Parameters

###Static parameters

Static parameters are values which can be defined before the evaluation of an expression. These parameters can be accesed using the Parameters dictionary of the Expression instance.

  Expression e = new Expression("2 * [x] ^ 2 + 5 * [y]");
  e.Parameters["x"] = 5;
  e.Parameters["y"] = 1;

  Console.WriteLine(e.Evaluate());

Parameters can be useful when a value is unknown at compile time, or when performance is important and the parsing can be saved for further calculations.

Expression parameters

Expressions can be splitted into several ones by defining expression parameters. Those parameters are not simple values but Expression instances themselves.

  Expression volume = new Expression("[surface] * h");
  Expression surface = new Expression("[l] * [L]");
  volume.Parameters["surface"] = surface;
  surface.Parameters["l"] = 1;
  surface.Parameters["L"] = 2;

Dynamic parameters

Sometimes parameters can be even more complex to evaluate and need a dedicated method to be evaluated. This can be done by intercepting there evaluation using the EvaluateParameter event published on Expression instances. Thus, each time a parameter is not defined in the dictionary, this event is called to try to resolve the value.

  Expression e = new Expression("Round(Pow([Pi], 2) + Pow([Pi], 2) + [X], 2)");

  e.Parameters["Pi2"] = new Expression("Pi * [Pi]");
  e.Parameters["X"] = 10;

  e.EvaluateParameter += delegate(string name, ParameterArgs args)
    {
      if (name == "Pi")
        args.Result = 3.14;
    };

Square brackets parameters

Parameters in between square brackets can contain special characters like spaces, dots, and also start with digits.

  Expression e = new Expression("[My First Parameter] + [My Second Parameter]");

Multi-valued parameters

When parameters are IEnumerable and the EvaluationOptions.IterateParameters is used, the result is an IList made of the evaluation of each value in the parameter.

 Expression e = new Expression("(a * b) ^ c", EvaluateOption.IterateParameters);
 e.Parameters["a"] = new int[] { 1, 2, 3, 4, 5 };
 e.Parameters["b"] = new int[] { 6, 7, 8, 9, 0 };
 e.Parameters["c"] = 3;

 foreach (var result in (IList)e.Evaluate())
 {
     Console.WriteLine(result);
 }

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