Interfaces are passed by reference

Program.cs

    class Program
    {
        static void Main(string[] args)
        {
            //If this is var, the result will be 0
            IHasNumber hasNumber = new StructWithNumber { Number = 0 };
            IncrementNumber(hasNumber);
            Console.WriteLine(hasNumber.Number);
        }

        private static void IncrementNumber(IHasNumber number)
        {
            number.Number++;
        }

    }

    public interface IHasNumber
    {
        int Number { get; set; }
    }

    public struct StructWithNumber : IHasNumber
    {
        public int Number { get; set; }
    }