FunctionCallNode.cs

namespace ExpressionParser
{
   // A function call node. Supports IF(), CONCAT(), and NOW().
   public class FunctionCallNode : ExpressionNode
   {
      private string _functionName;
      private List<ExpressionNode> _arguments;
      public FunctionCallNode(string functionName, List<ExpressionNode> arguments)
      {
         _functionName = functionName.ToUpperInvariant();
         _arguments = arguments;
      }
      public override object Evaluate(Dictionary<string, object> variables)
      {
         switch (_functionName)
         {
            case "IF":
               if (_arguments.Count != 3)
                  throw new Exception("IF function requires exactly three arguments.");
               object conditionObj = _arguments[0].Evaluate(variables);
               if (!(conditionObj is bool))
                  throw new Exception("The first argument to IF must be a boolean.");
               bool condition = (bool)conditionObj;
               return condition ? _arguments[1].Evaluate(variables) : _arguments[2].Evaluate(variables);

            case "CONCAT":
               {
                  string result = "";
                  foreach (var arg in _arguments)
                  {
                     object value = arg.Evaluate(variables);
                     if (value == null)
                        value = "";
                     else if (!(value is string))
                        value = value.ToString()!;
                     result += (string)value;
                  }
                  return result;
               }

            case "NOW":
               {
                  if (_arguments.Count != 0)
                     throw new Exception("NOW function does not accept any arguments.");
                  return DateTime.Now;
               }

            default:
               throw new Exception($"Unsupported function: {_functionName}");
         }
      }
   }
}