Binaryworld - Evaluate an expression at runtime ... [ VB -> Language ]

The .NET framework doesn't offer any direct way to evaluate an expression that has been entered by the end user when the application is running. However, it is quite simple to create a simple expression evaluator based on calculated columns in DataTable. The following routine does the trick:

Click here to copy the following block

Function EvalExpression(ByVal expr As String) As Double
  ' create a new DataTable containing a calculated column
  Dim dt As New DataTable()
  dt.Columns.Add("Expr", GetType(Double), expr)
  ' add a dummy row
  dt.Rows.Add(dt.NewRow)
  ' return the value of the calculated column
  Return CDbl(dt.Rows(0).Item("Expr"))
End Function

Here's an example that uses the above function:

Click here to copy the following block

Dim expr As String = "100 * (2 + 3)"
Console.WriteLine(EvalExpression(expr)) ' displays 500

You can also support variables by creating one or more column named after the variables, as in this function:

Click here to copy the following block

Function EvalExpression(ByVal expr As String, ByVal x As Double) As Double
  ' create a new DataTable
  Dim dt As New DataTable()
  ' first, add a column named after the variable
  dt.Columns.Add("x", GetType(Double))
  ' then add the calculated column
  dt.Columns.Add("Expr", GetType(Double), expr)

  ' add a dummy row
  dt.Rows.Add(dt.NewRow)
  ' set the value of the variable
  dt.Rows(0).Item("x") = x
  ' return the value of the calculated column
  Return CDbl(dt.Rows(0).Item("Expr"))
End Function