使用IF ELSE语句围绕SQL使用C＃

Question

这是我的程序，我需要验证用户名或密码是否已经存在，但是如果我为验证添加另一个使用语句，它仍会通过数据库。我应该如何使用IF内有如SQL语句（ “@用户名” == _f3txtuser.text）

private void _F3Register_Click(object sender, EventArgs e) 
    { 


     if (_F3txtUser.Text == "" || _F3txtPass.Text == "") 
      MessageBox.Show("Please Fill The Missing Informations"); 
     else if (_F3txtPass.Text != _F3txtCheckPass.Text) 
      MessageBox.Show("Password Do Not Match"); 

     else if (("@USERNAME" == _F3txtUser) || ("@PASSWORD" == _F3txtPass)) 
      MessageBox.Show("Username or Password already Exists"); 

     else 
     { 
      using (SqlConnection _sqlcon = new SqlConnection(connectionstring)) //Database Connection 
      { 
       _sqlcon.Open(); 


       String query2 = "Insert into LOGINFORM(USERNAME, PASSWORD, FNAME, LNAME, AGE) values(@USERNAME, @PASSWORD, @FNAME, @LNAME, @AGE)"; // Database Command for Insertion 

       using (SqlCommand _sqlcmd = new SqlCommand(query2, _sqlcon)) //using database connection and command in one method 
       { 
        _sqlcmd.Parameters.AddWithValue("@USERNAME", _F3txtUser.Text.Trim()); 
        _sqlcmd.Parameters.AddWithValue("@PASSWORD", _F3txtPass.Text.Trim()); 
        _sqlcmd.Parameters.AddWithValue("@FNAME", _txtFN.Text.Trim()); 
        _sqlcmd.Parameters.AddWithValue("@LNAME", _txtLN.Text.Trim()); 
        _sqlcmd.Parameters.AddWithValue("@AGE", _txtAge.Text.Trim()); 
        _sqlcmd.ExecuteNonQuery(); 
       } 
       MessageBox.Show("Registration Successfull"); 
       Clear(); 

      } 
     } 
    } 
    void Clear() 
    { 
     _F3txtUser.Clear(); 
     _F3txtPass.Clear(); 
     _txtFN.Clear(); 
     _txtLN.Clear(); 
     _txtAge.Clear(); 
     _F3txtCheckPass.Clear(); 
    } 
} 

} ELSE语句

Answer 1

您可以像下面的方法：

private bool UserNameOrPasswordExists(string userName, string password) 
    { 
     using (SqlConnection conn = new SqlConnection("your connection string")) 
     { 
      conn.Open(); 
      var query = "Select * from LOGINFORM where USERNAME='@USERNAME' or PASSWORD='@PASSWORD'"; 
      using (SqlCommand comm = new SqlCommand(query, conn)) 
      { 
       comm.Parameters.AddWithValue("@USERNAME", userName); 
       comm.Parameters.AddWithValue("@PASSWORD", password); 
       var result = comm.ExecuteNonQuery(); 
       return result > 0; 
      } 
     } 
    } 

然后用此代替第三，如果其他人：

else if (UserNameOrPasswordExists(_F3txtUser, _F3txtPass)) 
     MessageBox.Show("Username or Password already Exists");