Co- and Contra-Variance in .NET 3.5

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.