VB.NET Evaluate a string with variables

Question

I like to populate my database with different formulas.

When I have a formula in the database like "(2+4)*5" I get the result 40:

update DB.dbo.INFOGROUP SET formula = '(2+4)*5' where INFOGROUP = 'S42'

When I have a formula in the database like "(x+4)*5" I get the result 20. The x value is ignored even when I set the value of x to 10:

update DB.dbo.INFOGROUP SET formula = '(x+4)*5' where INFOGROUP = 'S42'

Private Sub BtnCalculate_Click(sender As Object, e As EventArgs) Handles BtnCalculate.Click
    Dim SC As New MSScriptControl.ScriptControl
    Dim x As Double = 10
    SQL.AddParam("@infogroup", "S42")
    SQL.ExecQuery($"SELECT formula FROM DB.dbo.INFOGROEP where infogroep=@infogroep")
    Dim Formula As String = <string_from_db>
    'SET LANGUAGE TO VBSCRIPT
    SC.Language = "VBSCRIPT"
    'ATTEMPT MATH
    Try
        Dim Result As Double = Convert.ToDouble(SC.Eval(Formula))
        'SHOW THAT IT WAS VALID
        MessageBox.Show("Math success, " & Formula & " equals " & Result.ToString)
    Catch ex As Exception
        'SHOW THAT IT WAS INVALID
        MessageBox.Show("Not a valid math formula for a double.")
    End Try
End Sub

Is this possible at all or do I need a third party script for that?

BTW: in Visual Foxpro there was a function like EVAL(db_string).

The variable `x` in your code is in a different environment to the one in `SC`. Have you tried replacing the string "x" in the string_from_db with the string "10" and then evaluating it? — Andrew Morton, Oct 07 '18 at 17:16

score 2 · Accepted Answer · answered Oct 07 '18 at 17:31

The script engine has no knowledge about variables in your methods. If you want to evaluate a formula that contains a variable, you need to tell it what value that variable has:

SC.Language = "VBSCRIPT"
sc.AddCode "Private Const x = 10"

Dim Result As Double = Convert.ToDouble(SC.Eval(Formula))

VB.NET Evaluate a string with variables

1 Answers1