Writing a good GetHashCode override C# with Example

GetHashCode has major performance effects on Dictionary<> and HashTable. 
Good GetHashCode Methods 
should have an even distribution 
every integer should have a roughly equal chance of returning for a random instance 
if your method returns the same integer (e.g. the constant '999') for each instance, you'll have bad 
performance 
should be quick 
These are NOT cryptographic hashes, where slowness is a feature 
the slower your hash function, the slower your dictionary 
must return the same HashCode on two instances that Equals evaluates to true 
if they do not (e.g. because GetHashCode returns a random number), items may not be found in a List, 
Dictionary, or similar. 
A good method to implement GetHashCode is to use one prime number as a starting value, and add the hashcodes 
of the fields of the type multiplied by other prime numbers to that: 
public override int GetHashCode() 
{ 
unchecked // Overflow is fine, just wrap 
{ 
int hash = 3049; // Start value (prime number). 
// Suitable nullity checks etc, of course  :) 
hash = hash * 5039 + field1.GetHashCode(); 
hash = hash * 883 + field2.GetHashCode(); 
hash = hash * 9719 + field3.GetHashCode(); 
return hash; 
} 
} 
Only the fields which are used in the Equals-method should be used for the hash function. 
If you have a need to treat the same type in different ways for Dictionary/HashTables, you can use 
IEqualityComparer.