Server.cs

public static class Server
{
    private static int _count = 0;

    private static readonly object Lock = new object();

    public static void AddToCount(int value)
    {
        lock (Lock)
            _count += value;
    }

    public static int GetCount()
    {
        lock (Lock)
            return _count;
    }
}

namespace Var2
{
    public static class Server
    {
        private static int _count = 0;

        public static void AddToCount(int value) => Interlocked.Add(ref _count, value); 

        public static int GetCount() => _count;
    }   
}