Visual Basic ScriptControl won't write to variable?

Question

I'm trying to make a (very simple) graphing calculator in Visual Basic, and I used ScriptControl so that the user can input their own equation and it'll use eval() to get the result.

Public Class Form1

    Dim sc As MSScriptControl.ScriptControl = New MSScriptControl.ScriptControl

    Private Sub Form1_KeyPressed(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Me.KeyPress, Me.Load

        sc.Language = "VBScript"
        sc.AddObject("x", 355, True)
        sc.AddCode("Sub setx(xvalo)" + vbCrLf + "x = xvalo" + vbCrLf + "End Sub")

        Me.CreateGraphics.DrawLine(Pens.Black, New Point(0, 200), New Point(5000, 200))
        Dim eq As String = InputBox("Please enter equation:")
        Dim y As Double = 0
        For i As Double = 0 To 50 Step 0.01
            sc.Run("setx", i)
            y = sc.Eval(eq)
            Me.CreateGraphics.DrawLine(Pens.Blue, New Point(i * 10, y * 10 + 200), New Point(i * 10 + 0.1, y * 10 + 0.1 + 200))
        Next

    End Sub
End Class

When I run this and type in x + 2 or even just x in the prompt box, I get an exception: NotSupportedException: Object doesn't support this property or method: 'x'

But I added x earlier... does anyone know how to fix this?

And another weird thing: It seems like whenever I try to edit the sc.AddCode() arguments, it says COMException was unhandled.

A quick google search led me here: http://en.kioskea.net/faq/18985-vb-evaluate-a-mathematical-expression-of-a-string, it's a VB.NET implementation of what you need. It probably does not support variables, but considering how it's implemented, should not be a big deal to add it there. There may be other (better) expression evaluation classes/libraries out there - just keep googling. — Victor Zakharov, Jan 06 '14 at 14:57
If you want to write one yourself, here are some more links: http://www.codeproject.com/Articles/9519/An-expression-evaluator-written-in-VB-NET and http://www.vb-helper.com/howto_net_evaluate_expressions.html — Victor Zakharov, Jan 06 '14 at 17:00

tinstaafl · Answer 1 · 2014-01-06T16:59:43.810

0

ScriptControl so that the user can input their own equation

Instead of an old legacy control not really designed for .net, you could try using the Compute method in DataTable:

            Dim dt As New DataTable()
            Try
               Dim result2 As Double = CDbl(dt.Compute(ExpressionString, ""))
            Catch ex As Exception
                MessageBox.Show(ex.Message)
            End Try

With the Try block invalid formulas won't stop execution

It will evaluate nested parentheses as well '(((3*(12.223+ 15)) - 7)*21)/7'.

edited Jan 06 '14 at 16:59

answered Jan 06 '14 at 16:43

tinstaafl

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Does [it](http://msdn.microsoft.com/en-us/library/system.data.datatable.compute(v=vs.110).aspx) support arbitrary variables? – Victor Zakharov Jan 06 '14 at 16:53
Build the string from the variables using the ToString method. But the OP was asking about inputting whole formulas not pieces. – tinstaafl Jan 06 '14 at 16:57

Visual Basic ScriptControl won't write to variable?

1 Answers1