public partial class MyPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //snip
        MyButton.Click += (o, i) =>
        {
            //snip
        }
    }
}

public partial class MyPage : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //snip
        MyButton.Click += new EventHandler(delegate (Object o, EventArgs a)
        {
            //snip
        });
    }
}

MyButton.Click -= (o, i) =>
{
    //snip
}

EventHandler handler = (s, e) => MessageBox.Show("Woho");

button.Click += handler;
button.Click -= handler;