"Object reference not set to an instance of an object" in Xamarin.android c#

Question

I was just trying to make a simple app with Xamarin.android from the tutorial shown https://learn.microsoft.com/en-us/learn/modules/create-a-mobile-app-with-xamarin-forms/5-exercise-create-phone-number-translator-app But when I build it, application crashes. I tried debugging and got an error in OnTranslate method as System.NullReferenceException: Object reference not set to an instance of an object.

I tried setting values of declared variables as null. Tried using null operator.

This is my code

    {
        Entry phoneNumberText = null;
        Button translateButton = null;
        Button callButton = null;
        string translatedNumber;
        public MainPage()
        {
           /*
               Some design code
           */
            translateButton.Clicked += OnTranslate;
            this.Content = panel;
        }

         void OnTranslate(object sender, EventArgs e)
        {
            string enterednum = phoneNumberText.Text;
            translatedNumber = Core.PhonewordTranslator.ToNumber(enterednum);

            if (!string.IsNullOrEmpty(translatedNumber))
            {
                callButton.IsEnabled = true;
                callButton.Text = "Call" + translatedNumber;
            }
            else
            {
                callButton.IsEnabled = false;
                callButton.Text = "Call";
            }
        }

I have set the values for entry and those two buttons in my design code. And this is the method to translate string to number

        {
            if (string.IsNullOrWhiteSpace(raw))
                return null;
            raw = raw?.ToUpperInvariant();
            var newNumber = new StringBuilder();

            foreach (var c in raw)
            {
                if (" -0123456789".Contains(c))
                    newNumber?.Append(c);
                else
                {
                    var result = TranslateToNumber(c);
                    if (result != null)
                        newNumber.Append(result);
                    //bad string?
                    else return null;
                }
            }
            return newNumber?.ToString();
        }

        static bool Contains(this string keystring, char c){
            return keystring?.IndexOf(c) >= 0;
            }

        static readonly string[] digits =
        {
            "ABC", "DEF", "GHI", "JKL", "MNO", "PQRS", "TUV", "WXYZ"
        };
        static int? TranslateToNumber(char c)
        {
            for(int i = 0; i<digits.Length; i++)
            {
                if (digits[i].Contains(c))
                    return 2 + i;
            }
            return null;
        }
    }

the stack trace of the exception object should tell you which specific line caused the exception, or you can use the debugger to step through the code until you identify the cause. This is basic c# debugging — Jason
– Jason, Commented Jul 11, 2019 at 7:36
debugging stops automatically at translateButton.Clicked += OnTranslate; — Vikesh Patil
– Vikesh Patil, Commented Jul 11, 2019 at 7:51
This doesn't depend on Xamarin, you're quite clearly calling something on a null object. You assigned null to it instead of an instance, in the first place. Consider having a look at the basics first. — StackLloyd
– StackLloyd, Commented Jul 11, 2019 at 8:12

Jason · Accepted Answer · 2019-07-11 07:55:46Z

2

debugging stops automatically at translateButton.Clicked += OnTranslate;

that should tell you something. Here you're declaring a button but never instantiating it

Button translateButton = null;

so later when you attempt to assign an event handler, your button is still null, which causes the exception

translateButton.Clicked += OnTranslate;

you need to instantiate the button first

translateButton = new Button();

answered Jul 11, 2019 at 7:55

Jason

89.5k15 gold badges137 silver badges151 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Vikesh Patil Over a year ago

Thanks this solved my NullReferenceException but my method still not working when I clicked on that button. I will look what I can do. Thank you for your time.

Collectives™ on Stack Overflow

"Object reference not set to an instance of an object" in Xamarin.android c#

1 Answer 1

1 Comment

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Linked

Related