C # Winforms

Question

I am new to C #
I have made one super market project
In that I used login form It working properly but it at one place it will not work properly
textbox---     'txtuser'
textbox----    'txtPass'
button----     'btnlogin'
  my form has contlos
defaulut I set my form propertis
accept-----btnlogin
txtUser ===== Auto complete costume source  some strings
while user enter in few Text(use) in  txtuser it will give some suggestion
like
user123
user1234
user234
while any one of selecting and press enter Textbox will select that username but in my application it accepts string and btnlogin event also
how to prevent that btnlogin event while i selecting that particular exception