<UseRandomizedStringHashAlgorithm> Element

Determines whether the common language runtime calculates hash codes for strings on a per application domain basis.

<configuration>
  <runtime>
    <UseRandomizedStringHashAlgorithm>

Syntax

<UseRandomizedStringHashAlgorithm
   enabled=0|1 />  

Attributes and Elements

The following sections describe attributes, child elements, and parent elements.

Attributes

enabled Attribute

Child Elements

None.

Parent Elements

Remarks

By default, the StringComparer class and the String.GetHashCode method use a single hashing algorithm that produces a consistent hash code across application domains. This is equivalent to setting the enabled attribute of the <UseRandomizedStringHashAlgorithm> element to 0. This is the hashing algorithm used in the .NET Framework 4.

The StringComparer class and the String.GetHashCode method can also use a different hashing algorithm that computes hash codes on a per application domain basis. As a result, hash codes for equivalent strings will differ across application domains. This is an opt-in feature; to take advantage of it, you must set the enabled attribute of the <UseRandomizedStringHashAlgorithm> element to 1.

The string lookup in a hash table is typically an O(1) operation. However, when a large number of collisions occur, the lookup can become an O(n²) operation. You can use the <UseRandomizedStringHashAlgorithm> configuration element to generate a random hashing algorithm per application domain, which in turn limits the number of potential collisions, particularly when the keys from which the hash codes are calculated are based on data input by users.

Example

The following example 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());
   }
}

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

        ' 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)
        display2.ShowStringHashCode()
    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
        Else
            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,
                          s.GetHashCode())
    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"?>  
<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  

See also

• Configure apps by using configuration files
• StringComparer.GetHashCode
• String.GetHashCode
• Object.GetHashCode