<Grid x:Name="main">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
        <MediaElement x:Name="me1" Grid.Row="0" Grid.Column="0" />
        <MediaElement x:Name="me2" Grid.Row="0" Grid.Column="1" />
        <MediaElement x:Name="me3" Grid.Row="0" Grid.Column="2" />
        <MediaElement x:Name="me4" Grid.Row="1" Grid.Column="0" />
        <MediaElement x:Name="me5" Grid.Row="1" Grid.Column="1" />
        <MediaElement x:Name="me6" Grid.Row="1" Grid.Column="2" />
    </Grid>

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            var action = new Action(() =>
           {
               var suffix = "wmv";
               var index = 1;
               foreach (var child in this.main.Children)
               {
                   if (child is MediaElement)
                   {
                       var mediaElement = child as MediaElement;
                       var uriString = string.Format(@"C:\Users\Public\Videos\Sample Videos\{0}.{1}", index, suffix);
                       var uri = new Uri(uriString);
                       mediaElement.Source = uri;
                       index++;
                   }
               }
           });
           this.Dispatcher.BeginInvoke(action);
        }