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Hashtable Constructor (Int32, IEqualityComparer)

Initializes a new, empty instance of the Hashtable class using the specified initial capacity and IEqualityComparer, and the default load factor.

Namespace:  System.Collections
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Public Sub New ( _
	capacity As Integer, _
	equalityComparer As IEqualityComparer _
)

Parameters

capacity
Type: System.Int32

The approximate number of elements that the Hashtable object can initially contain.

equalityComparer
Type: System.Collections.IEqualityComparer

The IEqualityComparer object that defines the hash code provider and the comparer to use with the Hashtable.

-or-

Nothing to use the default hash code provider and the default comparer. The default hash code provider is each key's implementation of Object.GetHashCode and the default comparer is each key's implementation of Object.Equals.

ExceptionCondition
ArgumentOutOfRangeException

capacity is less than zero.

Specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the Hashtable object. Capacity is automatically increased as required based on the load factor.

The load factor is the maximum ratio of elements to buckets. A smaller load factor means faster lookup at the cost of increased memory consumption.

When the actual load factor reaches the specified load factor, the number of buckets is automatically increased to the smallest prime number that is larger than twice the current number of buckets.

The IEqualityComparer object includes both the hash code provider and the comparer. If an IEqualityComparer is used in the Hashtable constructor, the objects used as keys in the Hashtable are not required to override the Object.GetHashCode and Object.Equals methods.

The hash code provider dispenses hash codes for keys in the Hashtable. The default hash code provider is the key's implementation of Object.GetHashCode.

The comparer determines whether two keys are equal. Every key in a Hashtable must be unique. The default comparer is the key's implementation of Object.Equals.

The IEqualityComparer enables scenarios such as doing lookups with case-insensitive strings.

This constructor is an O(n) operation, where n is the capacity parameter.

The following code example creates hash tables using different Hashtable constructors and demonstrates the differences in the behavior of the hash tables, even if each one contains the same elements.

Imports System
Imports System.Collections
Imports System.Globalization

Public Class myCultureComparer
    Implements IEqualityComparer

    Dim myComparer As CaseInsensitiveComparer

    Public Sub New()
        myComparer = CaseInsensitiveComparer.DefaultInvariant
    End Sub 

    Public Sub New(ByVal myCulture As CultureInfo)
        myComparer = New CaseInsensitiveComparer(myCulture)
    End Sub 

    Public Function Equals1(ByVal x As Object, ByVal y As Object) _
        As Boolean Implements IEqualityComparer.Equals

        If (myComparer.Compare(x, y) = 0) Then 
            Return True 
        Else 
            Return False 
        End If 
    End Function 

    Public Function GetHashCode1(ByVal obj As Object) _
        As Integer Implements IEqualityComparer.GetHashCode
        Return obj.ToString().ToLower().GetHashCode()
    End Function 
End Class 

Public Class SamplesHashtable   

   Public Shared Sub Main()

      ' Create a hash table using the default comparer. 
      Dim myHT1 As New Hashtable(3)
      myHT1.Add("FIRST", "Hello")
      myHT1.Add("SECOND", "World")
      myHT1.Add("THIRD", "!")

      ' Create a hash table using the specified IEqualityComparer that uses 
      ' the CaseInsensitiveComparer.DefaultInvariant to determine equality. 
      Dim myHT2 As New Hashtable(3, New myCultureComparer())
      myHT2.Add("FIRST", "Hello")
      myHT2.Add("SECOND", "World")
      myHT2.Add("THIRD", "!")

      ' Create a hash table using an IEqualityComparer that is based on 
      ' the Turkish culture (tr-TR) where "I" is not the uppercase 
      ' version of "i". 
      Dim myCul As New CultureInfo("tr-TR")
      Dim myHT3 As New Hashtable(3, New myCultureComparer(myCul))
      myHT3.Add("FIRST", "Hello")
      myHT3.Add("SECOND", "World")
      myHT3.Add("THIRD", "!")

      ' Search for a key in each hash table.
      Console.WriteLine("first is in myHT1: {0}", myHT1.ContainsKey("first"))
      Console.WriteLine("first is in myHT2: {0}", myHT2.ContainsKey("first"))
      Console.WriteLine("first is in myHT3: {0}", myHT3.ContainsKey("first"))

   End Sub 'Main 

End Class 'SamplesHashtable


'This code produces the following output. 
'Results vary depending on the system's culture settings. 

'first is in myHT1: False 
'first is in myHT2: True 
'first is in myHT3: False

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 8.1, Windows Server 2012 R2, 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.

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