Dynamic formula calculator in VB.NET

I was searching to find out the way to calculate a formula based value which the parameters come from a data storage.

For example, my customer wants to write/change his formula and calculate the value based on it: Value = Round((P1+P3)/P2)

I completes the job using MSScriptControl.ScriptControlClass, it helps me to evaluate the String using VB.
Here is the complete code:

Public Class Evaluator
Dim scriptControl As New MSScriptControl.ScriptControlClass
Private _Formula As String
Private _VariablesPreffix As String
Private _Operators As ArrayList
Private _CorrectedFormula As String
Private _FormulaVariables As ArrayList
Private _ParseCondition As ParseCondition = ParseCondition.RaiseErroForUndefinedVariable

Public ReadOnly Property Formula() As String
Get
Return Me._Formula
End Get
End Property

Public ReadOnly Property VariablesPreffix() As String
Get
Return Me._VariablesPreffix
End Get

End Property
Private Sub PreEvaluate(ByVal keyCode As String, ByVal keyValue As String)
Me._Formula = Me._Formula.Replace(keyCode.ToUpper, keyValue)
End Sub

Private Sub PreEvaluate(ByVal keyValuesDictionary As Hashtable)
For Each keyCode As String In keyValuesDictionary.Keys
Me.PreEvaluate(keyCode, keyValuesDictionary(keyCode))
Next
End Sub

Public Function Evaluate(ByVal values As Hashtable) As Double
Dim tmpFormula As String = Me._Formula
For Each var As String In Me._FormulaVariables
Select Case var.ToLower
Case "abs", "round", "fix", "sin", "exp", "cos", "int", "atn", "tan", "sqr", "sgn", "log"
Case Else
Dim index As Integer = CInt(var.Replace(Me._VariablesPreffix, ""))
If Not values(index) Is Nothing Then
tmpFormula = tmpFormula.Replace(var, values(index))
Else
If Me._ParseCondition = ParseCondition.RaiseErroForUndefinedVariable Then
Throw New Exception("Undefined variable!" & vbCrLf & "Base formula: " & Me._Formula & vbCrLf & "Corrected formula: " & Me._CorrectedFormula & vbCrLf & "On variable: " & var)
Else
tmpFormula = tmpFormula.Replace(var, 0)
End If
End If
End Select
Next
Try
'tmpFormula = tmpFormula.Replace("+0", "").Replace("-0", "")
Return CDbl(scriptControl.Eval(tmpFormula))
Catch ex As Exception
scriptControl.Error.Clear()
Return -1
'Throw New Exception(tmpFormula, ex)
End Try
End Function

Public Sub New(ByVal formula As String, ByVal variablePreffix As String, ByVal parseCondition As ParseCondition, Optional ByVal preEvaluateKeyValuesDictionary As Hashtable = Nothing)
scriptControl.Language = "vbscript"
Me._Formula = formula
Me._VariablesPreffix = variablePreffix
Me._ParseCondition = parseCondition
InitializeOperators()
If Not preEvaluateKeyValuesDictionary Is Nothing Then
Me.PreEvaluate(preEvaluateKeyValuesDictionary)
End If
InitializeFormula()
End Sub

Private Sub InitializeOperators()
Me._Operators = New ArrayList
Me._Operators.Add(CChar("+"))
Me._Operators.Add(CChar("-"))
Me._Operators.Add(CChar("/"))
Me._Operators.Add(CChar("*"))
Me._Operators.Add(CChar("^"))
Me._Operators.Add(CChar("("))
Me._Operators.Add(CChar(")"))
'Me._Operators.Add(CChar(","))
End Sub

Private Sub InitializeFormula()
Me._CorrectedFormula = Me._Formula.Replace(" ", "")
Me._FormulaVariables = New ArrayList
Dim arrOperators() As Char
Dim variables() As String = Me._CorrectedFormula.Split(CType(Me._Operators.ToArray(GetType(Char)), Char()))
For Each var As String In variables
var = var.Trim
If var.Length > 0 AndAlso Not IsNumeric(var) Then
Me._FormulaVariables.Add(var)
If Not IsNumeric(var.Replace(Me._VariablesPreffix, "")) Then
Select Case var.ToLower
Case "abs", "round", "fix", "sin", "exp", "cos", "int", "atn", "tan", "sqr", "sgn", "log"
Case Else
Throw New Exception("Wrong formula!" & vbCrLf & "Base formula: " & Me._Formula & vbCrLf & "Corrected formula: " & Me._CorrectedFormula & vbCrLf & "On variable: " & var)
End Select
End If
End If
Next
End Sub

Public ReadOnly Property Operators() As ArrayList
Get
Return Me._Operators
End Get
End Property

Public ReadOnly Property ParseCondition() As ParseCondition
Get
Return Me._ParseCondition
End Get
End Property

End Class

Public Enum ParseCondition
RaiseErroForUndefinedVariable
DoNotRaiseErroForUndefinedVariable
End Enum


Now, you can use it simply:

Dim calculator As New Evaluator("Round((P1+P3)/P2)", "P", ParseCondition.DoNotRaiseErroForUndefinedVariable)
Dim parameters As New Hashtable
parameters.Add(1, 12)
parameters.Add(2, 24)
parameters.Add(3, 6)
Dim value As Double = calculator.Evaluate(parameters)


Considerations:

1. You should define the Parameters Prefix to be recognized (like "P").
2. DO NOT use the predefined functions as Parameters Prefix.
3. If you do not assign a named parameter, it will place Zero to the parameter's value.
4. If an error occures, it will return -1.
5. If you want to add more function, you should add in Red lines of code!
6. You can add more additional parameters before evaluation.

I'm searching for another way to implement it in better performance...