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LINQ中的是空功能还是合并功能?

nhibernate linq

Chris · 技术社区 · 14 年前

我有以下LINQ查询:

return (from r in Repository.Query<Measurement>()
        where
            r.Postal.ToLowerInvariant() ==
                (string.IsNullOrEmpty(postalCode)
                    ? r.Postal : postalCode).ToLowerInvariant()
            &&
            r.Trait.ToLowerInvariant() ==
                (string.IsNullOrEmpty(trait)
                    ? r.Trait : trait).ToLowerInvariant()
        select r).ToList();

我的目标是模拟IsNull或Coalesce的功能,但是LINQ不喜欢它。我得到以下异常:

The unary operator Not is not defined for the type 'System.String'.
at System.Linq.Expressions.Expression.GetUserDefinedUnaryOperatorOrThrow(ExpressionType unaryType, String name, Expression operand)
   at System.Linq.Expressions.Expression.Not(Expression expression, MethodInfo method)
   at System.Linq.Expressions.Expression.Not(Expression expression)
   at NHibernate.Linq.Visitors.BinaryBooleanReducer.ProcessBinaryExpression(Expression exprToCompare, Expression exprToReturn, ExpressionType nodeType, Expression original)
   at NHibernate.Linq.Visitors.BinaryBooleanReducer.VisitBinary(BinaryExpression expr)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitBinary(BinaryExpression b)
   at NHibernate.Linq.Visitors.BinaryBooleanReducer.VisitBinary(BinaryExpression expr)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitBinary(BinaryExpression b)
   at NHibernate.Linq.Visitors.BinaryBooleanReducer.VisitBinary(BinaryExpression expr)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitLambda(LambdaExpression lambda)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitUnary(UnaryExpression u)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitList(ReadOnlyCollection`1 original)
   at NHibernate.Linq.Visitors.ExpressionVisitor.VisitMethodCall(MethodCallExpression m)
   at NHibernate.Linq.Visitors.ExpressionVisitor.Visit(Expression exp)
   at NHibernate.Linq.NHibernateQueryProvider.TranslateExpression(Expression expression)
   at NHibernate.Linq.NHibernateQueryProvider.Execute(Expression expression)
   at NHibernate.Linq.Query`1.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at HarvestMap.Website.DataService.GetData(String postalCode, String trait, String measureType, Boolean ffOnly, String apiKey) in D:\Data\Projects\Active\Clients\HarvestMap\src\Website\DataService.svc.cs:line 90
   at HarvestMap.Website.DataService.GetDataText3(String postalCode, String trait, String measureType, Boolean ffOnly) in D:\Data\Projects\Active\Clients\HarvestMap\src\Website\DataService.svc.cs:line 162
   at SyncInvokeGetDataText3(Object , Object[] , Object[] )
   at System.ServiceModel.Dispatcher.SyncMethodInvoker.Invoke(Object instance, Object[] inputs, Object[]& outputs)
   at System.ServiceModel.Dispatcher.DispatchOperationRuntime.InvokeBegin(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage5(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage41(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage4(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage31(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage3(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage2(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage11(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.ImmutableDispatchRuntime.ProcessMessage1(MessageRpc& rpc)
   at System.ServiceModel.Dispatcher.MessageRpc.Process(Boolean isOperationContextSet)

就这样。关于我做错了什么没有更多的线索。帮忙?

2 回复 | 直到 14 年前

Simon 14 年前

尝试重构IsNullOrEmpty条件,如下所示:

return (from r in Repository.Query<Measurement>()
    where
        (string.IsNullOrEmpty(postalCode)
            || r.Postal.ToLowerInvariant() == postalCode.ToLowerInvariant()
        )
        &&
        (string.IsNullOrEmpty(trait)
            || r.Trait.ToLowerInvariant() == trait.ToLowerInvariant()
        )
    select r).ToList();

这可能会导致LINQ在发送查询之前计算IsNullOrEmpty。如果不是,您可以手动预先计算它们,并将两个布尔变量放在它们的位置。

code4life 14 年前

你有没有试过通过调用 Repository.Query().ToList() 在where条款之前?我注意到NHibernate好像在试图改变 string.IsNullOrEmpty() 调用SQL语法(失败)。

return (from r in Repository.Query<Measurement>().ToList()
        where
            r.Postal.ToLowerInvariant() ==
                (string.IsNullOrEmpty(postalCode)
                    ? r.Postal : postalCode).ToLowerInvariant()
            &&
            r.Trait.ToLowerInvariant() ==
                (string.IsNullOrEmpty(trait)
                    ? r.Trait : trait).ToLowerInvariant()
        select r).ToList();