Right way to implement GetHashCode for this struct

Question

I want to use a date range (from one date to another date) as a key for a dictionary, so I wrote my own struct:

   struct DateRange
   {
      public DateTime Start;
      public DateTime End;

      public DateRange(DateTime start, DateTime end)
      {
         Start = start.Date;
         End = end.Date;
      }

      public override int GetHashCode()
      {
         // ???
      }
   }

What's the best way to implement GetHashCode so no two objects of a differing range will generate the same hash? I want hash collisions to be as unlikely as possible, though I understand Dictionary<> will still check the equality operator which I will also implement, but didn't want to pollute the example code too much. Thanks!

Answer 1

You can use the method from Effective Java as Jon Skeet shows here. For your specific type:

public override int GetHashCode()
{
    unchecked // Overflow is fine, just wrap
    {
        int hash = 17;
        hash = hash * 23 + Start.GetHashCode();
        hash = hash * 23 + End.GetHashCode();
        return hash;
    }
}

Answer 2

In C# 7 you can do this:

public override int GetHashCode() => (Start, End).GetHashCode();

The ValueTuple is available in .NET Framework 4.7 and .NET Core, or via NuGet.

Not sure how well it performs, but I would be surprised if any custom code would beat it.