Overriding Evaluation Behavior

NCalc provides a flexible way to evaluate expressions, and in some cases, you may need to customize its evaluation behavior. This can be done by overriding the default evaluation visitor using IEvaluationVisitorFactory.

Custom Evaluation Visitor

By creating a custom evaluation visitor, you can modify how specific expressions are interpreted. The following example overrides the behavior of value expressions, returning a special string when encountering the number 42:

private class CustomVisitor(ExpressionContext context) : EvaluationVisitor(context)
{
    public override object Visit(ValueExpression expression)
    {
        // My custom behavior.
        if(expression.Value is 42)
            return "The answer";

        // Use the normal behavior.
        return base.Visit(expression);
    }
}

Custom Evaluation Visitor Factory

To integrate the custom visitor with NCalc, implement the IEvaluationVisitorFactory interface:

private class CustomEvaluationVisitorFactory : IEvaluationVisitorFactory
{
    public EvaluationVisitor Create(ExpressionContext context)
    {
        return new CustomVisitor(context);
    }
}

Using WithEvaluationVisitorFactory

Use the WithEvaluationVisitorFactory method to specify the custom implementation of IEvaluationVisitorFactory. This ensures that the created evaluation visitor is used when evaluating expressions.

Example:

services.AddNCalc()
        .WithEvaluationVisitorFactory<CustomEvaluationVisitorFactory>();

By overriding the evaluation visitor, you gain complete control over how expressions are interpreted and processed, enabling customization to fit specific application needs.