelectMany(l => l.leftg.DefaultIfEmpty(), newrightrs);
}你可以像这样使用它：
var result = ROOMS.RightOuterJoin(stringMap, set => new { set.Status, AttributeName = "new_status" }, roomStatus => new { Status = roomStatus.AttributeValue, roomStatus.AttributeName }, (set, roomStatus) => new { set.Id, set.Name, roomStatus.Value });页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/43856834
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