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[Activity(Label = "MoneyBack", MainLauncher = true, Icon = "@drawable/icon")]
    public class MainActivity : Activity
    {
        protected override void OnCreate(Bundle bundle)
        {
            base.OnCreate(bundle);

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

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protected override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);

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

    _amount = 0.00m;
    _result = 0.00m;
    _numberOfPeople = 0;

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protected override void OnStart()
{
    base.OnStart();
    RefreshUserInputsFromVariables();
}
// ...
private void RefreshUserInputsFromVariables()
{
    _inputAmount.SetText(_amount.ToString(CultureInfo.InvariantCulture), TextView.BufferType.Editable);
    _txtResultDecimal.SetText(_result.ToString(CultureInfo.InvariantCulture), TextView.BufferType.Editable);

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protected override void OnResume()
{
    base.OnResume();
    InitializeUserControlsEvents();
    // TODO: retrieving variables (amount, number of people) from persistent storage (file, database)
}

// ...
private void InitializeUserControlsEvents()
{

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protected override void OnPause()
{
    base.OnPause();
    DetatchUserControlsEvents();
    // TODO: saving variables (amount, number of people) to persistent storage (file, database)
}
// ...
private void DetatchUserControlsEvents()
{
    _btnCalculate.Click -= _btnCalculate_Click;

dsibinski

private bool AddNewCity(string cityName, string countryIso)
{
    var city = CountriesCitiesDbService.GetNewCity();
    var europeanCountries = CountriesCitiesDbService.GetAllEuropeanCountries();
    var country = europeanCountries.FirstOrDefault(ec => ec.IsoCode.Equals(countryIso));

    if (country == null)
        throw new ArgumentException($"Country with ISO Code {countryIso} does not exist!");

    city.Name = cityName;

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private List<Country> _europeanCountries;

public List<Country> EuropeanCountries => 
    _europeanCountries ?? (_europeanCountries = CountriesCitiesDbService.GetAllEuropeanCountries());

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private bool AddNewCity(string cityName, string countryIso)
{
    var city = CountriesCitiesDbService.GetNewCity();

    var country = EuropeanCountries.FirstOrDefault(ec => ec.IsoCode.Equals(countryIso));
    // the rest of method's code...
}

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namespace MoneyBack.Entities
{
    [Table("People")]
    public class Person
    {
        [PrimaryKey, AutoIncrement]
        public int Id { get; set; }

        public string Name { get; set; }

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public class Constants
{
    public static readonly string DbFilePath = 
        Path.Combine(
                Environment.GetFolderPath(Environment.SpecialFolder.Personal), 
                "moneyback.db"
        );
}