Question

What’s wrong with the code?
Why on the second report shows a mistake?

string level;
int key;

command.CommandText = "SELECT * FROM user WHERE name = 'admin'";

connection.Open();
Reader = command.ExecuteReader();

while (Reader.Read())
{
    level = Convert.ToString(Reader["level"]);
    key = Convert.ToInt32(Reader["key"]);

    MessageBox.Show(level); //Work fine
}

MessageBox.Show(level); //Show error:  Use of unassigned local variable 'level'

Answer 1

The compiler has no way of knowing level got a value. For all it knows, Reader.Read() always returns false, thus leaving level without a value.

The most common solution to this is to just initialize level to null (or I agree with AdaTheDev, string.Empty might be a good choice here too)