WPF: Changing Resources (colors) from the App.xaml during runtime

It looks like you're trying to do some sort of skinning?

I'd recommend defining the resources in a Resource Dictionary contained in a separate file. Then in code (App.cs to load a default, then elsewhere to change) you can load the resources as so:

//using System.Windows
ResourceDictionary dict = new ResourceDictionary();
dict.Source = new Uri("MyResourceDictionary.xaml", UriKind.Relative);

Application.Current.Resources.MergedDictionaries.Add(dict);

You could also define the default resource dictionary in App.xaml and unload it in code just fine.

Use the MergedDictionaries object to change the dictionary you're using at runtime. Works like a charm for changing an entire interface quickly.

Changing application wide resources in runtime is like:

Application.Current.Resources("MainBackgroundBrush") = Brsh

About the InvalidOperationException, i guess WallStreet Programmer is right. Maybe you should not try to modify an existing brush, but instead create a new brush in code with all the gradientstops you need, and then assign this new brush in application resources.

Another Approach on changing the color of some GradientStops is to define those colors as DynamicResource references to Application Wide SolidColorBrushes like:

<LinearGradientBrush x:Key="MainBrush" StartPoint="0, 0.5" EndPoint="1, 0.5" >
<GradientBrush.GradientStops>
    <GradientStop Color="{DynamicResource FirstColor}" Offset="0" />
    <GradientStop Color="{DynamicResource SecondColor}" Offset="1" />
</GradientBrush.GradientStops>

and then use

Application.Current.Resources["FirstColor"] = NewFirstColorBrsh
Application.Current.Resources["SecondColor"] = NewSecondColorBrsh

HTH