Simple way to understand the MVVM architecture pattern.
Let us split the MVVM as
M V VM
(model) (View) (ViewModel)
(model.cs) (XAML file) (ViewModel.cs)
For an example let us consider , there are two combo box , in one combo box we are filling list of countries and on selection of first combobox second combobox will be filled with the country specific languages. In addition to it let us have one text box which is going to show the country currency.
Take a look on Designing the classes.
MODEL : We are going to write methods which brings countries lists.
VIEWMODEL: An another class which contains the Property of display member of MODEL class to show on UI (VIEW)
VIEW: XAML binding of VIEWMODEL.
<?xml version="1.0" encoding="utf-8"?>
<Window
x:Class="MvvmCombo.Window1" xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MvvmCombo"
Height="417"
Width="496">
<Grid
Name="mGrid"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Width="479"
Height="374">
<ComboBox
ItemsSource="{Binding Countries}"
SelectedItem="{Binding SelectedCountry}"
DisplayMemberPath="DisplayName"
Grid.Column="1"
Grid.Row="1"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="10.99999999687,1.5,0,0"
Width="208"
Height="27"
SelectedIndex="0" />
<ComboBox
ItemsSource="{Binding Lang}"
Grid.Column="1"
Grid.Row="2"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Margin="10.99999999687,3.50000000000006,0,0"
Width="208"
Height="26"
SelectedIndex="{Binding SelectedIndex}" />
<TextBox
Name="txtCur"
Text="{Binding Currency}"
IsReadOnly="True"
Height="29"
Width="208"
Margin="10.99999999687,3.50000000000007,0,0"
VerticalAlignment="Top"
HorizontalAlignment="Left"
Grid.Row="3"
Grid.Column="1" />
<Label
Content="Countries"
Grid.Column="0"
Grid.Row="1"
HorizontalAlignment="Left"
VerticalAlignment="Stretch"
Margin="8,1.5,0,5.49999999999994"
Width="65"
Height="27" />
<Label
Content="Language"
Grid.Column="0"
Grid.Row="2"
HorizontalAlignment="Left"
VerticalAlignment="Stretch"
Margin="8,5.00000000000006,0,5.49999999999993"
Width="65"
Height="24.5" />
<Label
Grid.Column="0"
Grid.Row="3"
HorizontalAlignment="Left"
VerticalAlignment="Stretch"
Margin="8,3.50000000000007,0,8.50000000000004"
Width="54"
Height="29"
Content="Currency" />
<TextBox
Name="txtCurSym"
IsReadOnly="True"
Text="{Binding CurrencySym}"
Grid.Column="1"
Grid.Row="3"
HorizontalAlignment="Stretch"
VerticalAlignment="Top"
Margin="251.635641460119,3.50000000000009,94.1004888627257,0"
Width="46"
Height="30" />
<Grid.ColumnDefinitions>
<ColumnDefinition
Width="0.182179268637068*" />
<ColumnDefinition
Width="0.817820731362932*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition
Height="0.0971428571428572*" />
<RowDefinition
Height="0.106124505217704*" />
<RowDefinition
Height="0.0992443324937027*" />
<RowDefinition
Height="0.102439726520331*" />
<RowDefinition
Height="0.0988269161568912*" />
<RowDefinition
Height="0.0856423173803527*" />
<RowDefinition
Height="0.410579345088161*" />
</Grid.RowDefinitions>
</Grid>
</Window> MODEL FILE
using System;
using System.Collections;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace MvvmCombo
{
class Model
{
public Model()
{
}
public List<RegionInfo> GetCountries()
{
RegionInfo country = new RegionInfo(new CultureInfo("en-US", false).LCID);
List<RegionInfo> countryNames = new List<RegionInfo>();
foreach (CultureInfo cul in CultureInfo.GetCultures(CultureTypes.SpecificCultures))
{
country = new RegionInfo(new CultureInfo(cul.Name, false).LCID);
countryNames.Add(country);
}
return countryNames.OrderBy(names => names.DisplayName).Distinct().ToList();
}
public List<CultureInfo> GetCountriesAll()
{
List<CultureInfo> list = new List<CultureInfo>();
foreach (System.Globalization.CultureInfo ci in
System.Globalization.CultureInfo.GetCultures(System.Globalization.CultureTypes.AllCultures))
{
list.Add(System.Globalization.CultureInfo.CreateSpecificCulture(ci.Name));
}
return list;
}
}
} ViewModel File
using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.ComponentModel;
using System.Collections.ObjectModel;
namespace MvvmCombo
{
class ViewModel :INotifyPropertyChanged
{
public event PropertyChangedEventHandler PropertyChanged;
private RegionInfo _ri;
private Model _m;
private void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
public ViewModel(Model m)
{
_m = m;
}
private List<CultureInfo> _allCul;
public List<RegionInfo> Countries
{
get
{
_allCul = _m.GetCountriesAll();
return _m.GetCountries(); ;
}
}
private List<string> _lang;
public List<string> Lang
{
get
{
return _lang;
}
}
public List<string> FillLanguage()
{
return _lang =
_allCul.Where(c => c.EnglishName.Contains(_ri.EnglishName))
.Select(c=>c.EnglishName.IndexOf("(")>-1?c.EnglishName.Substring(0,c.EnglishName.IndexOf("(")):
c.EnglishName).Distinct().ToList();
}
private string _currencySym;
public string CurrencySym
{
get
{
return _currencySym;
}
set
{
}
}
public object SelectedCountry
{
set
{
_ri = value as RegionInfo;
FillLanguage();
_currencySym = _ri.CurrencySymbol;
OnPropertyChanged("Lang");
OnPropertyChanged("CurrencySym");
}
}
}
} 
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