How to Unsubscribe from Lambda Event Handlers in C#?

Remove Lambda event handler in C#

In C#, Lambda expressions provide a concise way to define simple event handlers. For example, subscribing to a click event using a Lambda expression is very simple:

<code class="language-csharp">button.Click += (s, e) => MessageBox.Show("Woho");</code>

But how do I subsequently unsubscribe from this event?

Use delegate to unsubscribe

The C# specification states that Lambda expressions and anonymous delegates do not always generate the same delegate. Therefore, to ensure reliable unsubscription, you can explicitly track the delegate instance used:

<code class="language-csharp">EventHandler handler = (s, e) => MessageBox.Show("Woho");

button.Click += handler;
...
button.Click -= handler;</code>

Use independent methods

If you don't want to use a delegate, you can define a separate method for the event handler:

<code class="language-csharp">public void ShowWoho(object sender, EventArgs e)
{
     MessageBox.Show("Woho");
}

...

button.Click += ShowWoho;
...
button.Click -= ShowWoho;</code>

Self-removing Lambda handler

If you wish to use a Lambda expression to create a self-removing event handler, you can reference the delegate in the Lambda expression. However, you must first assign the variable to null:

<code class="language-csharp">EventHandler handler = null;
handler = (sender, args) =>
{
    button.Click -= handler; // 取消订阅
    // 在此处添加您的单次执行代码
};
button.Click += handler;</code>

Use delegate class for encapsulation

It is possible to encapsulate self-removal functionality into a delegate class, but this may be more complicated due to the abstract nature of the event:

<code class="language-csharp">button.Click += Delegates.AutoUnsubscribe<EventHandler>((sender, args) =>
{
    // 此处的单次执行代码
}, handler => button.Click -= handler);</code>

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