using System;

using System.Collections.Generic;

using System.Linq;

​

   public class Product

   {

       public Product(string name, double price)

       {

           Name = name;

           Price = price;

       }

       public string Name { get; set; }

       public double Price { get; set; }

   }

​

public static class Utilities {

  public static int IndexOf<T>(IEnumerable<T> source, Predicate<T> predicate)

  {

      if (source is null)

      {

          throw new ArgumentNullException(nameof(source));

      }

​

      if (predicate == null)

      {

          throw new ArgumentNullException(nameof(predicate));

      }

      int index = 0;

      foreach (T item in source) {

          if (predicate(item))

              return index;

​

          index += 1;   

      } 

​

      return -1;

  }

}

public class Program

{

​

    public static void Main()

    {

        var products = new Product[]

        {

            new Product("Product 1", 10.0d),

            new Product("Product 2", 20.0d),

            new Product("Product 3", 30.0d),

        };

        var productToFind = new Product("Product 3", 30.0d);

        int index = Utilities.IndexOf(products, product => product.Name == productToFind.Name &&

                                                           product.Price == productToFind.Price);

        Console.WriteLine(index);

    }

}