C# – Custom Control loads a user control; Postback events are not triggered

The SelectedIndexChanged event (on a listbox) and the TextChanged event (on a Textbox) will fire on a postback even if the Autopostback property is false.

Try putting this on a form:

   <asp:ListBox runat="server" ID="test" 
        onselectedindexchanged="test_SelectedIndexChanged">
<asp:ListItem >number1</asp:ListItem>
 <asp:ListItem >number2</asp:ListItem>     
</asp:ListBox>

<asp:TextBox runat="server" ID="text" ontextchanged="text_TextChanged" />
<asp:Button runat="server" Text="Click Me" />

and this in the code behind:

 protected void test_SelectedIndexChanged(object sender, EventArgs e)
    {
        Response.Write("SelectedIndexChanged");
    }

    protected void text_TextChanged(object sender, EventArgs e)
    {
        Response.Write("TextChanged");
    }

Then run it, change the selected item, type some text, and hit the button - both events fire. You can see the sequence of events using Reflector. For the Textbox, the RaisePostDataChangedEvent method is:

 protected virtual void RaisePostDataChangedEvent()
{
    if (this.AutoPostBack && !this.Page.IsPostBackEventControlRegistered)
    {
        this.Page.AutoPostBackControl = this;
        if (this.CausesValidation)
        {
            this.Page.Validate(this.ValidationGroup);
        }
    }
    this.OnTextChanged(EventArgs.Empty);
}

You do this via attributes on the properties, like this:

[Description("Test text displayed in the textbox"),Category("Data")] 
public string Text {
  get => myInnerTextBox.Text;
  set => myInnerTextBox.Text = value;
}

The category is the heading under which the property will appear in the Visual Studio Properties box. Here's a more complete MSDN reference, including a list of categories.