Matthew A. Thomas

Loading Loose XAML

June 03, 2022—How to parse and load XAML from an embedded resource

Suppose you have a custom thingy like this:

namespace LooseXaml;

sealed class CustomThingy
{
    public string Property { get; set; } = string.Empty;
}

…and you declare it in some XAML:

<CustomThingy xmlns="clr-namespace:LooseXaml">
    <CustomThingy.Property>Hello, world!</CustomThingy.Property>
</CustomThingy>

…and that XAML is in a file named “EmbeddedResource.xaml”, which is an embedded resource:

<Project Sdk="Microsoft.NET.Sdk">

    <PropertyGroup>
        <OutputType>WinExe</OutputType>
        <TargetFramework>net6.0-windows</TargetFramework>
        <Nullable>enable</Nullable>
        <UseWPF>true</UseWPF>
    </PropertyGroup>

    <ItemGroup>
      <Page Remove="EmbeddedResource.xaml" />
      <EmbeddedResource Include="EmbeddedResource.xaml" />
    </ItemGroup>

</Project>

Did you know you can parse and instantiate it?

namespace LooseXaml;

using System;
using System.Diagnostics;
using System.IO;
using System.Windows;

public partial class App
{
    protected override void OnStartup(StartupEventArgs e)
    {
        var type = GetType();
        var assembly = type.Assembly;
        using var stream = assembly.GetManifestResourceStream(type, "EmbeddedResource.xaml");
        if (stream is null)
            throw new Exception("Wrong resource name");
        using var reader = new StreamReader(stream);
        var loader = new Loader();
        var customThingy = loader.Load<CustomThingy>(
            reader,
            assembly,
            new [] { assembly }
        );
        if (customThingy is null)
            throw new Exception("Failed to load it. Are you sure that XAML declares a CustomThingy?");
        if (customThingy.Property != "Hello, world!")
            throw new Exception("Loaded it, but its property is wrong for some reason");
        Trace.WriteLine("It works!");
        Environment.Exit(0);
    }
}

The loader

Here’s how:

namespace LooseXaml;

using System.Collections.Generic;
using System.IO;
using System.Reflection;
using System.Xaml;

public sealed class Loader
{
    public T? Load<T>(
        TextReader textReader,
        Assembly localAssembly,
        IEnumerable<Assembly> assemblies)
        where T : class
    {
        var context = new XamlSchemaContext(assemblies);
        var settings = new XamlXmlReaderSettings
        {
            LocalAssembly = localAssembly,
            ProvideLineInfo = true,
            AllowProtectedMembersOnRoot = true
        };
        using var xamlXmlReader = new XamlXmlReader(
            textReader,
            context,
            settings
        );
        var root = XamlServices.Load(xamlXmlReader);
        return root as T;
    }
}

Things to remember

XAML only supports parameterless constructors

Well, technically you can use the x:FactoryMethod or x:Arguments directives.

If you do that then remember to include the xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" namespace in your XAML.

`XamlXmlReaderSettings.LocalAssembly` is important

According to Microsoft’s documentation you can omit the ;assembly=YourAssembly part of xmlns="clr-namespace:YourNamespace;assembly=YourAssembly".

When you do then it’ll assume you’re talking about the current assembly. Which is what XamlXmlReaderSettings.LocalAssembly points to. So you need to set that property to the current assembly.

The point

You can use XAML for more than just WPF!