Yes. You can do WPF only with code.

Here is a simple example from http://www.java2s.com/Tutorial/CSharp/0470__Windows-Presentation-Foundation/CodeOnlyWPFApplicationSample.htm

using System;
using System.Windows;
using System.Windows.Controls;

  public class App : Application {

    protected override void OnStartup(StartupEventArgs e) {
      base.OnStartup(e);
      MainWindow window = new MainWindow();
      window.Show();
    }

    [STAThread]
    public static void Main()
    {
        App app = new App();
        app.Run();
    }
  }

  public class MainWindow : Window {

    protected override void OnInitialized(EventArgs e) {
      base.OnInitialized(e);
      Button closeButton = new Button();
      closeButton.Content = "Close";
      this.Content = closeButton;
      closeButton.Click += delegate {
        this.Close();
      };
    }
  }

Also, it might be easier to use WinForms since it uses C# as code-behind and less XML.

I'm going to try and build up from the other example to get you compile-able code. To get WPF working, you'll need a project that builds with WPF support.

The simplest way is to create a .csproj ourself and use the previously posted code-only solution.

So, start with a minimal App.csproj file as so:

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <OutputType>WinExe</OutputType>
    <TargetFramework>net5.0-windows</TargetFramework>
    <UseWPF>true</UseWPF>
  </PropertyGroup>
</Project>

Then the app & window code in App.cs as before. I've added a couple controls to it, as you'll need a container anyway (I used StackPanel) if you want to start adding more stuff later.

using System;
using System.Windows;
using System.Windows.Controls;

public class App : Application {
    protected override void OnStartup(StartupEventArgs e)     {
        base.OnStartup(e);
        new MainWindow().Show();
    }

    [STAThread]
    public static void Main() => new App().Run();
}

public class MainWindow : Window {
    protected override void OnInitialized(EventArgs e) {
        base.OnInitialized(e);

        var panel = new StackPanel();
        this.Content = panel;

        var label = new Label { Content = "Click me:" };
        panel.Children.Add(label);
        
        var closeButton = new Button { Content = "Close", Height = 100 };
        closeButton.Click += (sender, args) => this.Close();
        panel.Children.Add(closeButton);

        (this.Width, this.Height) = (200, 200);
    }
}

You then build the whole project by using dotnet build. And you can run it with dotnet run or invoking the exe directly.

Alternatively, if you'd like to explore a WPF project with separate .xaml and .xaml.cs code, you can create your project using dotnet new wpf -o MyProjectName. This will produce a project, a xaml-based App, and a xaml-based form.