How to define datacontext in Wpf MVVM pattern in proper way

See more:

C#

WPF

C#4.0

MVVM

Hi ,

can any know how to define datacontext for viewmodel in xaml or code behind .
below is the code i tried then am getting error as" Object reference not set to an instance of an object" in xaml under windows.DataContext.

In xaml defined like this

<Window x:Class="Demo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:Demo.ViewModel"
        Title="Demo" Height="768" Width="1024" Loaded="OnWindowsLoaded" ResizeMode="NoResize"  WindowStartupLocation="CenterScreen">
    <Window.DataContext>
        <local:CompanyViewModel/> //here am getting object reference not set error.
    </Window.DataContext>
</Window>

In code behind

public partial class MainWindow : Window
    {        
        CompanyViewModel CompVM; //this is my viewmodel       
        public MainWindow()
        {
            InitializeComponent();
            CompVM = (CompanyViewModel)base.DataContext;
        }
}

Its working with no issues but why am getting this error couldnt get it. can any one tell me what was mistake made in the above code.

am newbie in Implementing MVVM pattern