Quite often I have wanted to adjust a bound value in my XAML by a mathematical formula.

For example, I might want to size a control to 50% of it’s parent’s size. Or I’ll want to position it 30% down and 20% left of a parent panel. Or I might even want to make a control’s width something like 50% of (WindowHeight – 200). Of course, I could always write converters for this, but I got tired of doing that repeatedly so sat down one day to write one big MathConverter.

### Using the Math Converter

Using the MathConverter is simple. All you have to do is pass the Converter a math equation as the ConverterParameter, substituting @VALUE for your bound value. The equation can consist of numbers, basic operators (+, -, *, /, %), parentheses, and @VALUE.

For example, if you wanted a control whose height was 50% of the current Window Size, you would create a binding that looked like this:

Height="{Binding ElementName=RootWindow, Path=ActualHeight, Converter={StaticResource MathConverter}, ConverterParameter=@VALUE/2}"

Or if you wanted to make the control’s width equal to 30% of (WindowWidth – 200) your binding might look like this:

Width="{Binding ElementName=RootWindow, Path=ActualWidth, Converter={StaticResource MathConverter}, ConverterParameter=((@VALUE-200)*.3)}"

### The Math Converter

Here’s the actual converter code.

It takes the ConverterParameter, substitutes the bound value for @VALUE, then starts reading the equation from left-to-right. If it encounters an operator, it performs the specified operation on the previous number and the next number in the list. If it encounters a parentheses, it handles the equation inside the group first, then replaces the grouped equation in the string with the result of the grouped equation. Any character it finds that is not a number, operator, or parentheses throws an exception.

// Does a math equation on the bound value. // Use @VALUE in your mathEquation as a substitute for bound value // Operator order is parenthesis first, then Left-To-Right (no operator precedence) public class MathConverter : IValueConverter { private static readonly char[] _allOperators = new[] { '+', '-', '*', '/', '%', '(', ')' }; private static readonly List<string> _grouping = new List<string> { "(", ")" }; private static readonly List<string> _operators = new List<string> { "+", "-", "*", "/", "%" }; #region IValueConverter Members public object Convert(object value, Type targetType, object parameter, CultureInfo culture) { // Parse value into equation and remove spaces var mathEquation = parameter as string; mathEquation = mathEquation.Replace(" ", ""); mathEquation = mathEquation.Replace("@VALUE", value.ToString()); // Validate values and get list of numbers in equation var numbers = new List<double>(); double tmp; foreach (string s in mathEquation.Split(_allOperators)) { if (s != string.Empty) { if (double.TryParse(s, out tmp)) { numbers.Add(tmp); } else { // Handle Error - Some non-numeric, operator, or grouping character found in string throw new InvalidCastException(); } } } // Begin parsing method EvaluateMathString(ref mathEquation, ref numbers, 0); // After parsing the numbers list should only have one value - the total return numbers[0]; } public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) { throw new NotImplementedException(); } #endregion // Evaluates a mathematical string and keeps track of the results in a List<double> of numbers private void EvaluateMathString(ref string mathEquation, ref List<double> numbers, int index) { // Loop through each mathemtaical token in the equation string token = GetNextToken(mathEquation); while (token != string.Empty) { // Remove token from mathEquation mathEquation = mathEquation.Remove(0, token.Length); // If token is a grouping character, it affects program flow if (_grouping.Contains(token)) { switch (token) { case "(": EvaluateMathString(ref mathEquation, ref numbers, index); break; case ")": return; } } // If token is an operator, do requested operation if (_operators.Contains(token)) { // If next token after operator is a parenthesis, call method recursively string nextToken = GetNextToken(mathEquation); if (nextToken == "(") { EvaluateMathString(ref mathEquation, ref numbers, index + 1); } // Verify that enough numbers exist in the List<double> to complete the operation // and that the next token is either the number expected, or it was a ( meaning // that this was called recursively and that the number changed if (numbers.Count > (index + 1) && (double.Parse(nextToken) == numbers[index + 1] || nextToken == "(")) { switch (token) { case "+": numbers[index] = numbers[index] + numbers[index + 1]; break; case "-": numbers[index] = numbers[index] - numbers[index + 1]; break; case "*": numbers[index] = numbers[index] * numbers[index + 1]; break; case "/": numbers[index] = numbers[index] / numbers[index + 1]; break; case "%": numbers[index] = numbers[index] % numbers[index + 1]; break; } numbers.RemoveAt(index + 1); } else { // Handle Error - Next token is not the expected number throw new FormatException("Next token is not the expected number"); } } token = GetNextToken(mathEquation); } } // Gets the next mathematical token in the equation private string GetNextToken(string mathEquation) { // If we're at the end of the equation, return string.empty if (mathEquation == string.Empty) { return string.Empty; } // Get next operator or numeric value in equation and return it string tmp = ""; foreach (char c in mathEquation) { if (_allOperators.Contains(c)) { return (tmp == "" ? c.ToString() : tmp); } else { tmp += c; } } return tmp; } }

One day I plan on expanding this further to use an IMultiValueConverter that accepts multiple bindings, but I haven’t had a need for that yet so it hasn’t gotten done.