A lot of the discussion about .NET 4.0 is revolving around the introduction of co-variance and contra-variance for generic types.

It’s important to remember, though, that these concepts aren’t entirely new - there has been some support for variance built into .NET for some time.

For example, consider this snippet of code:

    public Form1()
    {
        InitializeComponent();
        textBox1.KeyPress += Handler;
        button1.Click += Handler;
        textBox1.KeyPress += new KeyPressEventHandler(Handler);
        button1.Click += new EventHandler(Handler);
    }

    private void Handler(object sender, EventArgs e)
    {
        label1.Visible = true;
    }

You can see here the Handler() method being registered for both the TextBox.KeyPress event and the Button.Click event, even though these events have different parameters (Button.Click expects parameters object and EventArgs while TextBox.KeyPress expects object and KeyPressEventArgs).

What’s going on?

Without specific compiler support, this wouldn’t work. But, variance comes into play. The compiler recognises that the second parameter of Handler() is of type EventArgs - which is sufficiently general to accept any possible value that might be supplied by the event. Based on this observation, the compiler hooks up the event without complaint.

Comments

blog comments powered by Disqus
Next Post
TechEd New Zealand 2009  19 Sep 2009
Prior Post
Coming up to speed  10 Sep 2009
Related Posts
Caching without Race Conditions  13 Jun 2020
Improved Caching  09 May 2020
Caching Speech  25 Apr 2020
Speech Middleware  11 Apr 2020
Redux Middleware Implementation  28 Mar 2020
Redux Middleware  14 Mar 2020
Always review code you copy  29 Feb 2020
Speech API  15 Feb 2020
Logging Implementation  25 Jan 2020
Logging Demonstrated  11 Jan 2020
More csharp posts »
Archives
September 2009
2009