class Program
{
  public static void Main(string[] args)
  {
    Console.WriteLine(Enumerable.Range(0,5).Select(n=>new Foo()).Distinct().Count());
  }
}
class Foo
{
  public override bool Equals(object obj)=>throw new NotImplementedException();
}

5

class ObjectEqualityComparer<T>.Equals(object, object)

struct A.....

public override int GetHashCode()
{
  return base.GetHashCode();
}

сlass A
{
   public int Id {get;set;}
   public string Name {get;set;}
   public DateTime Date {get;set;}

   public override int GetHashCode()
   {
       unchecked
                {
                    var hashCode = Id.GetHashCode();
                    hashCode = (hashCode * 397) ^ Name.GetHashCode();
                    hashCode = (hashCode * 397) ^ Date.GetHashCode();
                    return hashCode;
                }
   }

}

[StructLayout(LayoutKind.Sequential, Size = 1024)]
struct S1
{
  static Random Rnd = new Random((int)DateTime.Now.Ticks);
  public static implicit operator S1(int i)
  {
    S1[] res = new S1[1];
    var arr = new byte[BitConverter.GetBytes(i).Length];
    Rnd.NextBytes(arr);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
  public static implicit operator S1(long i)
  {
    S1[] res = new S1[1]; ;
    var arr = new byte[BitConverter.GetBytes(i).Length];
    Rnd.NextBytes(arr);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
}

[StructLayout(LayoutKind.Sequential, Size = 1024)]
struct S1
{
  long Value;
  static Random Rnd = new Random((int)DateTime.Now.Ticks);
  public static implicit operator S1(int i)
  {
    S1[] res = new S1[1];
    var arr = new byte[BitConverter.GetBytes(i).Length];
    Rnd.NextBytes(arr);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
  public static implicit operator S1(long i)
  {
    S1[] res = new S1[1]; ;
    var arr = new byte[BitConverter.GetBytes(i).Length];
    Rnd.NextBytes(arr);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
}

[StructLayout(LayoutKind.Sequential, Size = 1024)]
struct S1
{
  long Value;
  static Random Rnd = new Random((int)DateTime.Now.Ticks);
  public static implicit operator S1(int i)
  {
    S1[] res = new S1[1];
    var arr = BitConverter.GetBytes(i);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
  public static implicit operator S1(long i)
  {
    S1[] res = new S1[1]; ;
    var arr = BitConverter.GetBytes(i);
    var x = GCHandle.Alloc(res, GCHandleType.Pinned);
    var adr = x.AddrOfPinnedObject();
    Marshal.Copy(arr, 0, adr, arr.Length);
    x.Free();
    return res[0];
  }
}

hashCode = typeID * 711650207 + 2506965631U;