Parameters act as variables inside the expression. You can add as many parameters as you want. You can also enclose each variable in square brackets ([]). Parameters are useful when a value is unknown at compile-time or when performance is important, and the parsing can be saved for further calculations.
There are various ways to use parameters while evaluating your expressions.
Static parameters are values that can be defined before the evaluation of an expression. You can access the parameters defined in the expression using the Expression.Parameters member of type Dictionary<string, object>.
public static void Example1()
{
string expression = "x + y";
Expression evaluator = new Expression(expression);
evaluator.Parameters["x"] = 3;
evaluator.Parameters["y"] = 4;
Console.WriteLine("{0} = {1}", expression, evaluator.Evaluate());
}
You can also use special characters like spaces, dots, and also start with digits, etc. in between square brackets ([]) when defining parameters.
public static void Example2()
{
string expression = "[my x] + [my y]";
Expression evaluator = new Expression(expression);
evaluator.Parameters["my x"] = 3;
evaluator.Parameters["my y"] = 4;
Console.WriteLine("{0} = {1}", expression, evaluator.Evaluate());
}
You can split an expression into several ones by defining expression parameters. Expression parameters are not simple values but Expression instances themselves.
The following example uses the expression as a parameter.
public static void Example3()
{
Expression surfaceExpression = new Expression("[width] * [length]");
Expression volumeExpression = new Expression("[surface] * height");
surfaceExpression.Parameters["width"] = 1.5;
surfaceExpression.Parameters["length"] = 2.25;
volumeExpression.Parameters["height"] = 2;
volumeExpression.Parameters["surface"] = surfaceExpression;
Console.WriteLine("{0}: {1}", "Volume", volumeExpression.Evaluate());
}
Sometimes, you may need a method to evaluate complex parameters. The Expression class provides an EvaluateParameter event to perform this type of evaluation. Thus, each time a parameter is not defined in the dictionary, this event is called to try to resolve the value.
public static void Example4()
{
Expression expression = new Expression("Round(Pow([Pi], 2) + Pow([Pi], 2) + [X], 2)");
expression.Parameters["Pi2"] = new Expression("Pi * [Pi]");
expression.Parameters["X"] = 10;
expression.EvaluateParameter += delegate (string name, ParameterArgs args)
{
if (name == "Pi")
args.Result = 3.14;
};
Console.WriteLine(expression.Evaluate());
}
When parameters are IEnumerable, and you need the result as a list after evaluating each value in the parameter, use the EvaluationOptions.IterateParameters.
public static void Example5()
{
Expression expression = new Expression("x ^ c + y", EvaluateOptions.IterateParameters);
expression.Parameters["x"] = new int[] { 1, 3, 5, 7, 9 };
expression.Parameters["y"] = new int[] { 0, 2, 4, 6, 8 };
expression.Parameters["c"] = 3;
var results = (IList)expression.Evaluate();
foreach (var result in results)
{
Console.WriteLine(result);
}
}