How to add code-behind to a User Control?

Question

I have a User Control (myUserControl.ascx) with the code:

<div id="content"> 
    TEST USER CONTROL
    <div>

that is called from a webpart:

protected override void OnLoad(EventArgs e) {
 Control c = Page.LoadControl(_ascxPath);
   if (c != null)
            {
                this.Controls.Add(c);
            }
}

since here it's all ok.

Now I want to add a code-behind to this User Control

1) I created myUserControl.ascx.cs

public partial class SP4ITeditXML : UserControl
    {

 public string select1 = "test text";
}

2) I changed the myUserControl.ascx:

<%@ Control Language="C#" CodeBehind="myUserControl.ascx.cs"%>
<div id="content"> 
        TEST USER CONTROL
 <% Response.Write(this.select1);  %>
        <div>

but then I get the error CS0117 (myUserControl.ascx not contains a definition for select1).

What I'm doing wrong?

Answer 1

Commented migrated to answer:

Add the User Control via the templates, do Project -> Add -> New Item -> User Control.

This creates the ASCX and the associated CS file.

Answer 2

1. Compile the myUserControl.ascx.cs to a signed assembly.
2. Reference the myUserControl.ascx.cs with the full qualified name in the myUserControl.ascx
3. Add the assembly to the GAC.