在LINQ上调用ToArray选择查询 [英] call ToArray on LINQ select query
问题描述
我有这个查询:
Dim test = result.GroupBy(Function(row) groupedindexes.Select(
Function(grpindex) row(grpindex)).ToArray, comp)
我正在构建一个表达式树.我已经在GroupBy
函数内部构建了零件,现在我想调用ToArray
方法.这是代码:
I'm building an expression tree. I have already build the part inside the GroupBy
function and now I would like to call the ToArray
method. Here is the code:
Public Function Grouping(ByVal result As IEnumerable(Of Object()), ByVal groupedindexes As List(Of Integer), ByVal comparer As compare) As Expression
Dim groupbyMethod = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).First(Function(m) m.Name = "GroupBy").MakeGenericMethod(GetType(Object()), GetType(System.Collections.Generic.IEqualityComparer(Of Object())))
Dim convertMethod As MethodInfo = Nothing
Dim rowParameter = Expression.Parameter(GetType(Object()), "Row")
Dim indexParameter = Expression.Parameter(GetType(Integer), "grpindex")
Dim methodstring As String
Dim expr As Expression = Nothing
Dim index As Integer
Dim grpindexes As Expression = Expression.Constant(groupedindexes, GetType(System.Collections.Generic.List(Of Integer)))
Dim selectMethod = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).First(Function(m) m.Name = "Select").MakeGenericMethod(GetType(Integer), GetType(Object))
Dim toarrayMethod2 = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).First(Function(m) m.Name = "ToArray").MakeGenericMethod(GetType(Object))
Dim cmp As Expression = Expression.Constant(comparer, GetType(compare))
Dim fieldselector As Expressions.LambdaExpression
fieldselector = Expression.Lambda(Expression.ArrayAccess(rowParameter, indexParameter), indexParameter)
Dim outerfieldselector As Expressions.LambdaExpression
outerfieldselector = Expression.Lambda(Expression.Call(selectMethod, grpindexes, fieldselector), rowParameter)
expr = Expression.Call(groupbyMethod, Expression.Call(outerfieldselector, toarrayMethod2), cmp)
Return expr
End Function
我在expr = ...
行收到一条错误消息:静态方法需要空实例,非静态方法需要非空实例.
I get an error message at the line expr = ...
: Static method requires null instance, non-static method requires non-null instance.
我已经知道此错误消息,但是我认为表达式调用是正确的:我在outerfieldselector
上调用了ToArray
方法.
I already know this error message, but I think, the expression call is correct: I call the ToArray
method on outerfieldselector
.
你能帮帮我吗?您可以复制并粘贴代码并对其进行测试.您可以根据需要删除比较类.
Could you help me out? You can copy&paste the code and test it. You can remove the compare class, if necessary.
谢谢.
推荐答案
您有几个错误:
- 您得到了错误的
GroupBy
方法. - 对静态方法方法(如
GroupBy
和ToArray
)的调用需要一个空实例,就像错误所说一样.
- You're getting the wrong
GroupBy
method. - Calls to static methods methods like
GroupBy
andToArray
need a null instance, just like the error says.
这是更正的代码.更改的行是以expr =
和outerfieldselector =
开头并获得GroupBy
方法的行:
Here's the corrected code. Lines changed are the lines that begin with expr =
and outerfieldselector =
and getting the GroupBy
method:
Public Function Grouping(ByVal result As IEnumerable(Of Object()), ByVal groupedindexes As List(Of Integer), ByVal comparer As compare) As Expression
Dim groupbyMethod = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).
Where(Function(m) m.Name = "GroupBy").
First(Function(m) m.GetParameters().Count() = 3).
MakeGenericMethod(GetType(Object()), GetType(Object()))
Dim convertMethod As MethodInfo = Nothing
Dim rowParameter = Expression.Parameter(GetType(Object()), "Row")
Dim indexParameter = Expression.Parameter(GetType(Integer), "grpindex")
Dim methodstring As String
Dim expr As Expression = Nothing
Dim index As Integer
Dim grpindexes As Expression = Expression.Constant(groupedindexes, GetType(System.Collections.Generic.List(Of Integer)))
Dim selectMethod = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).First(Function(m) m.Name = "Select").MakeGenericMethod(GetType(Integer), GetType(Object))
Dim toarrayMethod2 = GetType(Enumerable).GetMethods(BindingFlags.Public Or BindingFlags.Static).First(Function(m) m.Name = "ToArray").MakeGenericMethod(GetType(Object))
Dim cmp As Expression = Expression.Constant(comparer, GetType(compare))
Dim fieldselector As Expressions.LambdaExpression
fieldselector = Expression.Lambda(Expression.ArrayAccess(rowParameter, indexParameter), indexParameter)
Dim outerfieldselector As Expressions.LambdaExpression
outerfieldselector = Expression.Lambda(Expression.Call(Nothing, toarrayMethod2, Expression.Call(selectMethod, grpindexes, fieldselector)), rowParameter)
Dim test = Enumerable.GroupBy(result, Function(row) groupedindexes.Select(Function(grpindex) row(grpindex)).ToArray(), comparer)
expr = Expression.Call(Nothing, groupbyMethod, Expression.Constant(result), outerfieldselector, Expression.Constant(comparer))
Return expr
End Function
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