SumByCategory.cs

class Program
{
  static void Main()
  {
    var source = new [] { 
      new { Category = "A", Amount = 1000m },
      new { Category = "B", Amount = 1000m },
      new { Category = "B", Amount = 1000m },
      new { Category = "C", Amount = 1000m },
      new { Category = "C", Amount = 1000m },
      new { Category = "C", Amount = 1000m },
    };

    var results = source
      .GroupBy(x => x.Category)
      .Select(g => new { Category = g.Key, Amount = g.Sum(x => x.Amount)});

    foreach (var x in results)
    {
      Console.WriteLine("Category:{0}, Amount:{1}", x.Category, x.Amount);
    }
  }
}