SetFormatter method implementation for convert Number to string in Xamarin Android

My sample code below In OnCreateDialog event calling method below

SetNumericMonth(true, null, dialog);

private int monthSpinnerResId;
        public void SetNumericMonth(bool onCreateEventOrNot, DatePicker view, DatePickerDialog dialog)
        {
            if (DatePickerNumericMonthFormat == false)
            {
                monthSpinnerResId = Activity.Resources.GetIdentifier("android:id/month", null, null);
                if (monthSpinnerResId != 0)
                {
                    var numberPicker = (NumberPicker)dialog.DatePicker.FindViewById<NumberPicker>(monthSpinnerResId);
                    numberPicker.SetFormatter(new NumericMonthFormatter());
                    numberPicker.MinValue = 0;
                    numberPicker.MaxValue = 9;
                }
            }
        }


public class NumericMonthFormatter: Java.Lang.Object, NumberPicker.IFormatter
     {
         public string Format1(int value)
         {
             return string.Format("{0:D2}", value);
         }
     }

You can try to create your DatePickerDialog and set the month number picker's display values. Such as:

 public class MyDialog : DatePickerDialog
    {
        public MyDialog(Context context) : base(context)
        {
        }
        public override void OnDateChanged(DatePicker view, int year, int month, int dayOfMonth)
        {
            base.OnDateChanged(view, year, month, dayOfMonth);
            var monthSpinnerResId = Android.App.Application.Context.Resources.GetIdentifier("android:id/month", null, null);
            var numberPicker = (NumberPicker)(this.DatePicker.FindViewById<NumberPicker>(monthSpinnerResId));
            string[] arrays = new string[] { "01", "02", "03", "04", "05", "06", "07", "08", "09", "10", "11", "12" };
            numberPicker.SetDisplayedValues(arrays);
        }
    }

And in the main activity:

MyDialog dialog = new MyDialog(this);
dialog.Show();

And there are some point for your code:

1. The method name should be Format not Format 1:

public class NumericMonthFormatter: Java.Lang.Object, NumberPicker.IFormatter
     {
         public string Format(int value)
         {
             return string.Format("{0:D2}", value);
         }
     }

2. The DatePickerDialog'd default mode in the Android version heigher than Api 21 is calendar, so you need to change it as spinner.

In the styles.xml:

<resources>

    <!-- Base application theme. -->
    <style name="AppTheme" parent="Theme.MaterialComponents.Light.DarkActionBar">
        <!-- Customize your theme here. -->
        <item name="colorPrimary">@color/colorPrimary</item>
        <item name="colorPrimaryDark">@color/colorPrimaryDark</item>
        <item name="colorAccent">@color/colorAccent</item>
            <item name="android:datePickerStyle">@style/MySpinnerDatePicker</item>
    </style>
      <style name="MySpinnerDatePicker" parent="android:Widget.Material.DatePicker">
            <item name="android:datePickerMode">spinner</item>
      </style>
</resources>

If the default style is calendar, the var numberPicker = (NumberPicker)dialog.DatePicker.FindViewById<NumberPicker>(monthSpinnerResId); will get null.

In addition, if just use the formatter, the display values will be reset when the date changed. So I try to use the custom datepicker dialog.