Call Command from Code Behind

Preface: Without knowing more about your requirements, it seems like a code smell to execute a command from code-behind upon loading. There has to be a better way, MVVM-wise.

But, if you really need to do it in code behind, something like this would probably work (note: I cannot test this at the moment):

private void UserControl_Loaded(object sender, RoutedEventArgs e)     
{
    // Get the viewmodel from the DataContext
    MyViewModel vm = this.DataContext as MyViewModel;

    //Call command from viewmodel     
    if ((vm != null) && (vm.MyCommand.CanExecute(null)))
        vm.MyCommand.Execute(null);
} 

Again - try to find a better way...

Well, if the DataContext is already set you could cast it and call the command:

var viewModel = (MyViewModel)DataContext;
if (viewModel.MyCommand.CanExecute(null))
    viewModel.MyCommand.Execute(null);

(Change parameter as needed)

Try this:

private void UserControl_Loaded(object sender, RoutedEventArgs e)
{
    //Optional - first test if the DataContext is not a MyViewModel
    if( !this.DataContext is MyViewModel) return;
    //Optional - check the CanExecute
    if( !((MyViewModel) this.DataContext).MyCommand.CanExecute(null) ) return;
    //Execute the command
    ((MyViewModel) this.DataContext).MyCommand.Execute(null)
}

I have a more compact solution that I want to share. Because I often execute commands in my ViewModels, I got tired of writing the same if statement. So I wrote an extension for ICommand interface.

using System.Windows.Input;

namespace SharedViewModels.Helpers
{
    public static class ICommandHelper
    {
        public static bool CheckBeginExecute(this ICommand command)
        {
            return CheckBeginExecuteCommand(command);
        }

        public static bool CheckBeginExecuteCommand(ICommand command)
        {
            var canExecute = false;
            lock (command)
            {
                canExecute = command.CanExecute(null);
                if (canExecute)
                {
                    command.Execute(null);
                }
            }

            return canExecute;
        }
    }
}

And this is how you would execute command in code:

((MyViewModel)DataContext).MyCommand.CheckBeginExecute();

I hope this will speed up your development just a tiny bit more. :)

P.S. Don't forget to include the ICommandHelper's namespace too. (In my case it is SharedViewModels.Helpers)