Using ViewModel in Views (The MVVM way)

The article says how to use one/more ViewMiodels in a View. Also, multiple Views using same ViewModel. Read this post to create a ViewModel.

There are several ways of using ViewModel in a XAML file based on the different scenarios.

Case 1: Initialize a ViewModel without any constructor parameters.

Read this post. This article also expects the reader to be familiar with Expression Blend.

Case 2: Initialize a ViewModel with constructor parameters.

xmlns:local="clr-namespace:****your namespace***"

xmlns:sys="clr-namespace:System;assembly=mscorlib"

<ObjectDataProvider ObjectType="{x:Type local:ViewModel}" x:Key="theViewModel">            <ObjectDataProvider.ConstructorParameters>
                <sys:String>Uday</sys:String>
                <sys:Int32>25</sys:Int32>
</ObjectDataProvider.ConstructorParameters>       
        </ObjectDataProvider>

Notice the System namespace declaration. We can use any data type from System namespace as a constructor parameter. If you have custom data types you can pass them too as a constructor parameter; by declaring namespace in XAML file.

Case 3: In the above two cases, the object can be accessed on the C# side (as a Resource). However, if you update the object in C# side, the ViewModel values are NOT reflected on the XAML side. Now what??!  We can declare a ViewModel object on the C# side and declare it as DataContext for the XAML side. Here is how to do it.

public partial class MainWindow : Window
    {
        private ViewModel theViewModel = new ViewModel();

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = theViewModel;
        }

        private void TextBox_TextChanged(object sender, System.Windows.Controls.TextChangedEventArgs e)
        {
            theViewModel.FirstName = "I am still typing";
        }
    }

In this way, any updates from XAML side would be seen on C# side too. But, if you use Expression Blend, the ViewModel is not seen in the designer, so you have to write the binding code manually. I wouldn’t suggest this way, but sometime its not up to your choice!