Duration (Pacific Standard Time):
To (Pacific Standard Time):
  • None
User Action:
  • None
.NET Framework Class Library

String.GetHashCode Method

Returns the hash code for this string.

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
Public Overrides Function GetHashCode As Integer

Return Value

Type: System.Int32
A 32-bit signed integer hash code.

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.

Important note Important

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.

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.

Module Example
   Public Sub Main()
      ' Show hash code in current domain. 
      Dim display As New DisplayString()

      ' Create a new app domain and show string hash code. 
      Dim domain As AppDomain = AppDomain.CreateDomain("NewDomain")
      Dim display2 = CType(domain.CreateInstanceAndUnwrap(GetType(Example).Assembly.FullName, 
                                                          "DisplayString"), DisplayString)   
   End Sub 
End Module 

Public Class DisplayString : Inherits MarshalByRefObject

   Private s As String = "This is a string." 

   Public Overrides Function Equals(obj As Object) As Boolean 
      Dim s2 As String = TryCast(obj, String)
      If s2 Is Nothing Then 
         Return False 
         Return s = s2 
      End If 
   End Function 

   Public Overloads Function Equals(str As String) As Boolean 
      Return s = str
   End Function     

   Public Overrides Function GetHashCode() As Integer 
      Return s.GetHashCode()
   End Function 

   Public Overrides Function ToString() As String 
      Return s
   End Function 

   Public Sub ShowStringHashCode()
      Console.WriteLine("String '{0}' in domain '{1}': {2:X8}",
                        s, AppDomain.CurrentDomain.FriendlyName, 
   End Sub 
End Class

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"?>
      <UseRandomizedStringHashAlgorithm enabled="1" />

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
Important note Important

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.

Notes to Callers

The value returned by GetHashCode is platform-dependent. It differs on the 32-bit and 64-bit versions of the .NET Framework. It also can differ between versions of the .NET Framework.

Caution note Caution

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:

  • Do not serialize hash code values or store them in databases.

  • Do not use the hash code as the key to retrieve an object from a keyed collection.

  • Do not use the hash code instead of a value returned by a cryptographic hashing function. For cryptographic hashes, use a class derived from the System.Security.Cryptography.HashAlgorithm or System.Security.Cryptography.KeyedHashAlgorithm class.

  • Do not test for equality of hash codes to determine whether two objects are equal. (Unequal objects can have identical hash codes.) To test for equality, call the ReferenceEquals or Equals method.

The following example demonstrates the GetHashCode method using various input strings.

Module GetHashCode
    Sub Main()
    End Sub 

    Sub DisplayHashCode(Operand As String)
        Dim HashCode As Integer = Operand.GetHashCode()
        Console.WriteLine("The hash code for ""{0}"" is: 0x{1:X8}, {1}", 
                          Operand, HashCode)
    End Sub  
End Module  
' 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

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

XNA Framework

Supported in: 3.0, 2.0, 1.0

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Portable Class Library

Supported in: Portable Class Library