MVVM Standards

Purpose

MVVM keeps WPF and MAUI applications testable and maintainable.

Responsibilities

Component	Responsibility
View	Layout, visual states, bindings, controls
ViewModel	Presentation state, commands, UI workflow orchestration
Model / Domain	Business state and rules
Application service	Use case execution
Infrastructure service	External systems, storage, platform APIs

View Rules

• XAML should not contain business decisions.

• Code-behind should be limited to UI-specific behaviour.

• Avoid direct service calls from views.

• Bind to ViewModel properties and commands.

ViewModel Rules

ViewModels should:

• be testable without UI runtime

• expose immutable or observable state

• use async commands for I/O

• handle cancellation where practical

• validate user input

• expose user-friendly error state

• not depend directly on WPF controls or MAUI pages

Example ViewModel

public sealed partial class CaseSearchViewModel : ObservableObject
{
    private readonly ICaseSearchService _caseSearchService;

    [ObservableProperty]
    private string _query = string.Empty;

    [ObservableProperty]
    private bool _isBusy;

    public ObservableCollection<CaseSummary> Results { get; } = new();

    public CaseSearchViewModel(ICaseSearchService caseSearchService)
    {
        _caseSearchService = caseSearchService;
    }

    [RelayCommand]
    private async Task SearchAsync(CancellationToken cancellationToken)
    {
        IsBusy = true;
        try
        {
            var results = await _caseSearchService.SearchAsync(Query, cancellationToken);
            Results.Clear();
            foreach (var result in results)
            {
                Results.Add(result);
            }
        }
        finally
        {
            IsBusy = false;
        }
    }
}

MVVM Checklist

• Can ViewModel be unit tested?

• Are commands asynchronous where needed?

• Is UI state explicit?

• Are errors represented cleanly?

• Is business logic outside the ViewModel?

• Are platform-specific dependencies abstracted?