Question

I have a class that contains the following two properties:

public int Id      { get; private set; } public T[] Values  { get; private set; } 

I have made it IEquatable<T> and overriden the object.Equals like this:

public override bool Equals(object obj) {     return Equals(obj as SimpleTableRow<T>); }  public bool Equals(SimpleTableRow<T> other) {     // Check for null     if(ReferenceEquals(other, null))         return false;      // Check for same reference     if(ReferenceEquals(this, other))         return true;      // Check for same Id and same Values     return Id == other.Id && Values.SequenceEqual(other.Values); } 

When having override object.Equals I must also override GetHashCode of course. But what code should I implement? How do I create a hashcode out of a generic array? And how do I combine it with the Id integer?

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

Answer 1

Because of the problems raised in this thread, I’m posting another reply showing what happens if you get it wrong… mainly, that you can’t use the array’s GetHashCode(); the correct behaviour is that no warnings are printed when you run it… switch the comments to fix it:

using System; using System.Collections.Generic; using System.Linq; static class Program {     static void Main()     {         // first and second are logically equivalent         SimpleTableRow<int> first = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6),             second = new SimpleTableRow<int>(1, 2, 3, 4, 5, 6);          if (first.Equals(second) && first.GetHashCode() != second.GetHashCode())         { // proven Equals, but GetHashCode() disagrees             Console.WriteLine('We have a problem');         }         HashSet<SimpleTableRow<int>> set = new HashSet<SimpleTableRow<int>>();         set.Add(first);         set.Add(second);         // which confuses anything that uses hash algorithms         if (set.Count != 1) Console.WriteLine('Yup, very bad indeed');     } } class SimpleTableRow<T> : IEquatable<SimpleTableRow<T>> {      public SimpleTableRow(int id, params T[] values) {         this.Id = id;         this.Values = values;     }     public int Id { get; private set; }     public T[] Values { get; private set; }      public override int GetHashCode() // wrong     {         return Id.GetHashCode() ^ Values.GetHashCode();     }     /*     public override int GetHashCode() // right     {         int hash = Id;         if (Values != null)         {             hash = (hash * 17) + Values.Length;             foreach (T t in Values)             {                 hash *= 17;                 if (t != null) hash = hash + t.GetHashCode();             }         }         return hash;     }     */     public override bool Equals(object obj)     {         return Equals(obj as SimpleTableRow<T>);     }     public bool Equals(SimpleTableRow<T> other)     {         // Check for null         if (ReferenceEquals(other, null))             return false;          // Check for same reference         if (ReferenceEquals(this, other))             return true;          // Check for same Id and same Values         return Id == other.Id && Values.SequenceEqual(other.Values);     } }