Re: Key compare in Dictionary

I always override Object.GetHashCode and Object.Equals for these sorts
of things ...

// Ryan

On Thu, May 20, 2010 at 12:32 PM, David Nicholson <[email protected]> w=
rote:
> I am using a class as a key to a Dictionary, and the key comparisons are =
not working as I expect. The example below illustrates:
>
> // This is the key of the dictionary
> namespace DictionaryTest
> {
> =A0 =A0using System;
> =A0 =A0public class KeyData : IEquatable<KeyData>
> =A0 =A0{
> =A0 =A0 =A0 =A0public string F1 { get; set; }
> =A0 =A0 =A0 =A0public int F2 { get; set; }
>
> =A0 =A0 =A0 =A0public KeyData(string f1, int f2)
> =A0 =A0 =A0 =A0{
> =A0 =A0 =A0 =A0 =A0 =A0F1 =3D f1;
> =A0 =A0 =A0 =A0 =A0 =A0F2 =3D f2;
> =A0 =A0 =A0 =A0}
>
> =A0 =A0 =A0 =A0bool IEquatable<KeyData>.Equals(KeyData other)
> =A0 =A0 =A0 =A0{
> =A0 =A0 =A0 =A0 =A0 =A0if (String.Compare(F1, other.F1, true) !=3D 0)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return false;
>
> =A0 =A0 =A0 =A0 =A0 =A0if (F2 !=3D other.F2)
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0return false;
>
> =A0 =A0 =A0 =A0 =A0 =A0return true;
> =A0 =A0 =A0 =A0}
> =A0 =A0}
> }
>
> and I am using it like this:
>
> namespace DictionaryTest
> {
> =A0 using System;
> =A0 using System.Collections.Generic;
> =A0 =A0class Program
> =A0 =A0{
> =A0 =A0 =A0 =A0static void Main(string[] args)
> =A0 =A0 =A0 =A0{
> =A0 =A0 =A0 =A0 =A0 =A0var d1 =3D new Dictionary<KeyData, int>();
>
> =A0 =A0 =A0 =A0 =A0 =A0d1.Add(new KeyData("a", 1), 1);
> =A0 =A0 =A0 =A0 =A0 =A0d1.Add(new KeyData("b", 1), 1);
>
> =A0 =A0 =A0 =A0 =A0 =A0KeyData kd =3D new KeyData("a", 1);
>
> =A0 =A0 =A0 =A0 =A0 =A0string mesg;
> =A0 =A0 =A0 =A0 =A0 =A0if (d1.ContainsKey(kd))
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0mesg =3D "Found";
> =A0 =A0 =A0 =A0 =A0 =A0else
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0mesg =3D "Not found";
>
> =A0 =A0 =A0 =A0 =A0 =A0Console.WriteLine(mesg);
> =A0 =A0 =A0 =A0}
> =A0 =A0}
> }
>
> This always returns "Not found", which isn't what I expect. A breakpoint =
on KeyData.Equals is never reached. The documentation for dictionary includ=
es:
>
> "If type TKey implements the System .IEquatable <T> generic interface, th=
e default equality comparer uses that implementation"
>
> I wrote an IEqualityComparer<KeyData> and passed that to the dictionary c=
onstructor, and it works as I expect.
>
> Thanks
> David.
>
> =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
> View archives and manage your subscription(s) at http://peach.ease.lsoft.=
com/archives
>

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
View archives and manage your subscription(s) at http://peach.ease.lsoft.com/archives