VB.NET Expression Evaluator using CodeDom.

[conipto]

The below code is a port of this C# Codebank example to VB.NET, modified to compile VB.NET code instead of C#. This function returns a Double value after evaluating a valid VB Expression which would evaluate to a double. Possible applications include calculators, or anything which would need to perform complex math operations on the fly.

As with the C# Example, you will need to import a few namespaces or qualify some of the variable names in the example for this to work correctly.

Imports System.CodeDom.Compiler
Imports Microsoft.VisualBasic 'More than just VB6 Runtime functions in here!
Imports System.Reflection

And here is the function, converted to VB.NET

''' <summary>
    ''' A simple function using CodeDom to process an expression.
    ''' </summary>
    ''' <param name="command">A string expression to evaluate</param>
    ''' <returns>A double with the result of the evaluated command parameter</returns>
    ''' <remarks>Vulnerable to injection attacks.</remarks>
    Private Function ProcessCommand(ByVal command As String) As Double
        Dim MyProvider As New VBCodeProvider 'Create a new VB Code Compiler
        Dim cp As New CompilerParameters     'Create a new Compiler parameter object.
        cp.GenerateExecutable = False        'Don't create an object on disk
        cp.GenerateInMemory = True           'But do create one in memory.
        'If cp.OutputAssembly is used with a VBCodeProvider, it seems to want to read before it is executed.  
        'See C# CodeBank example for explanation of why it was used.
 
        'the below is an empty VB.NET Project with a function that simply returns the value of our command parameter.
        Dim TempModuleSource As String = "Imports System" & Environment.NewLine & _
                                         "Namespace ns " & Environment.NewLine & _
                                         "Public Class class1" & Environment.NewLine & _
                                         "Public Shared Function Evaluate()" & Environment.NewLine & _
                                         "Return " & command & Environment.NewLine & _
                                         "End Function" & Environment.NewLine & _
                                         "End Class" & Environment.NewLine & _
                                         "End Namespace"
        'Create a compiler output results object and compile the source code.
        Dim cr As CompilerResults = MyProvider.CompileAssemblyFromSource(cp, TempModuleSource)
        If cr.Errors.Count > 0 Then
            'If the expression passed is invalid or "", the compiler will generate errors.
            Throw New ArgumentOutOfRangeException("Invalid Expression - please use something VB could evaluate")
        Else
            'Find our Evaluate method.
            Dim methInfo As MethodInfo = cr.CompiledAssembly.GetType("ns.class1").GetMethod("Evaluate")
            'Invoke it on nothing, so that we can get the return value
            Return Convert.ToDouble(methInfo.Invoke(Nothing, Nothing))
        End If
    End Function

Sample usage and output:

MsgBox(ProcessCommand(TextBox1.Text)) 'SHows a message box with the result of an expression in TextBox1
        Console.WriteLine(ProcessCommand("1+1").ToString()) 'Displays 2
        Console.WriteLine(ProcessCommand("Math.PI").ToString()) 'Displays 3.14159265358979
        Console.WriteLine(ProcessCommand("Math.Abs(-22)").ToString()) 'Displays 22
        Console.WriteLine(ProcessCommand("3-4+6+7+22/3+66*(55)").ToString()) 'Displays 3649.333333333333333333

Bill