c# convert string expression to a boolean expression

Have a look at Flee (Fast Lightweight Expression Evaluator) on CodePlex.

I would use Irony, the .NET language kit. You could construct a simple grammar with Irony and then parse the string into executable command. There's a decent example of an arthmetic grammar in this tutorial and in the Expression Grammar Sample, its a pretty common request ;)

I definitely suggest using a proper compiler as opposed to Regex or a roll your own approach - it will be much more extensible if you ever want to add more rules.

If it follows all C# expression rules then compile it as dynamic code as per http://www.west-wind.com/presentations/dynamiccode/dynamiccode.htm