How to handle unhandled(nullexception) exception in button handlers in xamarin.android

Question

1.00/5 (1 vote)

See more:

, +

I was making a sample android app to test things out and the error is as follows on creating a button handler, though my build succeeded.

Please help

Error: Unhandled Exception: System.NullReferenceException: Object reference not set to an instance of an object. occurred

Code is as follows:

using Android.App;
using Android.Widget;
using Android.OS;

namespace Android_Picture
{
[Activity(Label = "Android Picture", MainLauncher = true, Icon = 
"@drawable/icon")]
public class MainActivity : Activity
{
    Button ButtonPrev;
    Button ButtonNext;
    TextView TextTitle;
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

        // Set our view from the "main" layout resource
        // SetContentView (Resource.Layout.Main);

        ButtonPrev = FindViewById<Button>(Resource.Id.buttonPrev);
        ButtonNext = FindViewById<Button>(Resource.Id.buttonNext);
        TextTitle = FindViewById<TextView>(Resource.Id.textTitle);

        ButtonPrev.Click += ButtonPrev_Click; //error
        ButtonNext.Click += ButtonNext_Click;
    }

    private void ButtonNext_Click(object sender, System.EventArgs e)
    {
        TextTitle.Text = "Next Clicked";
        //throw new System.NotImplementedException();
    }

    private void ButtonPrev_Click(object sender, System.EventArgs e)
    {
        TextTitle.Text = "Previous Clicked";
        //throw new System.NotImplementedException();
    }
}
}

My Main.axml is as follows:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<Button
    android:text="Prev"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/buttonPrev"
    android:layout_alignParentBottom="true" />
<Button
    android:text="Next"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/buttonNext"
    android:layout_alignParentRight="true"
    android:layout_alignParentBottom="true" />
<TextView
    android:text="Medium Text"
    android:textAppearance="?android:attr/textAppearanceMedium"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textTitle"
    android:layout_centerHorizontal="true"
    android:layout_alignParentBottom="true"
    android:layout_marginBottom="150dp" />
</RelativeLayout>

What I have tried:

I tried uncommenting the line

SetContentView (Resource.Layout.Main);

But still getting this error

Please help!!!

Posted 11-Apr-17 9:20am

Member 13121956

Updated 11-Apr-17 13:04pm

Add a Solution

Comments

Richard Deeming 11-Apr-17 16:15pm

The error message is pretty obvious: you're trying to modify a variable which is null.

That means the FindViewById<Button>(Resource.Id.buttonPrev) call returned null for some reason.

To avoid the error, test whether the button is null before trying to attach an event handler.

To resolve the problem, debug your code and find out why the button isn't being found.

Afzaal Ahmad Zeeshan 11-Apr-17 19:07pm

Because he had intentionally hid the button, his layout inflation was commented out in the code that he has shared. Which is why, the button never got inflated and not found; see my answer as well.

1 solution

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Afzaal Ahmad Zeeshan · Answer 1 · 2017-04-11T13:04:00

Although to handle the errors, you would need to use a try...catch block, whereas in this case it would be an if...else block to be used.

ButtonPrev = FindViewById<Button>(Resource.Id.buttonPrev);
ButtonNext = FindViewById<Button>(Resource.Id.buttonNext);
TextTitle = FindViewById<TextView>(Resource.Id.textTitle);

if(ButtonPrev != null) {
   ButtonPrev.Click += ButtonPrev_Click; // Would only execute, if not null.
}

// Same here.
// ButtonNext.Click += ButtonNext_Click;

This way, you will make sure that the event handlers are only associated if the objects exist, because if the buttons do not exist — there are several reasons for this, such as wrong ID being used, buttons not available in current layout, problem inflating the layout, ID different etc. — you do not need to even implement any handler on them. That would also make sure that the errors do not get raised.

The main error

Whereas, the main problem in your code is that you have commented out the layout; if the layout is included, and the button exists, there is no way that a button would end up null. Where as, in your code, you have intentionally done that.

// SetContentView (Resource.Layout.Main);

Uncomment this line, and it would work.