Program crashes if returned value is null

i am trying to retrieve some data from mysql(c#,mono), the problem is that if the returned value is null the program crashes, i know it has to do something with the returned null value, because if i want to retrieve something that is in the database it works,can anyone help me with this?

The code:

MySqlConnection dbcon;
dbcon = new MySqlConnection(conString);
try
{
    dbcon.Open();
}
catch (Exception)
{
    Console.WriteLine("MySQL Database Connection Problem !!!");

}

//readin开发者_JAVA技巧g data from mysql   
MySqlCommand dbcmd = new MySqlCommand(sql, dbcon);

MySqlDataReader reader = dbcmd.ExecuteReader();

while(reader.Read()){

    txtFirstname.Text = reader["first_name"].ToString();
    txtLastname.Text  = reader["last_name"].ToString();
    imgUser.File      = path+reader["photo"].ToString();
    expDate           = reader["expiration_datetime"].ToString();
    usrName           = reader["username"].ToString();

}

dbcon.Close();

You are calling the ToString method on the objects returned by MySQL.

If MySQL returns null, you'll call the ToString method on a null object, giving a NullReferenceException.

Assuming that the SQL is actually returning strings (not numbers or dates), you can simply cast to a string, like this: (string)reader["username"].
If the SQL is returning non-string datatypes, you can call Convert.ToString(reader["username"]).

Both of these will result in null if MySQL returns null.

If you want MySQL nulls to result in something other than null, use the null coalescing operator, like this: Convert.ToString(reader["username"]) ?? "It's Null!".

For instance, let's imagine your "first_name" is null at database; reader["first_name"] will return null and it doesn't have a .ToString() method, so your code fails.

Below, I changed way you get that files; I try to cast it to string and, if I got a null value, I use that ?? (null-coalescing operator) to return an empty string.

txtFirstname.Text = reader["first_name"  ] as string ?? "";
txtLastname.Text  = reader["last_name"   ] as string ?? "";
imgUser.File      = path + reader["photo"] as string ?? "";
expDate           = reader["expiration_datetime"] as string ?? "";
usrName           = reader["username"] as string ?? "";

HTH

Based on the exception in your comment, it looks like you have a completely different problem somewhere else.

I would guess that you're reading a .resources file and generating a filename from the SQL query. If so, nulls might cause you to generate an incorrect filename.

You need to debug your code.

so i guess it will help to post the whole code, here it is. i am just trying to activate an account in ldap, by scanning the barcode which is printed on a card. i also retrieve information about the user of that account, from mysql database.

using System;
using System.Collections;
using Gtk;
using MySql.Data.MySqlClient;
using Novell.Directory.Ldap;


public partial class MainWindow: Gtk.Window
{

    public MainWindow (): base (Gtk.WindowType.Toplevel)
    {
        Build ();
    }

protected void OnDeleteEvent (object sender, DeleteEventArgs a)
{
    Application.Quit ();
    a.RetVal = true;
}

protected virtual void OnEntry1FocusOutEvent (object o, Gtk.FocusOutEventArgs args)
{
    string conString = "server=localhost;database=ldap;User ID=someid;password=somepassword;";
    string sql = String.Format("Select * From user where barcode='{0}'",txtBarcode.Text);
    string path = "/home/path/to/project/Photo/";
    string expDate = "";
    string usrName = "";
    //creating connection with database
    MySqlConnection dbcon;
    dbcon = new MySqlConnection(conString);
    try

    {

        dbcon.Open();

    }

    catch (Exception ex)

    {
        Console.WriteLine(ex.ToString());

        //Console.WriteLine("MySQL Database Connection Problem !!!");

    }
    //reading data from mysql
    MySqlCommand dbcmd = new MySqlCommand(sql, dbcon);

    MySqlDataReader reader = dbcmd.ExecuteReader();

    while(reader.Read()){


        txtFirstname.Text = Convert.ToString(reader["first_name"]);
        txtLastname.Text  = Convert.ToString(reader["last_name"]);
        imgUser.File      = path+Convert.ToString(reader["photo"]);
        expDate           = Convert.ToString(reader["expiration_datetime"]);
        usrName           = Convert.ToString(reader["username"]);

        }

    dbcon.Close();

    //changeing time from sting to datetime
    DateTime dt = Convert.ToDateTime(expDate);
    DateTime now = DateTime.Now;
    txtStatus.Text = dt.ToString();
    //checking if the account has expired
    if(dt > now){
        //connecting with ldap server
        string server = "ip of the server";
        string dn = "cn=Manager,dc=itc";
        string up = "password";
        string dn1 = "uid="+usrName+",ou=people,dc=itc";

        // Create a new LdapConnection instance
        LdapConnection connl = new LdapConnection();

        // Connect to the server
        connl.Connect(server, 389);

        //Bind
        connl.Bind(dn,up);

        //Modify user in ldap entry
        ArrayList modList = new ArrayList();
        LdapAttribute attribute;
        //Replace the value of loginshell attribute
        attribute = new LdapAttribute( "loginShell", "/bin/sh");
        modList.Add( new LdapModification(LdapModification.REPLACE, attribute));

        LdapModification[] mods = new LdapModification[modList.Count];  
        //Type mtype=Type.GetType("Novell.Directory.LdapModification");
        mods = (LdapModification[])modList.ToArray(typeof(LdapModification));

        //Modify the entry in the directory
        connl.Modify ( dn1, mods );
        txtStatus.Text="Account Activated!";
        // Disconnect from server
        connl.Disconnect();
    }else{
        txtStatus.Text="Account Expired!";

    }
}

}