call ToArray on LINQ select query - vb.net

Question

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.

Answer 1

You have a couple of mistakes:
You're getting the wrong GroupBy method.
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