Xamarin FindViewById returns null

Question

I am trying to make an android app using Xamarin so C#. I made two layouts and in each one of them I made tow buttons to navigate between them.I tried like this:

using System;

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


namespace Example
{
    [Activity(Label = "Example", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);
            this.SetContentView(Resource.Layout.Main);

            this.FindViewById<Button>(Resource.Id.ForwardButton).Click += this.Forward;
            this.FindViewById<Button>(Resource.Id.BackButton).Click += this.Back;
        }

        public void Forward(object sender, EventArgs e)
        {
            this.SetContentView(Resource.Layout.Main2);
        }

        public void Back(object sender, EventArgs e)
        {
            this.SetContentView(Resource.Layout.Main);
        }
    }
}

But every time when I start the app I get this errror: System.NullReferenceException has been thrown.Object reference not set to an instance of an object. Any advice or better idea?

Answer 1

You are setting Main as the layout for your activity and then in the following lines of code you are asking to find a button named Back at runtime which is not part of this layout. This means the following line will return null:

FindViewById<Button>(Resource.Id.BackButton)

Now if you do FindViewById<Button>(Resource.Id.BackButton).Click, you will definitely get a System.NullReferenceException.

EDIT:

In view of the comments, here is what you should do to achieve what you are looking for:

Create two different activities (Main1 and Main2). In Main1 you do:

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        this.SetContentView(Resource.Layout.Main);

        this.FindViewById<Button>(Resource.Id.ForwardButton).Click += this.Forward;
    }

    public void Forward(object sender, EventArgs e)
    {
        this.StartActivity (typeof(Main2));
    }

Then in Main2, you do:

    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);
        this.SetContentView(Resource.Layout.Main2);

        this.FindViewById<Button>(Resource.Id.BackButton).Click += this.Back;
    }

    public void Back(object sender, EventArgs e)
    {
        this.StartActivity (typeof(Main));
    }

Answer 2

Since it just happened to me and almost drove me up the wall, here is yet another way FindViewById can return null and how to fix it. Probably not a mistake a more experienced Android developer is likely to make, but still, it might prevent some frustration for someone like me in the future.

My situation

• My control was in the correct view (not a fragment), the ID was assigned correctly, everything used to work just a couple of days ago when I last worked on the project.

• After building the project, the very first FindViewById call in OnCreate of my main view suddenly returned null when run on a physical device

• It turns out I had just run some updates through the Android SDK Manager, which introduced a new Android Version (in this case Android 7) to my system, and my device simply did not have that version installed yet.

The solution

• In the properties of my Android project, the "Compile using Android version" drop down was set to "Use Latest Platform", which now was pointing to the newly installed Android 7 -- simply setting it to the version of Android running on my test device (in my case 6) and recompiling fixed the null return value.

TL;DR Check that the version of Android you are compiling against is in fact supported by the device you are testing on.