Use DatePicker spinners in API 24

gistfile1.java

/**
 * Workaround for this bug: https://code.google.com/p/android/issues/detail?id=222208
 * In Android 7.0 Nougat, spinner mode for the DatePicker in DatePickerDialog is
 * incorrectly displayed as calendar, even when the theme specifies otherwise.
 *
 * Modified slightly from the equivalent fix for TimePicker from @jeffdgr8: 
 *     https://gist.github.com/jeffdgr8/6bc5f990bf0c13a7334ce385d482af9f
 */
private void fixSpinner(Context context, int year, int month, int dayOfMonth) {
    // The spinner vs not distinction probably started in lollipop but applying this
    // for versions < nougat leads to a crash trying to get DatePickerSpinnerDelegate
    if (Build.VERSION.SDK_INT == Build.VERSION_CODES.N) {
        try {
            // Get the theme's android:datePickerMode
            final int MODE_SPINNER = 1;
            Class<?> styleableClass = Class.forName("com.android.internal.R$styleable");
            Field datePickerStyleableField = styleableClass.getField("DatePicker");
            int[] datePickerStyleable = (int[]) datePickerStyleableField.get(null);
            final TypedArray a = context.obtainStyledAttributes(null, datePickerStyleable,
                    android.R.attr.datePickerStyle, 0);
            Field datePickerModeStyleableField = styleableClass.getField("DatePicker_datePickerMode");
            int datePickerModeStyleable = datePickerModeStyleableField.getInt(null);
            final int mode = a.getInt(datePickerModeStyleable, MODE_SPINNER);
            a.recycle();

            if (mode == MODE_SPINNER) {
                DatePicker datePicker = (DatePicker) findField(DatePickerDialog.class,
                        DatePicker.class, "mDatePicker").get(this);
                Class<?> delegateClass = Class.forName("android.widget.DatePicker$DatePickerDelegate");
                Field delegateField = findField(DatePicker.class, delegateClass, "mDelegate");
                Object delegate = delegateField.get(datePicker);

                Class<?> spinnerDelegateClass = Class.forName("android.widget.DatePickerSpinnerDelegate");

                // In 7.0 Nougat for some reason the datePickerMode is ignored and the
                // delegate is DatePickerCalendarDelegate
                if (delegate.getClass() != spinnerDelegateClass) {
                    delegateField.set(datePicker, null); // throw out the DatePickerCalendarDelegate!
                    datePicker.removeAllViews(); // remove the DatePickerCalendarDelegate views

                    Constructor spinnerDelegateConstructor = spinnerDelegateClass
                            .getDeclaredConstructor(DatePicker.class, Context.class,
                                    AttributeSet.class, int.class, int.class);
                    spinnerDelegateConstructor.setAccessible(true);

                    // Instantiate a DatePickerSpinnerDelegate
                    delegate = spinnerDelegateConstructor.newInstance(datePicker, context,
                            null, android.R.attr.datePickerStyle, 0);

                    // set the DatePicker.mDelegate to the spinner delegate
                    delegateField.set(datePicker, delegate);

                    // Set up the DatePicker again, with the DatePickerSpinnerDelegate
                    datePicker.updateDate(year, month, dayOfMonth);
                }
            }
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

private Field findField(Class objectClass, Class fieldClass, String expectedName) {
    try {
        Field field = objectClass.getDeclaredField(expectedName);
        field.setAccessible(true);
        return field;
    } catch (NoSuchFieldException e) {
        Timber.i(e); // ignore
    }

    // search for it if it wasn't found under the expected ivar name
    for (Field searchField : objectClass.getDeclaredFields()) {
        if (searchField.getType() == fieldClass) {
            searchField.setAccessible(true);
            return searchField;
        }
    }
    return null;
}

Thanks to lognaturel and Noisyfox.
Here is the c# Xamarin version of this fix:

class NougatFixDatePickerDialog : DatePickerDialog
    {
        public NougatFixDatePickerDialog(Context context, IOnDateSetListener listener, int year, int month, int dayOfMonth) 
            : base(context, listener, year, month, dayOfMonth)
        {
            FixSpinner(context, year, month, dayOfMonth);
        }

        private void FixSpinner(Context context, int year, int month, int dayOfMonth)
        {
            try
            {
                const int modeSpinner = 1;
                Class styleableClass = Class.ForName("com.android.internal.R$styleable");
                Field datePickerStyleableField = styleableClass.GetField("DatePicker");
                int[] datePickerStyleable = (int[]) datePickerStyleableField.Get(null);
                TypedArray a = context.ObtainStyledAttributes(
                    null, datePickerStyleable, Android.Resource.Attribute.DatePickerStyle, 0);
                Field datePickerModeStyleableField = styleableClass.GetField("DatePicker_datePickerMode");
                int datePickerModeStyleable = datePickerModeStyleableField.GetInt(null);
                int mode = a.GetInt(datePickerModeStyleable, modeSpinner);
                a.Recycle();

                if (mode != modeSpinner)
                {
                    return;
                }

                DatePicker datePicker = (DatePicker) FindField(
                    Java.Lang.Class.FromType(typeof(DatePickerDialog)),
                    Java.Lang.Class.FromType(typeof(Android.Widget.DatePicker)), 
                    "mDatePicker").Get(this);

                Class delegateClass = Class.ForName("android.widget.DatePicker$DatePickerDelegate");
                Field delegateField = FindField(
                    Java.Lang.Class.FromType(typeof(Android.Widget.DatePicker)),
                    delegateClass, "mDelegate");
                Object jDelegate = delegateField.Get(datePicker);

                Class spinnerDelegateClass;
                try
                {
                    // For android [5.0, 6.0], the DatePickerSpinnerDelegate is an inner class.
                    spinnerDelegateClass = Class.ForName("android.widget.DatePicker$DatePickerSpinnerDelegate");
                }
                catch (ClassNotFoundException)
                {
                    // For android [7.0,), the DatePickerSpinnerDelegate is moved out to a normal class.
                    spinnerDelegateClass = Class.ForName("android.widget.DatePickerSpinnerDelegate");
                }

                if (delegateClass.Class == spinnerDelegateClass)
                {
                    return;
                }

                delegateField.Set(datePicker, null);
                datePicker.RemoveAllViews();

                Constructor spinnerDelegateConstructor = spinnerDelegateClass
                    .GetDeclaredConstructor(new Class[]
                    {
                        Class.FromType(typeof(Android.Widget.DatePicker)),
                        Java.Lang.Class.FromType(typeof(Android.Content.Context)),
                        Java.Lang.Class.FromType(typeof(IAttributeSet)),
                        Integer.Type,
                        Integer.Type
                    });
                spinnerDelegateConstructor.Accessible = true;
                    
                jDelegate.Dispose();
                jDelegate = spinnerDelegateConstructor
                    .NewInstance(datePicker, context, null, Android.Resource.Attribute.DatePickerStyle, 0);

                delegateField.Set(datePicker, jDelegate);
                datePicker.CalendarViewShown = false;
                datePicker.UpdateDate(year, month, dayOfMonth);
            }
            catch (Exception e)
            {
                Console.Out.Write(e.Message);
                throw e;
            }
        }

        private Field FindField(Class objectClass, Class fieldClass, string expectedName)
        {
            try
            {
                Field field = objectClass.GetDeclaredField(expectedName);
                field.Accessible = true;
                return field;
            }
            catch (NoSuchFieldException) { }

            // search for it if it wasn't found under the expected ivar name
            foreach (Field searchField in objectClass.GetDeclaredFields())
            {
                if (searchField.Type == fieldClass)
                {
                    searchField.Accessible = true;
                    return searchField;
                }
            }

            return null;
        }
    }