String.GetHashCode Method
Returns the hash code for this string.
Namespace: System
Assembly: mscorlib (in mscorlib.dll)
The behavior of 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 GetHashCode.
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If two string objects are equal, the 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. For more information about hash codes, see Object.GetHashCode. |
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.
using System; public class Example { public static void Main() { // Show hash code in current domain. DisplayString display = new DisplayString(); display.ShowStringHashCode(); // Create a new app domain and show string hash code. AppDomain domain = AppDomain.CreateDomain("NewDomain"); var display2 = (DisplayString) domain.CreateInstanceAndUnwrap(typeof(Example).Assembly.FullName, "DisplayString"); display2.ShowStringHashCode(); } } public class DisplayString : MarshalByRefObject { private String s = "This is a string."; public override bool Equals(Object obj) { String s2 = obj as String; if (s2 == null) return false; else return s == s2; } public bool Equals(String str) { return s == str; } public override int GetHashCode() { return s.GetHashCode(); } public override String ToString() { return s; } public void ShowStringHashCode() { Console.WriteLine("String '{0}' in domain '{1}': {2:X8}", s, AppDomain.CurrentDomain.FriendlyName, s.GetHashCode()); } }
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>
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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 you 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.
Notes to CallersThe value returned by GetHashCode is platform-dependent. It differs on the 32-bit and 64-bit versions of the .NET Framework.
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A hash code is intended for efficient insertion and lookup in collections that are based on a hash table. A hash code is not a permanent value. For this reason:
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The following example demonstrates the GetHashCode method using various input strings.
using System; class GetHashCode { public static void Main() { DisplayHashCode( "" ); DisplayHashCode( "a" ); DisplayHashCode( "ab" ); DisplayHashCode( "abc" ); DisplayHashCode( "abd" ); DisplayHashCode( "abe" ); DisplayHashCode( "abcdef" ); DisplayHashCode( "abcdeg" ); DisplayHashCode( "abcdeh" ); DisplayHashCode( "abcdei" ); DisplayHashCode( "Abcdeg" ); DisplayHashCode( "Abcdeh" ); DisplayHashCode( "Abcdei" ); } static void DisplayHashCode( String Operand ) { int HashCode = Operand.GetHashCode( ); Console.WriteLine("The hash code for \"{0}\" is: 0x{1:X8}, {1}", Operand, HashCode ); } } /* This example displays output like the following: The hash code for "" is: 0x2D2816FE, 757602046 The hash code for "a" is: 0xCDCAB7BF, -842352705 The hash code for "ab" is: 0xCDE8B7BF, -840386625 The hash code for "abc" is: 0x2001D81A, 536991770 The hash code for "abd" is: 0xC2A94CB5, -1029092171 The hash code for "abe" is: 0x6550C150, 1699791184 The hash code for "abcdef" is: 0x1762906D, 392335469 The hash code for "abcdeg" is: 0x1763906D, 392401005 The hash code for "abcdeh" is: 0x175C906D, 391942253 The hash code for "abcdei" is: 0x175D906D, 392007789 The hash code for "Abcdeg" is: 0x1763954D, 392402253 The hash code for "Abcdeh" is: 0x175C954D, 391943501 The hash code for "Abcdei" is: 0x175D954D, 392009037 */
Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
