Make sure that the Blend 4 SDK is installed from the Microsoft Site here http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=3062

Let's create a new Silverlight Project SLSimpleCommands.

Let's add the following Dlls as references to the Silverlight project.

invoke command in silverlight

Create a ViewModel as shown below:

 public class ViewModel
 {
public ICommand PopupVM
{
get
 {
return new InvokeChildCommand();
}
}

public class InvokeChildCommand : ICommand

public bool CanExecute(object parameter)
{
if (parameter != null)
{
CanExecuteChanged.Invoke(parameter, new EventArgs());
}
return true;
}

public event EventHandler CanExecuteChanged;

public void Execute(object parameter)
{
//Push the ViewModel into the Popup
 Child child = new Child();
child.Show();
}
}
}

Add a new Child Window control to the project.

Let's move on to the MainPage.xaml.

<UserControl x:Class="SLSimpleCommands.MainPage"
 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
 xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
 xmlns:local="clr-namespace:SLSimpleCommands.ViewModels"
 mc:Ignorable="d"
 d:DesignHeight="300" d:DesignWidth="400">
<UserControl.Resources>
 <local:ViewModel x:Key="popup"></local:ViewModel>
 </UserControl.Resources>

<Grid x:Name="LayoutRoot" Background="White" DataContext="{Binding Source={StaticResource popup}}">
 <Ellipse Fill="#FFF4F4F5" Height="31" Margin="149,199,278,0" Stroke="Black" VerticalAlignment="Top">
 <i:Interaction.Triggers>
 <i:EventTrigger>
 <i:InvokeCommandAction Command="{Binding PopupVM}"/>
 </i:EventTrigger>
 </i:Interaction.Triggers>
 </Ellipse>
</Grid>
</UserControl>

Make sure you have added the namespaces as shown below:

xmlns:i="http://schemas.microsoft.com/expression/2010/interactivity"
 xmlns:local="clr-namespace:SLSimpleCommands.ViewModels"

Also important is how I have bound the ViewModel to the Grid.

Add the ViewModel as a Resource.
<UserControl.Resources>
 <local:ViewModel x:Key="popup"></local:ViewModel>
 </UserControl.Resources>

Bind the ViewModel to the Grid as shown below :

 <Grid x:Name="LayoutRoot" Background="White" DataContext="{Binding Source={StaticResource popup}}">

Now the ViewModel is available for any of the child controls of the Grid that want to use the the ViewModel.

Check out the use of InvokeCommandAction:

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

The Trigger is triggered when the Ellipse is loaded hence as soon as the page opens the Child window opens with it .

Let's give it a run.

invoke command in silverlight