Add one of the following nuget packages to your project:

• GoodbyeXAML.Wpf.Framework for the WPF .Net Framework version
• GoodbyeXAML.Wpf.Core for the WPF .Net Core version
• GoodbyeXAML.Avalonia for the Avalonia version

GoodbyeXAML is a collection of extension methods that lets helps you lay out WPF or Avalonia UIs using declarative C#, rather than XAML.

For example, here's a simple "Hello world" window in XAML:

    <Window x:Class="MyApp.MainWindow"
                xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
                xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
                xmlns:local="clr-namespace:MyApp"
                mc:Ignorable="d">
        <TextBlock Text="Hello world" />
    </Window>

With GoodbyeXAML, we can create the same window in pure C#:

    using System.Windows;
    using System.Windows.Controls;
    using GoodbyeXAML.Wpf.Shared;

    namespace MyApp
    {
        public class MainWindow : Window
        {
            public MainWindow()
            {
                this.Content = new TextBlock()
                    ._Text("Hello world!");
            }
        }
    }

_Text is an extension method that sets TextBlock's Text property. Every property on every Control has a corresponding extension method that sets its value and returns the Control you called it on. That means you can chain multiple calls like this:

new TextBlock()
    ._Text("Hello world!")
    ._Background(Brushes.Black)
    ._Foreground(Brushes.White);

Using this technique, you can create trees of Controls just like you can in XAML:

new StackPanel()
    ._Orientation(Orientation.Vertical)
    ._HorizontalAlignment(HorizontalAlignment.Center)
    ._VerticalAlignment(VerticalAlignment.Center)
    ._Children
    (
        new TextBlock()._Text("Hello"),
        new TextBlock()._Text("World"),
        new StackPanel()
            .Orientation(Orientation.Horizontal)
            ._Children
            (
                new TextBlock()._Text("I'm "),
                new TextBlock()._Text("a "),
                new TextBlock()._Text("nested "),
                new TextBlock()._Text("StackPanel!")
            )
    );

Unlike with XAML, though, you have access to all of the language constructs built into C#. For example, we can extract new TextBlock()._Text("Blah") into a separate function, since we keep repeating it so much:

public static TextBlock Label(string text)
{
    return new TextBlock()
        ._Text(text);
}

Or preferrably, we can use C#'s alternate syntax:

public static TextBlock Label(string text) => new TextBlock()
    ._Text(text);

Now our earlier code can be refactored to look like:

new StackPanel()
    ._Orientation(Orientation.Vertical)
    ._HorizontalAlignment(HorizontalAlignment.Center)
    ._VerticalAlignment(VerticalAlignment.Center)
    ._Children
    (
        Label("Hello"),
        Label("World"),
        new StackPanel()
            .Orientation(Orientation.Horizontal)
            ._Children
            (
                Label("I'm "),
                Label("a "),
                Label("nested "),
                Label("StackPanel!")
            )
    );

If we later want to change all of those TextBlocks (for example, to give them a dark theme), we only need to change the Label function. It is, after all, just a function!

public static TextBlock Label(string text) => new TextBlock()
    ._Text(text)
    ._Background(Brushes.Black)
    ._Foreground(Brushes.White);

Or maybe we don't want them to be TextBlocks after all-- maybe we want them to be buttons! Why? Because I said so, that's why!

public static Button Label(string text) => new Button()
    ._Content(text)
    ._Click((s, e) => MessageBox.Show(text))

Properties aren't the only things that have these extenion methods-- all public facing events do too! And because we're using C#, we can use a lambda expression as our event handler. That way we don't need to write all of the usual boilerplate just for a one-line event handler.

Of course, you can still use a delegate instead of a lambda expression, just like you would in XAML:

public static Button Label(string text) => new Button()
    ._Content(text)
    ._Click(Label_Click)

private static void Label_Click(object sender, RoutedEventArgs args)
{
    var btn = (Button)sender;
    var text = (string)(btn.Content);

    MessageBox.Show(text);
}

I hope now you can see just how flexible this technique is---far moreso than XAML!

GoodbyeXAML includes a feature I call "lambda binding." Essentially, it lets you bind a property to the result of a lambda expression.

    public static StackPanel PokemonStatsView(PokemonViewModel vm) =>
        new StackPanel()
            ._Orientation(Orientation.Vertical)
            ._Children
            (
                new TextBlock()._Text(() => vm.SpeciesName.ToUpper())
                new TextBlock()._Text(() => $"Level {vm.Level}"),
                new TextBlock()._Text(() => vm.IsMale
                    ? "Male"
                    : "Female"),
                })
            );

If you pass a parameterless lambda expression instead of a value, the property will be bound to the result of that lambda expression. The property will automatically update whenever the expression changes.

Under the hood, it does this by converting the lambda expression to an Expression Tree, and then searching it to find any references to a class implementing INotifyPropertyChanged. It will then subscribe(using a weak event pattern) to the relevant property.

GoodbyeXAML only looks at your lambda expression on a surface level; it will only subscribe to INotifyPropertyChanged objects that are directly referenced by the lambda expression. If your INotifyPropertyChanged reference is buried behind a function call, GoodbyeXAML won't detect it.

For example:

    // Assume CalculatorInput implements INotifyPropertyChanged
    var vm = new CalculatorInput();

    // This will NOT work:
    int AddNumbers() => vm.OperandA + vm.OperandB;
    var badResultText = new TextBlock()
        ._Text(() => AddNumbers());

    // But this WILL:
    var goodResultText = new TextBlock()
        ._Text(() => vm.OperandA + vm.OperandB);

In the first example, the lambda expression was just () => AddNumbers(). GoodbyeXAML will not dive into AddNumbers() when parsing it, so it won't see the part where you referenced vm.OperandA or vm.OperandB, and therefore it won't subscribe to vm's PropertyChanged event.

On the other hand, the second example WILL work because it references those two properties directly. They're not obscured behind a function call.

Before you can build any of the projects in the GoodbyeXAML folder, you first need to run CodeGenerator. Open CodeGenerator/CodeGenerator.sln in Visual Studio and click "run". This will generate a bunch of shared projects and dump them into the folder GeneratedExtensionMethods.

Now you can open GoodbyeXAML/GoodbyeXAML.sln in Visual Studio and build the individual projects at your leisure.

At some point, I will write a script or something to automated this.