How to do MSAccess EVAL() function in VB6 and VB.net?
To get Eval functionality in .NET, the general approach is to call
JavaScript from your VB.NET project. The following should explain the
process:
Build an Evalute Function
=================
One of the more useful features in prior versions of Visual Basic was the
Eval() function. Eval would evaluate the supplied string and return a
result. This resulted in the ability to evaluate arbitrary statements at
runtime. Unfortunately, neither C# nor Visual Basic .NET have the same
functionality.
Fortunately, the power of .NET, specifically with the ability to call
components authored in other .NET languages makes this fairly simple to
implement. Our first piece of the puzzle is JScript.NET, the .NET version of
javascript. JScript.NET does include an Eval function. The question is how
to get to that function from our C# or Visual Basic .NET projects.
To build your own Eval equivalent, follow these steps.
Step 1: Create a JScript Wrapper DLL
-------------------------------------------
1. Open Notepad and type in the following code:
class JScriptEval
{
function Evaluate(evalString : String)
{
return eval(evalString);
}
Quote:
}
2. Save the file as jScriptEval.js and close NotePad.
3. Compile the file using the .NET JavaScript compiler. The following
command line shows how to do this. Note that your path will need to the
directory where the compiler lives. Typically, this is something like:
C:\WINDOWS\Microsoft.NET\Framework\v1.0.3705
jsc /t:library jScriptEval.js
4. If everything worked according to plan, your directory should now contain
a file called jScriptEval.dll.
Step 2: Create References in your Visual Studio .NET project
-----------------------------------------------------------------
1. Back in your Visual Basic.NET or C# project, create a reference to your
new DLL. To do this, right-click on your project, in the Solution Explorer,
Select the .NET tab, press the Browse button to locate JScriptEval.DLL and
press [OK]
2. You will also need a reference to the Microsoft JScript library. This is
typically located in
c:\windows\microsoft.net\framework\v1.0.3705\Microsoft.JScipt.dll.
Step 3: Test your new eval code
Test your eval library like this:
Dim j As New JScriptEval()
Dim s1 As String = "1 + 2 * 3"
MsgBox(j.Evaluate(s1))
Security Issues
---------------
You should be aware that using Eval statements can lead to potential
security risks. Eval can execute just about any program code. There are two
ways to minimize this. First, you can use a regular expression evaluator to
make sure that the string passed to Eval contains only letters, operators
and numbers, not words. Secondly, consider using .NET security features to
disallow any potentially dangerous operations before code is executed by
Eval.
Hope this helps,
----------------------------------------
Dan Haught, FMS Inc.
- Total .NET SourceBook: The ULTIMATE .NET code library
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Download today at: www.fmsinc.com/dotnet
----------------------------------------
Quote:
> I have one Access applications using EVAL() function and I would like to
> know if there is an EVAL() function available in VB6 or VB.net.
> EVAL() is an evaluate expression function like EVAL('1+2') will get 3.
> Thank you,
> Rakkiat
