Why can I not set a SolidColorBrush resource value from code?

Why can I not set a SolidColorBrush resource value from code?

I have a resource defined in my xaml :
<...Resources>
<SolidColorBrush x:Key="..." />
</...Resources>
I am trying to set it from code :
this.Resources["..."] = new SolidColorBrush(Colors.Red);
But I get the error :
The method or operation is not implemented.