PosInformatique.Moq.Analyzers

PosInfoMoq2019: RaiseAsync() must be used only for events with async handlers (returning Task)

Property	Value
Rule ID	PosInfoMoq2019
Title	RaiseAsync() must be used only for events with async handlers (returning Task)
Category	Compilation
Default severity	Error

Cause

Mock<T>.RaiseAsync() must only be used with events whose delegate type returns Task (i.e., async events). Using RaiseAsync() on events whose handlers return void (or any non-Task type) is invalid.

Rule description

Moq provides RaiseAsync() to trigger asynchronous events. This requires that the event delegate returns Task (or Task<T>). If the event delegate returns void (like EventHandler or EventHandler<T>), RaiseAsync() is not appropriate and will cause runtime issues.

Delegate examples

// Async event delegate
public delegate Task AsyncEventHandler(object sender, EventArgs e);

// Classic sync delegates (return void)
public delegate void VoidEventHandler(object sender, EventArgs e);
public class DataEventArgs : EventArgs { public int Value { get; } public DataEventArgs(int v) => Value = v; }

Mocked type with both kinds of events

public class Service
{
    public event AsyncEventHandler AsyncChanged;                 // returns Task
    public event EventHandler SyncChanged;                       // returns void
    public event EventHandler<DataEventArgs> SyncDataChanged;    // returns void
}

Correct usage (async delegate returning Task)

var serviceMock = new Mock<Service>();

// Use RaiseAsync with async event (Task-returning delegate)
await serviceMock.RaiseAsync(s => s.AsyncChanged += null, EventArgs.Empty);

Incorrect usage (void-returning delegates)

var serviceMock = new Mock<Service>();

// EventHandler returns void — using RaiseAsync is invalid
await serviceMock.RaiseAsync(s => s.SyncChanged += null, EventArgs.Empty); // ❌

// EventHandler<T> returns void — using RaiseAsync is invalid
await serviceMock.RaiseAsync(s => s.SyncDataChanged += null, new DataEventArgs(42)); // ❌

How to fix violations

• If the event delegate returns void, use Raise() instead of RaiseAsync().
• If you need async semantics, change the event to use a Task-returning delegate, for example AsyncEventHandler.

Fix examples

// Using Raise() for void-returning events
serviceMock.Raise(s => s.SyncChanged += null, EventArgs.Empty);
serviceMock.Raise(s => s.SyncDataChanged += null, new DataEventArgs(42));

// Using RaiseAsync() for Task-returning events
await serviceMock.RaiseAsync(s => s.AsyncChanged += null, EventArgs.Empty);

When to suppress warnings

Do not suppress this rule. If disabled, Moq may throw a runtime exception (NullReferenceException) due to a known bug when RaiseAsync() is used on non-async events, instead of providing a clear error message.

Reference: https://github.com/devlooped/moq/issues/1568

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