Setting the DataContext of a View to a ViewModel in MVVM

Setting the DataContext of a View to a ViewModel can be done in various ways. It can be done in the constructor of the View

public LoginView()
{
 InitializeComponent();
 DataContext = new LoginViewModel();
}

or

public LoginView(LoginViewModel viewModel)
{
 InitializeComponent();
 DataContext = viewModel;
}

or directly in the XAML

<UserControl x:Class="MvvmPassword.LoginView"
      x:Name="This"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MvvmPassword">
    <UserControl.Resources>
        <local:LoginViewModel x:Key="ViewModel" />
    </UserControl.Resources>
    <Grid DataContext="{Binding Source={StaticResource ViewModel}}"> 

        ... 

    </Grid>
</UserControl>

or

<UserControl x:Class="MvvmPassword.LoginView"
      x:Name="This"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="clr-namespace:MvvmPassword"> 
 <UserControl.DataContext>
  <local:LoginViewModel x:Name="ViewModel" />
 </UserControl.DataContext>
 <Grid> 

           ...

        </Grid>
</UserControl>