In a few WPF projects that I have been on I have needed to load in loose ResourceDictionary files to provide different runtime styling. When there isn’t any external content being referenced (ie. images, videos, etc…) it is as simple as calling XamlReader.Load and then merging the dictionary into the application’s resources:

var resourceDictionary = XamlReader.Load(fileStream) as ResourceDictionary;
if(resourceDictionary != null)
{
    Resources.MergedDictionaries.Add(resourceDictionary)
}

When external content is being referenced there is the possibility that the XamlReader won’t be able to find those files, even if they are in the exact same folder as the ResourceDictionary files. To solve this issue, you will need to create a ParserContext and set its BaseUri to the root folder containing the files. For example, if the ResourceDictionary and content files are located in a folder called RuntimeResources, which is in the same folder as the application’s exe, then the BaseUri needs to point to that RuntimeResources folder. Then when the XamlReader loads the ResourceDictionary, it uses the provided ParserContext to know where to look for the actual files. NOTE: In the ResourceDictionary you will still need to include the root folder (RuntimeResources) at the beginning of the uri otherwise it will look in the parent directory for the desired file.

Below is the related code:

C#

var applicationDirectory = Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location);
if(!String.IsNullOrEmpty(applicationDirectory ))
{

    var runtimeResourcesDirectory = Path.Combine(applicationDirectory , "RuntimeResources");
    var pc = new ParserContext
    {
        BaseUri = new Uri(runtimeResourcesDirectory , UriKind.Absolute)
    };
    if(Directory.Exists(runtimeResourcesDirectory ))
    {
        foreach (string resourceDictionaryFile in Directory.GetFiles(runtimeResourcesDirectory , "*.xaml"))
        {
            using (Stream s = File.Open(resourceDictionaryFile, FileMode.Open, FileAccess.Read))
            {
                try
                {
                    var resourceDictionary = XamlReader.Load(s, pc) as ResourceDictionary;
                    if (resourceDictionary != null)
                    {
                        Resources.MergedDictionaries.Add(resourceDictionary);
                    }
                }
                catch
                {
                    MessageBox.Show("Invalid xaml: " + resourceDictionaryFile);
                }
            }
        }
    }
}

Resource Dictionary

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

	<ImageBrush x:Key="LogoImage"
			  ImageSource="RuntimeResources/logo.png" />

</ResourceDictionary>

Now you’ve got the ability to change your WPF application’s styling at runtime.