How to fire a Command when a window is loaded in wpf

To avoid code behind on your View, use the Interactivity library (System.Windows.Interactivity dll which you can download for free from Microsoft - also comes with Expression Blend).

Then you can create a behavior that executes a command. This way the Trigger calls the Behavior which calls the Command.

<ia:Interaction.Triggers>
    <ia:EventTrigger EventName="Loaded">
        <custombehaviors:CommandAction Command="{Binding ShowMessage}" Parameter="I am loaded"/>
    </ia:EventTrigger>
</ia:Interaction.Triggers>

CommandAction (also uses System.Windows.Interactivity) can look like:

public class CommandAction : TriggerAction<UIElement>
{
    public static DependencyProperty CommandProperty = DependencyProperty.Register("Command", typeof(ICommand), typeof(CommandAction), null);
    public ICommand Command
    {
        get
        {
            return (ICommand)GetValue(CommandProperty);
        }
        set
        {
            SetValue(CommandProperty, value);
        }
    }


    public static DependencyProperty ParameterProperty = DependencyProperty.Register("Parameter", typeof(object), typeof(CommandAction), null);
    public object Parameter
    {
        get
        {
            return GetValue(ParameterProperty);
        }
        set
        {
            SetValue(ParameterProperty, value);

        }
    }

    protected override void Invoke(object parameter)
    {
        Command.Execute(Parameter);            
    }
}

A more generic way using behaviors is proposed at AttachedCommandBehavior V2 aka ACB and it even supports multiple event-to-command bindings,

Here is a very basic example of use:

<Window x:Class="Example.YourWindow"
        xmlns:local="clr-namespace:AttachedCommandBehavior;assembly=AttachedCommandBehavior"
        local:CommandBehavior.Event="Loaded"
        local:CommandBehavior.Command="{Binding DoSomethingWhenWindowIsLoaded}"
        local:CommandBehavior.CommandParameter="Some information"
/>

This is much easier to do now. Simply include the following namespace:

xmlns:i="http://schemas.microsoft.com/xaml/behaviors"

And leverage it like this:

<i:Interaction.Triggers>
    <i:EventTrigger EventName="Loaded">
        <i:InvokeCommandAction Command="{Binding CommandInViewModel}"/>
    </i:EventTrigger>
</i:Interaction.Triggers>

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
       ApplicationCommands.New.Execute(null, targetElement); 
       // or this.CommandBindings[0].Command.Execute(null); 
    }

and xaml

    Loaded="Window_Loaded"