Conversion From String to Boolean Vb.net

Question

I want to evaluate a String with Boolean Values and Binary Logical Operators (And/Or). When i write Dim Expression = ((True And False) Or True) then the value of "Expression" Will be true. However, when i write Dim Expression = "((True And False) Or True)" and try to convert the string to Boolean using Convert.ToBoolean() like MsgBox(Convert.ToBoolean(Expression)) then i got an error stating: String was not recognized as a valid Boolean. as in the picture. All previous posts deals with strings containing only "True" or "False". but rarely i found a post dealing with an Expression containing brackets and more than one operator as "((True Or False) And True)" or much more complicated expressions. is this supported in the convert.ToBoolean function and if yes why i am getting this error?

You're asking it to convert the _string_ `"((True And False) Or True)"` directly into a boolean. Booleans can only be `True` or `False` and the only strings which match to those are `"True"` and `"False"`. It won't evaluate your string as if it was an actual piece of code and compute the result, it just treats it literally like a string, because that's what it is. And since it doesn't directly match a string version of a boolean, you get the error. — ADyson, Feb 27 '18 at 10:53
It's clearly documented here: https://msdn.microsoft.com/en-us/library/86hw82a3(v=vs.110).aspx - "For a successful conversion to occur, the value parameter must equal either Boolean.TrueString, a constant whose value is True, Boolean.FalseString, a constant whose value is False, or it must be null". If you read the documentation it answers your question for you already. — ADyson, Feb 27 '18 at 10:54
so isn't there a way to convert such a string into a Boolean Value? — Mohammad M Moshawrab, Feb 27 '18 at 12:18
No because it's not a boolean, it's effectively a random string. Just because it looks like code which might evaluate to a boolean if it was code is not really relevant (because it's not code, it's a string). — ADyson, Feb 27 '18 at 12:19
However, if you're looking for how to interpret an arbitrary string as code and execute it, this is a wider topic. The following may turn out to be of interest to you (just from a quick google search, you can search for more): http://www.nullskull.com/articles/20030908.asp, https://stackoverflow.com/questions/1511376/c-sharp-execute-a-string-as-code, https://stackoverflow.com/questions/4629/how-can-i-evaluate-c-sharp-code-dynamically — ADyson, Feb 27 '18 at 12:22

score 1 · Answer 1 · answered Feb 27 '18 at 19:07

There is a round-about way of evaluating an expression using Microsoft script control ..

Right click on your project and choose Add

In the sub menu, choose Reference

In the window that appears, on the left click on COM

Scroll down until you see MicrosoftScriptControl 1.0 and double click it.

Then click Ok on the lower right of the window

This function takes a string and using VBScript, evaluates the string expression. You can of course tweak it to your needs.

Private Function EvaluateBoolean(formula As String) As Boolean
    Dim sc As New MSScriptControl.ScriptControl
    'SET LANGUAGE TO VBSCRIPT
    sc.Language = "VBSCRIPT"
    'ATTEMPT MATH
    Try
        Return Convert.ToBoolean(sc.Eval(formula))
    Catch ex As Exception
        'SHOW THAT IT WAS INVALID
        MessageBox.Show("Invalid Boolean expression")
    End Try
End Function

Conversion From String to Boolean Vb.net

1 Answers1