Two ways of setting datacontext

Both of these examples are with default value, you will find it in the constructor of model "Hello world!"

XAML datacontext

Add new WPF application project, add new model like:

namespace HelloWorldDataContext
{
    public class HelloWorldDataContextModel
    {
        public string HelloWorld { get; set; }

        public HelloWorldDataContextModel()
        {
            HelloWorld = "Hello world!";
        }
    }
}

Then XAML:

<Window x:Class="HelloWorldDataContext.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:HelloWorldDataContext"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <local:HelloWorldDataContextModel x:Key="HelloWorldDataContext" />
    </Window.Resources>
    <Grid DataContext="{StaticResource HelloWorldDataContext}">
        <TextBox HorizontalAlignment="Left" Height="23" Margin="222,127,0,0" TextWrapping="Wrap" Text="{Binding HelloWorld}" VerticalAlignment="Top" Width="120"/>
    </Grid>
</Window>

Important lines in XAML on which you should take care are:

xmlns:local="clr-namespace:HelloWorldDataContext"

Where name space is declared in model like:

namespace HelloWorldDataContext

<local:HelloWorldDataContextModel x:Key="HelloWorldDataContext" />

<Grid DataContext="{StaticResource HelloWorldDataContext}">

Example project you can download from here.

Programmatically setting datacontext

XAML:

<Window x:Class="ProgramaticallyDataContext.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBox HorizontalAlignment="Left" Height="23" Margin="147,78,0,0" TextWrapping="Wrap" Text="{Binding HelloWorld}" VerticalAlignment="Top" Width="120"/>
    </Grid>
</Window>

Test Model is same like in above example, code behind (MainWindow.xaml.cs) is like:

public MainWindow()
{
	InitializeComponent();

	HelloWorldDataContextModel objHW = new HelloWorldDataContextModel();
	objHW.HelloWorld = "Hello world";
	this.DataContext = objHW;
}

Example you can download from here.