call ToArray on LINQ select query

问题

I have this query:

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

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

I get an error message at the line expr = ...: Static method requires null instance, non-static method requires non-null instance.

I already know this error message, but I think, the expression call is correct: I call the ToArray method on outerfieldselector.

Could you help me out? You can copy&paste the code and test it. You can remove the compare class, if necessary.

Thanks.

回答1:

You have a couple of mistakes:

1. You're getting the wrong GroupBy method.
2. Calls to static methods methods like GroupBy and ToArray need a null instance, just like the error says.

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

来源：https://stackoverflow.com/questions/28191256/call-toarray-on-linq-select-query