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Binding System uses Reflection to find Properties in DataContext then What is the use of Implementing Interface and IOC/DI in ViewModel
namespace ViewModel
{
public class MyViewModel : IMyViewModel, INotifyPropertyChanged
{
    public MyViewModel()
    {
        FirstName = "Harish";
        LastName = "Chauhan";
    }
    private string _firstName;
    private string _lastName;

    public string FirstName
    {
        get
        {
            return _firstName;
        }
        set
        {
            _firstName = value;
            Notify("FirstName");
        }

    }
    public string LastName
    {
        get
        {
            return _lastName;
        }
        set
        {
            _lastName = value;
            Notify("LastName");
        }

    }

    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;

    #endregion
    private void Notify(string propName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
    }
}
//Interface
public interface IMyViewModel
{
    string FirstName { get; set; }
}

<Grid>
    <Grid.RowDefinitions>
        <RowDefinition Height="70"/>
        <RowDefinition Height="70"/>
    </Grid.RowDefinitions>
    <TextBox x:Name="tbx1" Text="{Binding FirstName, Mode=TwoWay}"/>
    <TextBox x:Name="tbx2" Text="{Binding LastName, Mode=TwoWay}" Grid.Row="1"/>
</Grid>