InstantiationException: Unable to instantiate fragment make sure class name exists, is public, and has an empty constructor that is public

Question

I have a project that I originally developed using Android Studio. I decided to convert it to Xamarin (Visual Studio 2015).

After hours of porting all the code over, everything works except for my Settings activity (PreferenceActivity). I have a few PreferenceFragments that make up the settings, but all of them give me "Unable to instantiate fragment". Here is the exception I am getting:

Java.Lang.RuntimeException: Unable to start activity ComponentInfo{test.mypackagename/md50d00e677e41fc49f8b3c16e79df2b77f.SettingsActivity}: android.app.Fragment$InstantiationException: Unable to instantiate fragment test.mypackagename.GeneralPreferenceFragment: make sure class name exists, is public, and has an empty constructor that is public

I have been looking online for a solution but I just cant seem to find one. Everywhere I look they say make sure there is an empty public constructor, if its an inner class it has to be static. But I have the empty constructor and its not an inner class, its in its own file.

Here is my SettingsActivity.cs:

namespace test.mypackagename
{
    public class SettingsActivity : PreferenceActivity
    {
        protected override void OnPostCreate(Bundle savedInstanceState)
        {
            base.OnPostCreate(savedInstanceState);
        }

        public override void OnBuildHeaders(IList<Header> target)
        {
            LoadHeadersFromResource(Resource.Xml.pref_headers, target);
        }
    }
}

Here is my GeneralPreferenceFragment.cs:

namespace test.mypackagename
{
    public class GeneralPreferenceFragment : PreferenceFragment
    {
        public GeneralPreferenceFragment() { }

        public override void OnCreate(Bundle savedInstanceState)
        {
            base.OnCreate(savedInstanceState);
            AddPreferencesFromResource(Resource.Xml.pref_general);
        }
    }
}

And here is my pref_headers.xml:

<preference-headers xmlns:android="http://schemas.android.com/apk/res/android">

    <header android:fragment="test.mypackagename.GeneralPreferenceFragment"
        android:title="@string/pref_header_general" />

    <header android:fragment="test.mypackagename.OtherPreferenceFragment1"
        android:title="@string/pref_header_other1" />

    <header android:fragment="test.mypackagename.OtherPreferenceFragment2"
        android:title="@string/pref_header_other2" />

    <header android:fragment="test.mypackagename.OtherPreferenceFragment3"
        android:title="@string/pref_header_other3" />

    <header android:fragment="test.mypackagename.OtherPreferenceFragment4"
        android:title="@string/pref_header_other4" />

</preference-headers>

This was working fine before so Im not sure what the issue could be. Any help would be much appreciated.

Worldlywise

If this continues to be an issue, you may want to upload a simple reproduction of this. – Matthaus

Answer 1

I think you are running into this problem because when not using the [Register] attribute on your PreferenceFragment then its name appended with a MD5 sum by Xamarin.

So in order to actually have the namespace you expect it to in the pref_headers.xml you need to attribute your class:

[Register("test.mypackagename.GeneralPreferenceFragment")]
public class GeneralPreferenceFragment: PreferenceFragment
{
    // code here
}

EDIT:

I've just tested the code and it works fine on my machine. I am not using any support packages or anything.

pref_general.xml

<?xml version="1.0" encoding="utf-8" ?> 
<PreferenceScreen
        xmlns:android="http://schemas.android.com/apk/res/android">
    <PreferenceCategory
            android:title="durr">
        <CheckBoxPreference
                android:key="checkbox_preference"
                android:title="herp"
                android:summary="derp" />
    </PreferenceCategory>
</PreferenceScreen>

pref_headers.xml

<?xml version="1.0" encoding="utf-8" ?>
<preference-headers xmlns:android="http://schemas.android.com/apk/res/android">
  <header android:fragment="test.mypackagename.GeneralPreferenceFragment"
      android:title="general" />
</preference-headers>

SettingsActivity.cs

[Activity(Label = "SettingsActivity")]
public class SettingsActivity : PreferenceActivity
{
    public override void OnBuildHeaders(IList<Header> target)
    {
        LoadHeadersFromResource(Resource.Xml.pref_headers, target);
    }
}

GeneralPreferenceFragment.cs

[Register("test.mypackagename.GeneralPreferenceFragment")]
public class GeneralPreferenceFragment : PreferenceFragment
{
    public override void OnCreate(Bundle savedInstanceState)
    {
        base.OnCreate(savedInstanceState);

        AddPreferencesFromResource(Resource.Xml.pref_general);
    }
}

This works fine and the app shows up the SettingsActivity with first a general option, after clicking on that it shows up a Title and CheckBox.

This worked even without providing any ctor to the GeneralPreferenceFragment. However, you could try add these:

public GeneralPreferenceFragment()
{
}

public GeneralPreferenceFragment(IntPtr javaRef, JniHandleOwnership transfer)
    : base(javaRef, transfer)
{
}

The latter ctor is often needed when the app is coming back from background or when the Java world is invoking the class somehow.

Nethermost

Thank you but unfortunately that did not solve the issue. I did however notice if I put the Register attribute on the SettingsActivity, the exception no longer contains the MD5 sum. But as I said the problem still happens. – Worldlywise

How are you starting SettingsActivity? – Nethermost

From my MainActivity I have an options menu and I am overriding OnOptionsItemSelected, from there Im calling StartActivity(new Intent(this, typeof(SettingsActivity))); – Worldlywise

I'd love to see a reproducible sample. Because I can't get it to break here from the code I've provided in the answer. – Nethermost

Wow I feel really dumb, When I added the Register attribute, I had a typo in the word "Fragment". So sorry to waste your time with that. But your original answer was correct! Thank you very much – Worldlywise

The hardcoded strings is bad practice. I use 'nameof' keyword: [Register(nameof(GeneralPreferencesFragment))] public class GeneralPreferencesFragment : PreferenceFragment {} – Haile