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Attribute.GetHashCode Method

Returns the hash code for this instance.

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

'Declaration
Public Overrides Function GetHashCode As Integer

Return Value

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

The following code example illustrates the use of GetHashCode in the context of Attribute.


Imports System
Imports System.Reflection
Imports System.Collections

Module HashCodeVB

    ' Visual Basic requires that the AttributeUsage be specified.
    ' A custom attribute to allow two authors per method.
    <AttributeUsage(AttributeTargets.All.Method)> _
    Public Class AuthorsAttribute
        Inherits Attribute

        Public Sub New(ByVal name1 As String, ByVal name2 As String)
            myAuthorName1 = name1
            myAuthorName2 = name2
        End Sub

        Protected myAuthorName1 As String
        Protected myAuthorName2 As String

        Public Property AuthorName1() As String
            Get
                Return myAuthorName1
            End Get
            Set(ByVal Value As String)
                myAuthorName1 = AuthorName1
            End Set
        End Property

        Public Property AuthorName2() As String
            Get
                Return myAuthorName2
            End Get
            Set(ByVal Value As String)
                myAuthorName2 = AuthorName2
            End Set
        End Property

        ' Use the hash code of the string objects and Xor them together.
        Public Overrides Function GetHashCode() As Integer
            Return myAuthorName1.GetHashCode() Xor myAuthorName2.GetHashCode()
        End Function

    End Class

    ' Provide the author names for each method of the class.
    Public Class TestClass
        <Authors("Immanuel Kant", "Lao Tzu")> _
        Public Sub Method1()
        End Sub

        <Authors("Jean-Paul Sartre", "Friedrich Nietzsche")> _
        Public Sub Method2()
        End Sub

        <Authors("Immanuel Kant", "Lao Tzu")> _
        Public Sub Method3()
        End Sub

        <Authors("Jean-Paul Sartre", "Friedrich Nietzsche")> _
        Public Sub Method4()
        End Sub

        <Authors("Immanuel Kant", "Friedrich Nietzsche")> _
        Public Sub Method5()
        End Sub
    End Class

    Sub Main()
        ' Get the class type to access its metadata.
        Dim clsType As Type = GetType(TestClass)
        ' Use a hash table to store the hash codes and methods.
        Dim hTable As Hashtable = New Hashtable()
        Dim mInfo As MethodInfo
        ' Iterate through all the methods of the class.
        For Each mInfo In clsType.GetMethods()
            ' Get the unique hash code for these authors.
            Dim attr As Attribute = Attribute.GetCustomAttribute(mInfo, GetType(AuthorsAttribute))
            If Not attr Is Nothing Then
                Dim authAttr As AuthorsAttribute = CType(attr, _
                    AuthorsAttribute)
                Dim hCode As Integer = authAttr.GetHashCode()
                ' See if it's already been added to the hash table.
                If hTable.Contains(hCode) Then
                    Dim o As Object = hTable(hCode)
                    If TypeOf o Is ArrayList Then
                        ' If so, retrieve the array using the Item syntax.
                        Dim al As ArrayList = CType(o, ArrayList)
                        ' Add the method to the array list.
                        al.Add(mInfo.Name)
                        ' Put the edited array list back into the table.
                        hTable(hCode) = al
                    End If
                ' This is the first occurrence of this hash code.
                Else
                    ' Create a new array list.
                    Dim al As ArrayList = New ArrayList()
                    ' Add the method name as the first entry.
                    al.Add(mInfo.Name)
                    ' Add the new hash code and array list to the table.
                    hTable.Add(hCode, al)
                End If
            End If
        Next

        ' Get the hash table's enumerator and iterate through the table.
        Dim dictEnum As IDictionaryEnumerator = hTable.GetEnumerator
        While dictEnum.MoveNext()
            ' Get the array list for this iteration.
            Dim o As Object = dictEnum.Value
            If TypeOf o Is ArrayList Then
                Dim al As ArrayList = CType(o, ArrayList)
                Dim i As Integer
                ' Access each element of the array.
                for i = 0 to al.Count - 1
                    ' For the first element, retrieve the author names.
                    If i = 0 Then
                        Dim methodName as Object = al(i)
                        ' Get the metadata for the method.
                        Dim mInfo2 As MethodInfo = _
                            clsType.GetMethod(CType(methodName, String))
                        ' Retrieve the Authors attribute again.
                        Dim attr As Attribute = Attribute.GetCustomAttribute( _
                            mInfo2, GetType(AuthorsAttribute))
                        If Not attr Is Nothing And _
                            TypeOf attr Is AuthorsAttribute Then
                            ' Convert the attribute to access its data.
                            Dim authAttr As AuthorsAttribute = _
                                CType(attr, AuthorsAttribute)
                            ' Display the author's names.
                            Console.WriteLine("Authors: {0}, {1}", _
                                authattr.AuthorName1, authattr.AuthorName2)
                        Else
                            Console.WriteLine("The authors could not " + _
                                "be retrieved.")
                        End If
                    End If
                    ' Display each method authored by this team.
                    Console.WriteLine(al(i))
                Next i
                Console.WriteLine("")
            End If
        End While
    End Sub
End Module


.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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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