ErrorEventHandler デリゲートとは何？わかりやすく解説 Weblio辞書

.NET Framework クラスライブラリリファレンス

索引トップ用語の索引ランキング

ErrorEventHandler デリゲート

FileSystemWatcher オブジェクトの Error イベントを処理するメソッドを表します。

名前空間: System.IO
アセンブリ: System (system.dll 内)
構文

Visual Basic (宣言)

Public Delegate Sub ErrorEventHandler
 ( _
    sender As Object, _
    e As ErrorEventArgs _
)

Visual Basic (使用法)

Dim instance As New ErrorEventHandler(AddressOf
 HandlerMethod)

public delegate void ErrorEventHandler (
    Object sender,
    ErrorEventArgs e
)

C++

public delegate void ErrorEventHandler (
    Object^ sender, 
    ErrorEventArgs^ e
)

/** @delegate */
public delegate void ErrorEventHandler (
    Object sender, 
    ErrorEventArgs e
)

JScript

JScript では、デリゲートは使用できますが、新規に宣言することはできません。

パラメータ

sender: イベントのソース。

e: イベントデータを格納している ErrorEventArgs オブジェクト。

解説

ErrorEventHandler デリゲートを作成する場合は、イベントを処理するメソッドを識別してください。イベントとイベント処理メソッドを関連付けるには、デリゲートのインスタンスをイベントに追加します。デリゲートを削除しない限り、そのイベントが発生すると常にイベントハンドラメソッドが呼び出されます。イベントハンドラデリゲートの詳細については、「イベントとデリゲート」を参照してください。

使用例

ディスクドライブ上でのファイルの変更 (作成、削除、名前変更、変更) を監視する FileSystemWatcher オブジェクトを作成するコード例を次に示します。この例では、エラーの通知を正しく受け取る方法も示されています。

Visual Basic

Imports System.IO

Module Module1
    Sub Main()
        ' Create a FileSystemWatcher to monitor all files on drive C.
        Dim fsw As New FileSystemWatcher("C:\")

        ' Watch for changes in LastAccess and LastWrite times, and
        ' the renaming of files or directories. 
        fsw.NotifyFilter = (NotifyFilters.LastAccess Or NotifyFilters.LastWrite
 _ 
            Or NotifyFilters.FileName Or NotifyFilters.DirectoryName)

        ' Register a handler that gets called when a 
        ' file is created, changed, or deleted.
        AddHandler fsw.Changed, New FileSystemEventHandler(AddressOf
 OnChanged)

        ' The commented line of code below is a shorthand of the above
 line.
        ' AddHandler fsw.Changed, AddressOf OnChanged

        ' NOTE: The shorthand version is used in the remainder of this
 code.
        ' FileSystemEventHandler
        AddHandler fsw.Created, AddressOf OnChanged
 
        ' FileSystemEventHandler
        AddHandler fsw.Deleted, AddressOf OnChanged

        ' Register a handler that gets called when a file is renamed.
        ' RenamedEventHandler
        AddHandler fsw.Renamed, AddressOf OnRenamed

        ' Register a handler that gets called if the 
        ' FileSystemWatcher needs to report an error.
        ' ErrorEventHandler
        AddHandler fsw.Error, AddressOf OnError

        ' Begin watching.
        fsw.EnableRaisingEvents = True

        ' Wait for the user to quit the program.
        Console.WriteLine("Press 'Enter' to quit the sample.")
        Console.ReadLine()
    End Sub

    ' This method is called when a file is created, changed, or deleted.
    Private Sub OnChanged(ByVal
 source As Object, ByVal
 e As FileSystemEventArgs)

        ' Show that a file has been created, changed, or deleted.
        Dim wct As WatcherChangeTypes = e.ChangeType
        Console.WriteLine("File {0} {1}", e.FullPath,
 wct.ToString())
    End Sub

    ' This method is called when a file is renamed.
    Private Sub OnRenamed(ByVal
 source As Object, ByVal
 e As RenamedEventArgs)

        ' Show that a file has been renamed.
        Dim wct As WatcherChangeTypes = e.ChangeType
        Console.WriteLine("File {0} {2} to {1}", e.OldFullPath,
 e.FullPath, wct.ToString())
    End Sub

    ' This method is called when the FileSystemWatcher detects an error.
    Private Sub OnError(ByVal
 source As Object, ByVal
 e As ErrorEventArgs)

        ' Show that an error has been detected.
        Console.WriteLine("The FileSystemWatcher has detected
 an error")

        ' Give more information if the error is due to an internal buffer
 overflow.
        If TypeOf e.GetException Is
 InternalBufferOverflowException Then
            ' This can happen if Windows is reporting many file system
 events quickly 
            ' and internal buffer of the  FileSystemWatcher is not large
 enough to handle this
            ' rate of events. The InternalBufferOverflowException error
 informs the application
            ' that some of the file system events are being lost.
            Console.WriteLine( _
                "The file system watcher experienced an internal
 buffer overflow: " _
                + e.GetException.Message)
        End If
    End Sub
End Module