Program crashes if returned value is null

Question

hi, 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 !!!");

}

//reading 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();

Answer 1

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

Answer 2

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!".

Answer 3

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.