You can apply the VB.Net for each loop to loop over each row item of GridView control and use the VB.Net FindControl method to get the checkbox control placed inside the ASP.net GridView control. Set the Checked property of each checkbox retrieved using VB.Net FindControl method from each row of GridView equal to true that will select all the checkboxes placed inside the GridView control.
VB.Net Methods Used
1. For each loop
2. FindControl method
