StackOverflow2013

Note that there are some explanatory texts on larger screens.

plurals

PO
text
Body
copied!<blockquote> <p>Does anyone have an alternative solution to this problem?</p> </blockquote> <p>Hash code of the object must not be changed during the object life-time. You shouldn't use mutable data for hash code calculation.</p> <p><strong>Update</strong>.</p> <p>I didn't expect, that my answer will cause such discussion. Here's some detail explanation, may be it will help OP.</p> <p>Let's look at some mutable entity type defined in your code, which overrides <code>GetHashCode</code> and, of course, <code>Equals</code>. The equality is based on <code>Id</code> equality:</p> <pre><code>class Mutable : IEquatable<Mutable> { public int Id { get; set; } public override int GetHashCode() { return Id.GetHashCode(); } public override bool Equals(object obj) { if (obj == null) { return false; } var mutable = obj as Mutable; if (mutable == null) { return false; } return this.Equals(mutable); } public bool Equals(Mutable other) { return Id.Equals(other.Id); } } </code></pre> <p>Somewhere in your code you've created several instances of this type:</p> <pre><code> // here's some mutable entities with hash-code, calculated using mutable data: var key1 = new Mutable { Id = 1 }; var key2 = new Mutable { Id = 2 }; var key3 = new Mutable { Id = 3 }; </code></pre> <p>And this is some external code, which uses <code>Dictionary<Mutable, string></code> for its internal purposes:</p> <pre><code> // let's use them as a key for the dictionary: var dictionary = new Dictionary<Mutable, string> { { key1, "John" }, { key2, "Mary" }, { key3, "Peter" } }; // everything is ok, all of the keys are located properly: Console.WriteLine(dictionary[key1]); Console.WriteLine(dictionary[key2]); Console.WriteLine(dictionary[key3]); </code></pre> <p>Again, your code. Suppose, you've changed <code>Id</code> of <code>key1</code>. The hash-code was changed too:</p> <pre><code> // let's change the hashcode of key1: key1.Id = 4; </code></pre> <p>And again, external code. Here it tries to locate some data by <code>key1</code>:</p> <pre><code>Console.WriteLine(dictionary[key1]); // ooops! key1 was not found in dictionary </code></pre> <p>Of course, you <em>can</em> design mutable types, which overrides <code>GetHashCode</code> and <code>Equals</code>, and calculate hash-code on mutable data. But you shouldn't do that, really (except the cases, when you definitely know, what are you doing). </p> <p>You <em>can't</em> guarantee, that any external code won't use <code>Dictionary<TKey, TValue></code> or <code>HashSet<T></code> internally.</p>

Querying!

Guidance

An individual column

Larger individual text columns get their own page to allow for proper reading.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload