Question

我有这个问题：

Dim test = result.GroupBy(Function(row) groupedindexes.Select(
                                      Function(grpindex) row(grpindex)).ToArray, comp)

我正在构建表达式树。我已经在GroupBy函数中构建了部件，现在我想调用ToArray方法。这是代码：

    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

我在第expr = ...行收到错误消息：静态方法需要空实例，非静态方法需要非空实例。

我已经知道此错误消息，但我认为，表达式调用是正确的：我在ToArray上调用了outerfieldselector方法。

你可以帮帮我吗？您可以复制并粘贴代码并进行测试。如有必要，您可以删除比较类。

感谢。

Answer 1

你有几个错误：

您的错误GroupBy方法。
调用静态方法方法（如GroupBy和ToArray需要一个空实例，就像错误一样。

这是更正后的代码。更改的行是以expr =和outerfieldselector =开头并获取GroupBy方法的行：

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

在LINQ选择查询上调用ToArray

1 个答案: