How do I solve a csharp object reference required error?

Quote:

This is an error that says objectreference is not on an object, i get this on the

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!

Start by fixing the SQL Injection[^] vulnerability in your code.

If you want to invoke the method using the class name, rather than an instance of the class, then the method needs to be static.

You won't be able to access the instance fields conn and medewerkers from a static method. You could just make those members static as well, but that would lead to problems if multiple threads called your method at the same time. Instead, since they're only used within the method, make them local variables.

Since everything in the class is now static, you can mark the class itself as static. This will prevent you from accidentally adding instance members to it.

You should avoid using SELECT * FROM ...; instead, you should explicitly list the columns you want to load. I've assumed that the column names match the property names on your class.

static class Verzamel
{
    private static readonly string connectionString = ConfigurationManager.ConnectionStrings["TimeReg.Properties.Settings.TimeRegConnectionString"].ConnectionString;
    
    public static List<Medewerkers> GetAllMedewerkers(string filter)
    {
        List<Medewerkers> medewerkers = new List<Medewerkers>();
        
        using (SqlConnection conn = new SqlConnection(connectionString))
        using (SqlCommand cmd = new SqlCommand("", conn))
        {
            string query = "SELECT M.Personeelsnummer, M.LoginID, M.Voornaam, M.Tussenvoegsel, M.Achternaam FROM Medewerkers M";
            
            if (!string.IsNullOrEmpty(filter))
            {
                query += " WHERE M.Voornaam LIKE @filter OR M.Achternaam LIKE @filter";
                cmd.Parameters.AddWithValue("@filter", "%" + filter + "%");
            }
            
            cmd.CommandText = query;
            
            conn.Open();
            using (SqlDataReader reader = cmd.ExecuteReader())
            {
                while (reader.Read())
                {
                    medewerkers.Add(new Medewerkers
                    {
                        Personeelsnummer = reader.GetInt32(0),
                        LoginID = reader.GetInt32(1),
                        Voornaam = reader.GetString(2),
                        Tussenvoegsel = reader.GetString(3),
                        Achternaam = reader.GetString(4),
                    });
                }
            }
        }
        
        return medewerkers;
    }
}

---

Everything you wanted to know about SQL injection (but were afraid to ask) | Troy Hunt[^]
How can I explain SQL injection without technical jargon? | Information Security Stack Exchange[^]
Query Parameterization Cheat Sheet | OWASP[^]

I was to early

Now i get an error:

System.NullReferenceException
  HResult=0x80004003
  Message=De objectverwijzing is niet op een exemplaar van een object ingesteld.
  Source=TimeReg
  StackTrace:
   at TimeReg.Medewerker.LoadMedewerkers() in 
\source\repos\TimeReg\Medewerker.cs:line 33
   at TimeReg.Medewerker..ctor() in 
\source\repos\TimeReg\Medewerker.cs:line 22
   at TimeReg.Form1.button3_Click(Object sender, EventArgs e) in \source\repos\TimeReg\Form1.cs:line 201
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
   at System.Windows.Forms.Application.Run(Form mainForm)
   at TimeReg.Program.Main() in 

This is an error that says objectreference is not on an object, i get this on the

List<medewerkers> medewerkers = Verzamel.GetAllMedewerkers(filter);