Question

I am just trying my hand at some WinForm Applications and was creating a simple event handler, but I get an error message. Code:

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public delegate void MyHandler1(object sender, EventArgs e);

        public Form1()
        {
            InitializeComponent();

            List<string> names = new List<string>();
            names.Add("S");
            names.Add("I");
            names.Add("G");

            MyHandler1 onClicked = new MyHandler1(clicked);

            listBox1.DataSource = names;
            listBox1.Click += onClicked;


        }

        public void clicked(object sender, EventArgs e)
        {
            label1.ResetText();
            label1.Text = listBox1.SelectedItem.ToString();
        }
    }

}

Error:

Error   1   Cannot implicitly convert type 'WindowsFormsApplication1.Form1.MyHandler1' to 'System.EventHandler'

Answer 1

The reason that your code doesn’t compile is that implicit conversions do not exist between different delegate-types, even when the signatures are ‘compatible’.

Try either of these:

// Implicit method-group conversion, should work from C# 2.0 or later.
// Essentially shorthand for listBox1.Click += new EventHandler(clicked);
listBox1.Click += clicked; 

// Creating a delegate-instance from a 'compatible' delegate,
// a trick I recently learnt from his highness Jon Skeet
listBox1.Click += new EventHandler(onClicked);

As an aside, unless the intention is to learn how to use delegates, I suggest you don’t create your own delegate-type when one that comes with the framework will do the job.