在 LINQ 表达式异常中检测到循环

Question

我得到了错误：

在 LINQ 表达式中检测到循环。

在ToList() 中尝试执行以下操作：

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds)
{
    entityIds =
        MyObjectContext.CreateObjectSet<TEntity>()
            .Where(x => x.ClientId == _clientId)
            .Where(x => entityIds.Contains(x.Id))
            .Select(x => x.Id);

    return entityIds.ToList();
}

然而，这不会引发任何异常并且工作正常：

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds)
{
    entityIds =
        MyObjectContext.CreateObjectSet<TEntity>()
            .Where(x => x.ClientId == _clientId)
            .Where(x => entityIds.Contains(x.Id))
            .Select(x => x.Id)
            .ToList();

    return entityIds;
}

（当然这是简化版）。

有人知道为什么会发生这种奇怪的行为吗？

编辑：

这是堆栈跟踪：

   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.Funcletize(Expression expression, Func`1& recompileRequired)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.InlineExpression(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.InlineObjectQuery(ObjectQuery inlineQuery, Type expressionType)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.InlineValue(Expression expression, Boolean recompileOnChange)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.EntityExpressionVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitLambda(LambdaExpression lambda)
   at System.Linq.Expressions.EntityExpressionVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitUnary(UnaryExpression u)
   at System.Linq.Expressions.EntityExpressionVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.EntityExpressionVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitExpressionList(ReadOnlyCollection`1 original)
   at System.Linq.Expressions.EntityExpressionVisitor.VisitMethodCall(MethodCallExpression m)
   at System.Linq.Expressions.EntityExpressionVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.FuncletizingVisitor.Visit(Expression exp)
   at System.Data.Objects.ELinq.Funcletizer.Funcletize(Expression expression, Func`1& recompileRequired)
   at System.Data.Objects.ELinq.ExpressionConverter..ctor(Funcletizer funcletizer, Expression expression)
   at System.Data.Objects.ELinq.ELinqQueryState.CreateExpressionConverter()
   at System.Data.Objects.ELinq.ELinqQueryState.GetExecutionPlan(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.GetResults(Nullable`1 forMergeOption)
   at System.Data.Objects.ObjectQuery`1.System.Collections.Generic.IEnumerable<T>.GetEnumerator()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at ...FilterIdsByClient...

编辑2：

应该注意，在这种情况下，IEnumerable<int> entityIds 是来自 ajax 请求的列表，而不是来自某个地方的查询。

Answer 1

这种行为看起来很奇怪，因为您没有正确考虑闭包语义。请参阅下面的 cmets：

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds) 
{ 
    // The variable entityIds points to whatever was passed in: A List, according to the edited question.

    entityIds =                                    //this is an assignment, changing the referent of entityIds
        MyObjectContext.CreateObjectSet<TEntity>() 
            .Where(x => x.ClientId == _clientId) 
            .Where(x => entityIds.Contains(x.Id))  //this lambda closes over the variable entityIds
            .Select(x => x.Id); 

    // The query now has a reference to the *variable* entityIds, not to the object that entityIds pointed to originally.
    // The value of entityIds has been changed; it now points to the query itself!
    // The query is therefore operating on itself; this causes the "cycle detected" message.
    // Because of delayed execution, the query is not executed until the next line of code:

    return entityIds.ToList();
}

Answer 2

你为什么要分配给你的参数？为什么不

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds) 
{ 
    return
        MyObjectContext.CreateObjectSet<TEntity>() 
            .Where(x => x.ClientId == _clientId) 
            .Where(x => entityIds.Contains(x.Id)) 
            .Select(x => x.Id)
            .ToList(); 
} 

Answer 3

答案是不将 LINQ 查询分配给 entityIds。请参阅@Stu 的答案以获取解决方案。

Answer 4

当然有一个循环。您在 Where Linq Extension 方法中使用 entityIds，它是正在构建的查询本身。不修改输入的 IEnumerable，而是返回一个新的查询，如下所示：

private IEnumerable<int> FilterIdsByClient(IEnumerable<int> entityIds)
{
    var query =
        MyObjectContext.CreateObjectSet<TEntity>()
            .Where(x => x.ClientId == _clientId)
            .Where(x => entityIds.Contains(x.Id))
            .Select(x => x.Id);

    return query.ToList();
}