This article is for beginners who are learning. NET. This will be really helpful for them. Beginners will be wondered how to pass values between User Controls and ASPX page. In this article, I have listed some of the ways through which you can access and set the User control values in an ASPX page. I started of with some of the basics, advantages, and disadvantages of user controls.
User Control
User Control is the custom, reusable controls. User Controls offers a way to partition and reuse User Interface (UI) functionality across ASP.NET Applications.
Advantages of User Control
Disadvantages of User Control
Adding User Controls to a WebForms Page
At the top of the .aspx page, add the below line above <Html> tag.
  1. <%@ Register TagPrefix="Test" TagName="TestControl" Src="Test.ascx" %>
This directive registers the control so that it can be recognized when the page is processed. TagPrefix determines the unique namespace of the control, TagName is the name of the user control and Src is the path of the user control.
Declare user controls like
  1. <Test:TestControl id="TestControl" runat="Server"/>
Accessing and Setting User Controls Values in the .aspx Page
User can access and set the values of the User Control from .aspx page through properties, using javascript and in code-behind of aspx page. The details of it are shown below
1) Using Properties
If the test.ascx control has two textboxes and submit button. You can access the values of the textboxes in the control from a .aspx page by declaring public property in the .ascx page.
  1. public string FirstName
  2. {
  3. get{return txtFirstName.Text;}
  4. set{txtFirstName.Text = value;}
  5. }
In .aspx page,you can access FirstName using
TestControl.FirstName
You can set the FirstName of the control from aspx page using
TestControl.FirstName = "Suzzanne"
2) Using Javascript
You can set the values of the controls declared in the .ascx page by
  1. document.forms[0]['TestControl:txtFirstName'].value ="Suzzanne";
You can get the values of the controls declared in the .ascx page by
  1. document.forms[0]['TestControl:txtFirstName'].value
3) In ASPX code behind file
  1. TestControl objTestControl = (TestControl)Page.FindControl("TestControl");
  2. TextBox objTextBox = objTestControl.FindControl("txtFirstName");
  3. string strFirstName = objTextBox.Text;
This will find the control named TestControl declared in the aspx page and look for the TextBox named txtFirstName in the user control and instantiate object of it. In this way also you can access the value of the user control. This is not recommended as it consumes more memory.