The behavior of string.GetHashCode is dependent on its implementation, which might change from one version of the common language runtime to another. A reason why this might happen is to improve the performance of string.GetHashCode.
If two string objects are equal, the string.GetHashCode method returns identical values. However, there is not a unique hash code value for each unique string value. Different strings can return the same hash code.
The hash code itself is not guaranteed to be stable. Hash codes for identical strings can differ across versions of the .NET Framework and across platforms (such as 32-bit and 64-bit) for a single version of the .NET Framework. In some cases, they can even differ by application domain.
As a result, hash codes should never be used outside of the application domain in which they were created, they should never be used as key fields in a collection, and they should never be persisted.
Finally, do not use the hash code instead of a value returned by a cryptographic hashing function if you need a cryptographically strong hash. For cryptographic hashes, use a class derived from the System.Security.Cryptography.HashAlgorithm or System.Security.Cryptography.KeyedHashAlgorithm class.
For more information about hash codes, see object.GetHashCode.
In desktop apps, you can use the <UseRandomizedStringHashAlgorithm> element to generate unique hash codes on a per application domain basis. This can reduce the number of collisions and improve the overall performance of insertions and lookups that use hash tables. The following example shows how to use the <UseRandomizedStringHashAlgorithm> element. It defines a DisplayString class that includes a private string constant, s, whose value is "This is a string." It also includes a ShowStringHashCode method that displays the string value and its hash code along with the name of the application domain in which the method is executing.
code reference: System.String.GetHashCode#2
When you run the example without supplying a configuration file, it displays output similar to the following. Note that the hash codes for the string are identical in the two application domains.
String 'This is a string.' in domain 'PerDomain.exe': 941BCEAC String 'This is a string.' in domain 'NewDomain': 941BCEAC
However, if you add the following configuration file to the example's directory and then run the example, the hash codes for the same string will differ by application domain.
<?xml version ="1.0"?> <configuration> <runtime> <UseRandomizedStringHashAlgorithm enabled="1" /> </runtime> </configuration>
When the configuration file is present, the example displays the following output:
String 'This is a string.' in domain 'PerDomain.exe': 5435776D String 'This is a string.' in domain 'NewDomain': 75CC8236
Hash codes are used to insert and retrieve keyed objects from hash tables efficiently. However, hash codes do not uniquely identify strings. Identical strings have equal hash codes, but the common language runtime can also assign the same hash code to different strings. In addition, hash codes can vary by version of the .NET Framework, by platform within a single version, and by application domain. Because of this, you should not serialize or persist hash code values, nor should you use them as keys in a hash table or dictionary.
For additional information about the use of hash codes and the GetHashCode method, see object.GetHashCode.