Need a method to Validate Mathematical expression which contains unknown variables

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C#

I am writing a tool which contains a textbox and Save button.
In the textbox user will enter mathematical expression like "2*(UpperBound - LowerBound)" and then clicks the save button to save the expression in xml.
But before saving the expression into xml want to validate it.
For this i tried to create a method like ValidateExpression(string expresion,List<string> unknownvariables) here the list contains the strings "UpperBound ","LowerBound" ant etc.

Is there any method like this to validate the expression containing unknown variables.