Code

public class CustomFunctions : IExpressionEvaluator {
        int a = 0;

        public object EvaluateIdentifier(IExpressionEvaluatorContext context, string identifier) {
            return null;
        }

public object InvokeFunction(IExpressionEvaluatorContext context, string name, IList parameters) {
            if(name == "FUNC_A") {

                // HERE: get cell row and column where this formula is running

           }

// BLA BLA BLA .... CODE