Are we on the same page?

Unfortunately, despite being on its 5th printing my copy of the official Exam Ref book for 70-483 Programming in C# is missing page 201 (but on the up side contains two page 202s).

If you are also missing it, here's the text.

NB - I make no claims to this - the copyright belongs entirely to the author Wouter de Kort and Microsoft Press.

hash codes must be as random as possible. This is why the set implementation uses the GetHashCode method on each object to calculate in which bucket it should go. Now your items are distributed over a hundred buckets instead of one single bucket. When you see whether an item exists, you first calculate the hash code, go to the corresponding bucket, and look for the item. This technique is used by the Hashtable and Dictionary classes in the .NET Framework. Both use the hash code to store and access items. Hashtable is nongeneric collection; Dictionary is a generic collection. A couple of important principles can be deduced from this. First of all, equal items should have equal hash codes. This means that you can check to determine whether two items are equal by checking their hash codes. It also means that your implementation of GetHashCode should return the same value during time. It shouldn’t depend on changing values such as the current date or time. MORE INFO IMPLEMENTING GETHASHCODE For more information on how to implement GetHashCode correctly, see http://msdn.microsoft.com/enus/library/system.object.gethashcode(v=vs.110).aspx. These properties are important when looking at hashing in a security context. If you hash a paragraph of text and change only one letter, the hash code will change, so hashing is used to check the integrity of a message. For example, let’s say that Alice and Bob want to send a message to each other. Alice creates a hash of the message and sends both the hash and the message to Bob. Bob creates a hash of the message he has received from Alice and compares the two hash codes with each other. If they match, Bob knows he has received the correct message. Of course, without any additional encryption, a third party can still tamper with the message by changing both the message and the hash code. Combined with the encryption technologies that the .NET Framework offers, hashing is an important technique to validate the authenticity of a message. The .NET Framework offers a couple of classes to generate hash values. The algorithms that the .NET Framework offers are optimized hashing algorithms that output a significantly different hash code for a small change in the data. Listing 3-23 shows an example of using the SHA256Managed algorithm to calculate the hash code for a piece of text. LISTING 3-23 Using SHA256Managed to calculate a hash code UnicodeEncoding byteConverter = new UnicodeEncoding(); SHA256 sha256 = SHA256.Create(); string data = “A paragraph of text”; byte[] hashA = sha256.ComputeHash(byteConverter.GetBytes(data));