R3Command

Languages

简体中文

Apply [R3Command] (MvvmAIO.R3) to instance methods on a partial class or struct. The generator emits a public ReactiveCommand property on the same partial type.

Prerequisites

Topic	Detail
Partial type	The declaring type must be partial — otherwise R3SG0001
Method kind	Instance methods only (not static)
Attribute	MvvmAIO.R3.R3Command or MvvmAIO.R3.R3CommandAttribute
Output	Public properties such as SaveCommand or a custom CommandName
CanExecute	Optional; must be R3.Observable<bool> or IObservable<bool> on the same partial type

Observable event APIs do not require partial on the event source type — only command generation does.

Quick start

public partial class ShellViewModel
{
    private readonly Observable<bool> _canSave = new(true);

    [R3Command(CanExecute = nameof(_canSave))]
    private async Task Save() { /* ... */ }

    [R3Command]
    private void ExecuteBasic() { }
}

After build, the type gains properties like SaveCommand and ExecuteBasicCommand. Bind them in WPF or Avalonia like any ICommand / command property (for example Command="{Binding SaveCommand}").

How generation works

[R3Command] on partial method Save()
        │
        ▼
public ReactiveCommand SaveCommand { get; }
        │
        ▼
Handler wires method body (sync, async, or with argument)

• Default property name: {MethodName}Command
• CommandName = "Submit" → property Submit
• CanExecute = nameof(_field) → passes the observable into the ReactiveCommand constructor

Method signature matrix

Parameters	Return type	Generated property	When to use
none	void	ReactiveCommand	Simple sync command
one (T)	void	ReactiveCommand<T>	Sync with argument
none	Task	ReactiveCommand	Async (ValueTask inside handler)
one (T)	Task	ReactiveCommand<T>	Async with argument
none	ValueTask	ReactiveCommand	Async without result
one (T)	ValueTask	ReactiveCommand<T>
one (T)	Task<TResult>	ReactiveCommand<T, TResult>	Async with result
one (T)	ValueTask<TResult>	ReactiveCommand<T, TResult>
none	Task<TResult> / ValueTask<TResult>	—	R3SG1001
two or more parameters	any	—	R3SG1001
static method	any	—	R3SG1001

CanExecute

public partial class ShellViewModel
{
    private readonly Observable<bool> _canSave = new(true);

    [R3Command(CanExecute = nameof(_canSave))]
    private async Task Save() { }
}

Issue	Diagnostic
Member name not found	R3SG1002
Member is not Observable<bool> / IObservable<bool>	R3SG1003

The CanExecute field or property must exist on the same partial type as the command method.

Async commands

Task and ValueTask handlers are supported; the generator wraps them appropriately for ReactiveCommand.

[R3Command]
private async Task ExecuteAsync()
{
    await Task.Delay(1000);
}

Commands that return Task<TResult> / ValueTask<TResult> require one parameter and produce ReactiveCommand<T, TResult>.

Troubleshooting

Id	Fix
R3SG0001	Add partial to the declaring type
R3SG1001	Change method to a supported signature (see matrix)
R3SG1002	Fix CanExecute name or add the member
R3SG1003	Use Observable<bool> or IObservable<bool> for CanExecute
R3SG1004	Rename methods or set distinct CommandName values

Full reference: Diagnostics.

Try it in the samples

MvvmAIO.R3.SourceGenerators.Samples — navigation [R3Command] · Commands.

The sample demonstrates sync, parameterized, and async commands:

[MvvmAIO.R3.R3Command]
private void ExecuteBasic() { /* ... */ }

[MvvmAIO.R3.R3Command]
private void ExecuteWithParam(string message) { /* ... */ }

[MvvmAIO.R3.R3Command]
private async Task ExecuteAsync() { /* ... */ }

The same view model also uses FromEventHandlers() on INotifyPropertyChanged for property-change streams — see Observable events.

Further reading

• Getting started
• Observable events
• Diagnostics reference