NullReferenceException while using String.StartsWith()

Question

I was trying to make a program that compiles a piece of code written in a BASIC variation i'm making.

I tried to figure out the problem but I couldn't. I changed the "String.StartsWith()" to String == "something" and it worked, but I need to use StartsWith in this part.

        private String ComprobarCmd(String txt)
        {
            String texto = txt;
            if (texto.StartsWith("CLS")) return "1";
            if (texto.StartsWith("SETCURSOR")) return "2";
            if (texto.StartsWith("PRINT")) return "3";
            if (texto.StartsWith("END")) return "1000";
            return "F";
        }

I expected it to return a string, but it just gave me this error (it's in Spanish):

************** Texto de la excepción **************
System.NullReferenceException: Referencia a objeto no establecida como instancia de un objeto.
   en JBasic_Compiler.Form1.ComprobarCmd(String txt) en D:\Programas\Proyectos VB\JBasic Compiler\JBasic Compiler\JBasic Compiler\Form1.cs:línea 67
   en JBasic_Compiler.Form1.Compilar(String path) en D:\Programas\Proyectos VB\JBasic Compiler\JBasic Compiler\JBasic Compiler\Form1.cs:línea 54
   en JBasic_Compiler.Form1.Form1_Load(Object sender, EventArgs e) en D:\Programas\Proyectos VB\JBasic Compiler\JBasic Compiler\JBasic Compiler\Form1.cs:línea 28
   en System.Windows.Forms.Form.OnLoad(EventArgs e)
   en System.Windows.Forms.Form.OnCreateControl()
   en System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
   en System.Windows.Forms.Control.CreateControl()
   en System.Windows.Forms.Control.WmShowWindow(Message& m)
   en System.Windows.Forms.Control.WndProc(Message& m)
   en System.Windows.Forms.ScrollableControl.WndProc(Message& m)
   en System.Windows.Forms.Form.WmShowWindow(Message& m)
   en System.Windows.Forms.Form.WndProc(Message& m)
   en System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   en System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   en System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

If `texto` is null, what do you think will happen when you call `StartsWith` against it? You might need to check your inputs first. — Brendan Green, Jul 24 '19 at 01:27

score 0 · Accepted Answer · answered Jul 24 '19 at 01:31

You need to handle case when txt is null. Add at the beginning of method: if (txt == null) return "text to return if txt is null"; And that will be all.

Plus, read more about nulls and objects.

Double eqaulity sign worked because you can compare static string to null. But startsWith is a method on string object and you need to have this object instantiated to call it.

NullReferenceException while using String.StartsWith()

1 Answers1