This effective solution has been thoroughly tested and proven to succeed by utilizing solely JavaScript (without the need for jQuery).

1. C# Part (Within Page_Load Method)

To implement this functionality, a minor tweak is required in the Page_Load:

foreach(GridViewRow row in YourGridViewControlID.Rows)
{
    if (row.RowType == DataControlRowType.DataRow )
    {
      ((CheckBox) row.FindControl("YourCheckBoxID")).Attributes.Add("onchange", "javascript:TextboxAutoEnableAndDisable(" + (row.RowIndex ) + ");");
    }
}

This enhancement ensures that the JavaScript function is invoked on the OnChange event of each individual CheckBox within the GridView. The ability to pass the Row Index as a parameter distinguishes this approach from traditional HTML methods.

2. Key Considerations for the HTML Component

It is crucial to maintain static IDs for both the Checkbox and Textbox controls, as well as the GridView Control using the attribute ClientIDMode="Static" as illustrated below:

<asp:CheckBox ID="YourCheckBoxID" runat="server" ClientIDMode="Static"/>
<asp:TextBox ID="YourTextBoxID" TextMode="SingleLine" runat="server" ClientIDMode="Static" />

For the GridView control:

<asp:GridView ID="YourGridViewControlID" ...... ClientIDMode="Static" runat="server">

3. Javascript Component

In your JavaScript file/code:

function TextboxAutoEnableAndDisable(Row) {
  // Obtain the current active row within the GridView.
  var GridView = document.getElementById('YourGridViewControlID');
  var currentGridViewRow = GridView.rows[Row + 1];

  // Identify the relevant controls.
  var rowTextBox = currentGridViewRow.cells[2].getElementsByTagName("input")[0];
  var rowCheckbox = currentGridViewRow.cells[0].getElementsByTagName("input")[0];

  // Handle scenario when checkbox is unchecked.
  if( !rowCheckbox.checked) {
    // Empty textbox and disable it
    rowTextBox.value = "";
    rowTextBox.disabled = true;
    return;
  }

  // Enable the textbox if checkbox is checked.
  rowTextBox.disabled = false;
}

4. Additional Notes for Implementation

• Note that in the JavaScript code, at the line:
  

  var currentGridViewRow = GridView.rows[Row + 1];

  
  the [Row + 1] is critical for proper functioning and should remain unchanged.
• Lastly:

The following lines require attention:

var rowTextBox = currentGridViewRow.cells[2].getElementsByTagName("input")[0];
var rowCheckbox = currentGridViewRow.cells[0].getElementsByTagName("input")[0];

The values within .cells[2] and .cells[0] may vary based on your setup, necessitating correct adjustment within the brackets.
Typically, these numbers correspond to the column positions within your GridView, starting from 0.

For instance, if the CheckBox resides in the first column of the GridView, use .cells[0].
Similarly, if the TextBox occupies the second column, employ .cells[1] (in my example, the TextBox was situated in the third column, hence .cells[2] was utilized).

Upon further investigation, I discovered that using the statement if (GridView.rows[Row].cell[2].type == "checkbox") led to an error where GridView.rows[Row].cell[2].type is undefined. My current code implementation is shown below:

    var grid = document.getElementById('<%=grdResults.ClientID%>');
    if (grid.rows.length > 0) {
        for (row = 1; row < grid.rows.length; row++) {
            if (grid.rows[row].cells[0].childNodes[0].checked) {
                // performing certain actions here
                alert('function executed for row ' + row);
            }                                  
        }

The grid can be defined in the following manner:

 <div>
   <asp:GridView ID="GridView1" runat="server"  Width = "550px"
    AutoGenerateColumns = "false" Font-Names = "Calibri" 
    Font-Size = "12pt" HeaderStyle-BackColor = "LightYellow" AllowPaging ="true"  ShowFooter = "true"  OnPageIndexChanging = "OnPaging" PageSize = "10" >
   <Columns>
      <asp:TemplateField ItemStyle-Width = "100px"  HeaderText = "Name">
        <ItemTemplate>
         <asp:TextBox ID="txtPeriod" runat="server" CssClass="css1 mycss" Text='<%# Eval("Period")%>' 
             onblur="SetPostingPeriod(this)"></asp:TextBox>
        </ItemTemplate>                       
    </asp:TemplateField>   
   </Columns> 
   <AlternatingRowStyle BackColor="#C2D69B"  />
</asp:GridView> 
</div>

For the associated Javascript function:

<script language="javascript" type="text/javascript">
     /* Populating same data to all the textboxes inside grid, 
     once change of text for one textbox - by using jquery
     */
     function SetPostingPeriod(obj) {

         var cntNbr = $("#" + obj.id).val();
      // var cntNbr = document.getElementById(obj.id).value;
      // alert(cntNbr);

         //Access Grid element by using name selector
         $("#<%=GridView1.ClientID %> input[name*='txtPeriod']").each(function (index) {

             if ($.trim($(this).val()) != "")
                 if (!isNaN($(this).val())) {
                     $(this).val(cntNbr);
                 }
         });
     }
 </script>

This function is triggered by the onblur event of the textbox and accepts the textbox id as a parameter.

Inside the function, we retrieve the value of the textbox using the provided parameter:

      var cntNbr = $("#" + obj.id).val();

We then loop through each "txtPeriod" control within the grid and assign the current textbox value to them:

 $("#<%=GridView1.ClientID %> input[name*='txtPeriod']").each(function (index) {

 });

In this loop, we check if the value is not null or NaN before assigning it to other "txtPeriod" textboxes:

               if ($.trim($(this).val()) != "")
                 if (!isNaN($(this).val())) {
                     $(this).val(cntNbr);
                 }

Greetings! Here is a simple solution for you.

Imagine your grid has the following structure:

<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False" Height="64px"
    Width="389px" EnableViewState="False">
    <Columns>
        <asp:TemplateField HeaderText="EmployeeId">
            <ItemTemplate>
                <asp:Label ID="lblEmployeeId" runat="server" Text=""></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="FirstName">
            <ItemTemplate>
                <asp:Label ID="lblFirstName" runat="server" Text=""></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
        <asp:TemplateField HeaderText="LastName">
            <ItemTemplate>
                <asp:Label ID="lblLastName" runat="server" Text=""></asp:Label>
            </ItemTemplate>
        </asp:TemplateField>
    </Columns>
    <HeaderStyle BackColor="#FF66FF" ForeColor="#660033" />
</asp:GridView>

Additionally, here is the JavaScript code to locate controls within your grid:

   <script language="javascript" type="text/javascript">
        $(document).ready(function () {
            $("#btnAddToGrid").click(function () {
                var $grid = $('#<%=GridView1.ClientID %>');
                var $row = $grid.find('tr:last').clone().appendTo($grid);
                var employeeId = $row.find("span[id*='lblEmployeeId']").text();
                var firstname =  $row.find("span[id*='lblFirstName']").text();
                var lastname = $row.find("span[id*='lblLastName']").text();
              alert("ID :" + employeeId +"\n" + "Name :" + firstname +" "+ lastname );
            });
        });
    </script>

Locate and access controls within a grid using Java Script:

for (var r = 1; r < grid.rows.length; ++r) {

    var indexValue = 0; //adjust for different browsers like IE8
    var txtPeriod= grid.rows[r].cells[2].childNodes[indexValue];
    if (typeof(txtPeriod.value) == "undefined")//check for IE9                      
        indexValue = 1;
     var txtPeriod= grid.rows[r].cells[2].childNodes[indexValue];
     alert(txtPeriod.value);
}

let elements = document.getElementById('<%=grdResults.ClientID%>').querySelectorAll("input");
let count = 0;
for (let j = 0; j < elements.length; j++) {
     if(elements[j].type == "checkbox" && elements[j].checked) {
          count++;
     }
}
alert("Number of checked items: " + count);