Variables are user defined identifiers to which you can assign a variant value. The variables which the NFormulaEngine recognizes are stored in an instance of the NVariableCollection class a reference to which can be obtained from the Variables property.
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NFormulaEngine engine = new NFormulaEngine();
NVariableCollection variables = engine.Variables;
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Dim engine As New NFormulaEngine Dim variables As NVariableCollection = engine.Variables |
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In essence the variables collection is a hashtable, which assigns a name to a user provided variant value. Then the variant value can be used in the formula expression with its name. For example:
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// create an expression string expression = "a + b"; // define variables variables.Add("a", new NVariant(10)); variables.Add("b", new NVariant(20)); // evaluate expression - result is 30 NVariant res = engine.ParseAndEvaluate(expression); // change a variable value variables["a"] = new NVariant(20); // evaluate expression - result is 40 res = engine.ParseAndEvaluate(expression); |
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' create an expression Dim expression As String = "a + b" ' define variables variables.Add("a", New NVariant(10)) variables.Add("b", New NVariant(20)) ' evaluate expression - result is 30 Dim res As NVariant = engine.ParseAndEvaluate(expression) ' change a variable value variables.Item("a") = New NVariant(20) ' evaluate expression - result is 40 res = engine.ParseAndEvaluate(expression) |
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