The name 'txtBAvail' does not exist in the current context

Hi all,
      every time I drag a control from tool bar and if I use it by it's default name like TextBox1,TextBox2,TextBox3...so on (for every type of control for particular this webform) it's working fine, but as I change these control's ID to my desired ID like txtName, txtLastName.....
it shows error
The name 'control's ID(which I have defined)' does not exist in the current context.
for every control which I rename  ID
help me out pls...
thanks

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