WPF: Changing MediaElement source with ResourceDictionary

I want to use background music in my WPF Application. Like you can see here: How to do background music for my WPF-Application开发者_如何学编程?

So I use a MediaElement.

Now I want to change the source of it while running the Application.

I'm already doing something similar with some background pictures. There I have different ResourceDictionaries that I'm switching to show different "themes".

One of my dictionaries looks like this:

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

    <ImageBrush x:Key="Backgroundpic" ImageSource="picture.png"/>

         ...

</ResourceDictionary>

So I can use it in the xaml like this:

...
<Grid x:Name="Bg" Background="{DynamicResource Backgroundpic}"/>
...

But HOW can I do that with my MediaElement-Source that I can use it like this:

 <MediaElement x:Name="myMediaElement" Source="{DynamicResource ???}" />

I just don't know what to write into my ResourceDictionary.

Source is a Uri, so you need your resource to be a Uri. (Note that System.Uri is in the System assembly, not mscorlib, so it needs a different XML namespace than you would use for types like String):

<ResourceDictionary
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sys="clr-namespace:System;assembly=System">

    <sys:Uri x:Key="mediaSource">something.mp3</sys:Uri>

Then you can reference it with Source={DynamicResource mediaSource}.