How to get the Value of a ReadOnly Asp:TextBox in the Code Behind.

The above one you can do it in 2 ways:

1. Create a hidden field and bind the value to it using Jquery and access that in C#.

2. The Second Method is simply adding a Read Only attribute to the Text Box
in the Code behind during !IsPostback in the page Load Event.

 See the Example:--
In the aspx file: 

 <asp:TextBox ID="txtCheckin" runat="server"></asp:TextBox>

In the Code behind(C#):

protected void Page_Load(object sender, EventArgs e)
{ 

   if (!IsPostBack)
   {
      txtCheckin.Attributes.Add("readonly", "readonly");

}

}

Now you can access the value of the txtCheckin with ReadOnly. 

Note: Don't Write ReadOnly="True" in the asp:TextBox Tag.