so if you have a checkbox for each row of the grid using template field, and upon checking the checkbox you want to show a textbox as visible? try this solution below, you can convert it to c#.
For Each row As GridViewRow In GridView.Rows
Dim cbb As CheckBox = DirectCast(row.FindControl("CheckBox1"), CheckBox)
Dim txt As Textbox = DirectCast(row.FindControl("textbox1"), textbox)
If checkbox.Checked Then
Dim rowid As Integer = Convert.ToInt32(GridView.DataKeys(row.RowIndex).Value)
If cbb.Checked = False Then
txt.Visible = False
End If
Next