expression.call value cannot be null - vb.net

Question

I'm trying to code this LINQ query with expression trees:
Result = Result.Where(Function(Row) Convert.ToInt32(Row(2)) <= 10)
Result is declared as Dim Result As IEnumerable(Of Object()).
I have this code so far:
Dim queryiabledata As IQueryable(Of Object()) = Result.AsQueryable
Dim pe As ParameterExpression = Expression.Parameter(GetType(String), "Row(2)")
Dim left As expression = Expression.Call(pe, GetType(String).GetMethod("Convert.ToInt32", System.Type.EmptyTypes))
Dim right As Expression = Expression.Constant(10)
Dim e1 As Expression = Expression.LessThanOrEqual(left, right)
Dim predicatebody As Expression = e1
Dim wherecallexpression As MethodCallExpression = Expression.Call(
GetType(Queryable), "Where", New Type() {queryiabledata.ElementType}, queryiabledata.Expression,
Expression.Lambda(Of Func(Of Object(), Boolean))(predicatebody, New ParameterExpression() {pe}))
Result = queryiabledata.Provider.CreateQuery(Of Object())(wherecallexpression)
But if I run the query, I get an ArgumentNullException (Value cannot be null. Parameter name: method) at Expression.Call.
I tried to change "Convert.ToInt32" to "Value", but I got the same error.
How can I fix it?
Are the another code lines right to get the desired result?

Answer 1

I'm more used to C#, though I do VB.NET occasionally. Reflection in VB.NET is downright ugly. Getting the Where method is a bit of a hack. Here's the code:
shortForm and longForm should be identical.
Dim result As IEnumerable(Of Object()) = New List(Of Object())()
Dim queryiabledata As IQueryable(Of Object()) = result.AsQueryable()
Dim shortForm As Expression = queryiabledata.Where(Function(Row) Convert.ToInt32(Row(2)) <= 10).Expression
Dim whereMethod = GetType(Queryable).GetMethods(BindingFlags.Public Or BindingFlags.Static).
First(Function(m) m.Name = "Where").
MakeGenericMethod(GetType(Object()))
Dim convertMethod = GetType(System.Convert).GetMethod("ToInt32", New Type() {GetType(Object)})
Dim rowParameter = Expression.Parameter(GetType(Object()), "Row")
Dim longform As Expression =
Expression.Call(
whereMethod,
queryiabledata.Expression,
Expression.Lambda(
Expression.LessThanOrEqual(
Expression.Call(
convertMethod,
Expression.ArrayAccess(
rowParameter,
Expression.Constant(2)
)
),
Expression.Constant(10)
),
rowParameter
)
)
result = queryiabledata.Provider.CreateQuery(longform)

Answer 2

This line seems suspect to me:
GetType(String).GetMethod("Convert.ToInt32", System.Type.EmptyTypes)
GetType(String) returns the runtime type String. You then try to get a method called "Convert.ToInt32" which doesn't exist on the string type. I suspect that is returning null which is the source of your exception.
Perhaps you need to use something like this:
GetType(Convert).GetMethod("ToInt32", new Type() {GetType(Object)})
Since there are multiple overloads of the ToInt32 method of the Convert class, you need to specify which of the overloads you want by providing an array of Type as the second parameter. In other words, you a saying "give me the overload that takes an Object type as it's parameter".