Checking User is present in Db or Not
Select Usenname from Users ;
Then Compare With User entered Username with Db UserName
Like This
Sqlconnection con=new sqlconnection(Your connection string);
Sqldataadapter da=new Sqldataadapter("Select Usenname from Users",con);
dataset ds=new dataset();
if(textbox1.text==ds.tables[0].rows[0][0].tostring())
{
lblmsg.Text="Please Choose another User"
}
else
{
}