Parameters

Static parameters

Static parameters are values which can be defined before the evaluation of an expression. These parameters can be accessed using the <xref:NCalc.Expression.Parameters> property of the Expression instance.

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

  Console.WriteLine(expression.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 split 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 using the ExpressionParameter delegate.

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

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

expression.DynamicParameters["Pi"] = _ => {
    Console.WriteLine("I'm evaluating π!");
    return 3.14;
};

Square brackets parameters

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

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

Curly braces parameters

You can also use a curly braces as alternative to square brackets.

  var expression = new Expression("{PageState} ==  'List'");

Multi-valued parameters

When parameters are IEnumerable and the IterateParameters is used, the result is a List<object?> made of the evaluation of each value in the parameter.

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

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

 //  216
 //  2744
 //  13824
 //  46656
 //  0

Using Event Handlers

You can also use event handlers to handle parameters.

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

Compare with null parameters

When parameter is null and AllowNullParameter is used, comparison of values to null is allowed.

var expression = new Expression("'a string' == null", ExpressionOptions.AllowNullParameter);
(bool)expression.Evaluate();

 //  False

Getting all parameters from an expression

	var expression = new Expression ("if(x=0,x,y)"); 
    expression.Parameters["x"] = 1;
    expression.Parameters["y"] = "pan"
    var parameters = expression.GetParametersNames(); 
 //  x
 //  y

Case Sensitivity

See case_sensitivity for more info.