Event handling in asp.net

Yes and no. You can actually hook events in the code behind. Use visual studio you get a little help creating the handlers. So on page load to hook the click event to the button you would do:

ButtonObject.Click+=new EventHandler(Button2_Click);

Now where the help comes in is after you type "+=" you press tab twice which auto generates the event handler.
Now the event handler is generated without an access modifier it's just:

void Button2_Click(object sender, EventArgs e)
    {
        throw new NotImplementedException();
    }

no modifier means that it's private

You can keep it as is and remove the onclick part from the aspx page because the event handler hooks in the .cs file OR do what I do. Delete the line that hooks the event which is: "ButtonObject.Click+=new EventHandler(Button2_Click);" and add the protected modifier to the handler so it looks like this:

protected void Button2_Click(object sender, EventArgs e)
    {
        throw new NotImplementedException();
    }

and on the aspx have onclick="Button2_Click"

Yes it is more work in C# but there is some help. In a windows application this is even easier as you can look in the control's properties in the designer and double click the event you want.

If I double click, it opens aspx  source file,where I can say that "onclick= Button1_click. But I define Button1_click in aspx.cs file, right? So is there any way to generate Button1_click definition body ?

something like this:

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles

end sub()

and I write my code in between?

Wow that was a great explanation omega. Thank you very much. SO I  think that is more of a Vb and C# difference rather than a desktop application and web application difference. I  have one more question related to previous post:

1.  where do I write "ButtonObject.Click+=new EventHandler(Button2_Click);" and "void Button2_Click(object sender, EventArgs e)" in my aspx.cs file EXACTLY?
   

page load event

But I tested sammy197's solution and it does work, I never knew that. It seems it would be easier to go with his.

Ok I have done somehing like this:

aspx:

asp:Button ID="Button1" runat="server"
        style="z-index: 1; left: 277px; top: 277px; position: absolute"
        Text="Button" onclick="Button1_Click" />

aspc.cs:

namespace MyFirstWebApplication
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            Button1.Click += new EventHandler(Button1_Click);
        }
        private void Button_click(object sender, EventArgs e) {
            Button1.Text = " hello";  
        }

    }

It tells me that Button1_click  Does nto exist in current context. Hows that?

oh I'm sorry where I say
"so change it to:
<asp:Button ID="Button1" runat="server"
        style="z-index: 1; left: 277px; top: 277px; position: absolute"
        Text="Button" onclick="Button1_Click" /> "

I meant
so change it to:
<asp:Button ID="Button1" runat="server"
        style="z-index: 1; left: 277px; top: 277px; position: absolute"
        Text="Button" />

Okie I did exactly as you said in scenario 1:

aspx:

<asp:Button ID="Button1" runat="server"
        style="z-index: 1; left: 277px; top: 277px; position: absolute"
        Text="Button" />  
</div>

aspx.cs

namespace MyFirstWebApplication
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        { Button1.Click += new EventHandler(Button1_Click); }


        protected void Button1_Click(object sender, EventArgs e)
        {
            Button1.Text = " hello";  
        }
    }      
}  


Good news : No error
Bad news: Text property of the button is not changed when I click it on runtime