如何：觸發並處理事件

本文中的範例示範如何處理事件。 其中包括EventHandler委派、EventHandler<TEventArgs>委派和自定義委派的範例，以說明具有數據和不具有數據的事件。

這些範例使用 事件 一文中所述的概念。

範例 1

第一個例子展示了如何在沒有資料的情況下，觸發並處理事件。 其中包含名為 Counter 的類別，其具有名為 ThresholdReached的事件。 當計數器值等於或超過臨界值時，就會引發這個事件。 委派 EventHandler 與事件相關聯，因為未提供任何事件數據。

class ProgramOne
{
    static void Main()
    {
        Counter c = new(new Random().Next(10));
        c.ThresholdReached += c_ThresholdReached;

        Console.WriteLine("press 'a' key to increase total");
        while (Console.ReadKey(true).KeyChar == 'a')
        {
            Console.WriteLine("adding one");
            c.Add(1);
        }
    }

    static void c_ThresholdReached(object? sender, EventArgs e)
    {
        Console.WriteLine("The threshold was reached.");
        Environment.Exit(0);
    }
}

class Counter(int passedThreshold)
{
    private readonly int _threshold = passedThreshold;
    private int _total;

    public void Add(int x)
    {
        _total += x;
        if (_total >= _threshold)
        {
            ThresholdReached?.Invoke(this, EventArgs.Empty);
        }
    }

    public event EventHandler? ThresholdReached;
}

Module Module1

    Sub Main()
        Dim c As New Counter(New Random().Next(10))
        AddHandler c.ThresholdReached, AddressOf c_ThresholdReached

        Console.WriteLine("press 'a' key to increase total")
        While Console.ReadKey(True).KeyChar = "a"
            Console.WriteLine("adding one")
            c.Add(1)
        End While
    End Sub

    Sub c_ThresholdReached(sender As Object, e As EventArgs)
        Console.WriteLine("The threshold was reached.")
        Environment.Exit(0)
    End Sub
End Module

Class Counter
    Private ReadOnly _threshold As Integer
    Private _total As Integer

    Public Sub New(passedThreshold As Integer)
        _threshold = passedThreshold
    End Sub

    Public Sub Add(x As Integer)
        _total += x
        If (_total >= _threshold) Then
            RaiseEvent ThresholdReached(Me, EventArgs.Empty)
        End If
    End Sub

    Public Event ThresholdReached As EventHandler
End Class

範例 2

第二個範例展示了如何觸發並處理一個提供資料的事件。 委派 EventHandler<TEventArgs> 與事件相關聯，並提供自定義事件數據對象的實例。

class ProgramThree
{
    static void Main()
    {
        Counter c = new(new Random().Next(10));
        c.ThresholdReached += c_ThresholdReached;

        Console.WriteLine("press 'a' key to increase total");
        while (Console.ReadKey(true).KeyChar == 'a')
        {
            Console.WriteLine("adding one");
            c.Add(1);
        }
    }

    static void c_ThresholdReached(object? sender, ThresholdReachedEventArgs e)
    {
        Console.WriteLine($"The threshold of {e.Threshold} was reached at {e.TimeReached}.");
        Environment.Exit(0);
    }
}

class Counter(int passedThreshold)
{
    private readonly int _threshold = passedThreshold;
    private int _total;

    public void Add(int x)
    {
        _total += x;
        if (_total >= _threshold)
        {
            ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
            args.Threshold = _threshold;
            args.TimeReached = DateTime.Now;
            OnThresholdReached(args);
        }
    }

    protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
    {
        ThresholdReached?.Invoke(this, e);
    }

    public event EventHandler<ThresholdReachedEventArgs>? ThresholdReached;
}

public class ThresholdReachedEventArgs : EventArgs
{
    public int Threshold { get; set; }
    public DateTime TimeReached { get; set; }
}

Module Module3

    Sub Main()
        Dim c As New Counter(New Random().Next(10))
        AddHandler c.ThresholdReached, AddressOf c_ThresholdReached

        Console.WriteLine("press 'a' key to increase total")
        While Console.ReadKey(True).KeyChar = "a"
            Console.WriteLine("adding one")
            c.Add(1)
        End While
    End Sub

    Sub c_ThresholdReached(sender As Object, e As ThresholdReachedEventArgs)
        Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached)
        Environment.Exit(0)
    End Sub
End Module

Class Counter
    Private ReadOnly _threshold As Integer
    Private _total As Integer

    Public Sub New(passedThreshold As Integer)
        _threshold = passedThreshold
    End Sub

    Public Sub Add(x As Integer)
        _total += x
        If (_total >= _threshold) Then
            Dim args As New ThresholdReachedEventArgs With {
                .Threshold = _threshold,
                .TimeReached = Date.Now
            }
            OnThresholdReached(args)
        End If
    End Sub

    Protected Overridable Sub OnThresholdReached(e As ThresholdReachedEventArgs)
        RaiseEvent ThresholdReached(Me, e)
    End Sub

    Public Event ThresholdReached As EventHandler(Of ThresholdReachedEventArgs)
End Class

Class ThresholdReachedEventArgs
    Inherits EventArgs

    Public Property Threshold As Integer
    Public Property TimeReached As Date
End Class

範例 3

第三個範例展示了如何為某個活動宣布代表。 此代表的名稱為 ThresholdReachedEventHandler。 此範例只是圖例。 一般而言，您不需要為事件定義委派，因為您可以使用 EventHandler 或 EventHandler<TEventArgs> 委派。 您應該只在罕見的情況下宣告委派，例如讓類別可供無法使用泛型的舊有程式碼使用。

class ProgramFour
{
    static void Main()
    {
        Counter c = new(new Random().Next(10));
        c.ThresholdReached += c_ThresholdReached;

        Console.WriteLine("press 'a' key to increase total");
        while (Console.ReadKey(true).KeyChar == 'a')
        {
            Console.WriteLine("adding one");
            c.Add(1);
        }
    }

    static void c_ThresholdReached(Object sender, ThresholdReachedEventArgs e)
    {
        Console.WriteLine($"The threshold of {e.Threshold} was reached at {e.TimeReached}.");
        Environment.Exit(0);
    }
}

class Counter
{
    private readonly int _threshold;
    private int _total;

    public Counter(int passedThreshold)
    {
        _threshold = passedThreshold;
    }

    public void Add(int x)
    {
        _total += x;
        if (_total >= _threshold)
        {
            ThresholdReachedEventArgs args = new();
            args.Threshold = _threshold;
            args.TimeReached = DateTime.Now;
            OnThresholdReached(args);
        }
    }

    protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
    {
        ThresholdReached?.Invoke(this, e);
    }

    public event ThresholdReachedEventHandler ThresholdReached;
}

public class ThresholdReachedEventArgs : EventArgs
{
    public int Threshold { get; set; }
    public DateTime TimeReached { get; set; }
}

public delegate void ThresholdReachedEventHandler(
    object sender,
    ThresholdReachedEventArgs e);

Module Module4

    Sub Main()
        Dim c As New Counter(New Random().Next(10))
        AddHandler c.ThresholdReached, AddressOf c_ThresholdReached

        Console.WriteLine("press 'a' key to increase total")
        While Console.ReadKey(True).KeyChar = "a"
            Console.WriteLine("adding one")
            c.Add(1)
        End While
    End Sub

    Sub c_ThresholdReached(sender As Object, e As ThresholdReachedEventArgs)
        Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached)
        Environment.Exit(0)
    End Sub
End Module

Class Counter
    Private ReadOnly _threshold As Integer
    Private _total As Integer

    Public Sub New(passedThreshold As Integer)
        _threshold = passedThreshold
    End Sub

    Public Sub Add(x As Integer)
        _total += x
        If (_total >= _threshold) Then
            Dim args As New ThresholdReachedEventArgs With {
                .Threshold = _threshold,
                .TimeReached = Date.Now
            }
            OnThresholdReached(args)
        End If
    End Sub

    Protected Overridable Sub OnThresholdReached(e As ThresholdReachedEventArgs)
        RaiseEvent ThresholdReached(Me, e)
    End Sub

    Public Event ThresholdReached As ThresholdReachedEventHandler
End Class

Public Class ThresholdReachedEventArgs
    Inherits EventArgs

    Public Property Threshold As Integer
    Public Property TimeReached As Date
End Class

Public Delegate Sub ThresholdReachedEventHandler(sender As Object, e As ThresholdReachedEventArgs)

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• 事件