CommunityToolKit.Mvvm 8.4.0 and MVVM Toolkit warning MVVMTK0045

Question

I am following the excellent Learn .NET MAUI - Full Course for Beginners from James Montemagno https://github.com/dotnet-presentations/dotnet-maui-workshop?WT.mc_id=dotnet-29192-cxa

In this tutorial he explains how to use the CommunityToolKit.Mvvm I have some problems using it (I am using V8.4.0) For a basic usage you declare something like this:

using CommunityToolkit.Mvvm.ComponentModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace DreamMachineApp.ViewModel
{
    public partial class BaseViewModel : ObservableObject
    {
        [ObservableProperty]
        [NotifyPropertyChangedFor(nameof(IsNotBusy))]
        bool isBusy;

        [ObservableProperty]
        private string? title;

        bool IsNotBusy => !IsBusy;
    }
}

The first problem is that I get the following warning for the declaration of isBusy and title: MVVM Toolkit warning MVVMTK0045 : The field DreamMachineApp.ViewModel.BaseViewModel.isBusy using [ObservableProperty] will generate code that is not AOT compatible in WinRT scenarios (such as UWP XAML and WinUI 3 apps), and a partial property should be used instead (as it allows the CsWinRT generators to correctly produce the necessary WinRT marshalling code)

It is only a warning but I would like to get rid of it. I have tried what is advice here but its even worse. What puzzle me is in the project provided by James (using the exact same code) I do not see the error? May be some settings to be done?

The second problem is that if I look in the dependencies I see tons of warning in the toolkit

In fact you can see that in ObservablePropertyGenerator has been working successfully and the application works OK but This looks scary! I am on the lates version of Visual Studio using .net9.0 (very new release – may be the problem?)

I have tried what is recommended here: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/errors/mvvmtk0045 but it does not help

I would appreciate if someone has a solution and can explain why on the project from James there is no error ?

Answer 1

I created a new .net 9.0 maui project and reproduced your problem with the CommunityToolKit.Mvvm version 8.4.0. And you can get rid of the warning by the following two ways:

1. Use the CommunityToolKit.Mvvm version 8.3.2 instead of the CommunityToolKit.Mvvm version 8.4.0. This can make the warning disappeared directly.
2. According to the official documment about MVVM Toolkit warning MVVMTK0045 and MVVM Toolkit error MVVMTK0041.

You can add the following code into the csproj file:

<PropertyGroup>
      <LangVersion>preview</LangVersion>
</PropertyGroup>

And then change your code to:

public partial class BaseViewModel : ObservableObject
 {
     [ObservableProperty]
     public partial bool Isbusy { get; set; }
     [ObservableProperty]
     public partial string? Title { get; set; }
 }