VSTHRD013 Apply CompletedTaskAttribute only to immutable members

Microsoft.VisualStudio.Threading.CompletedTaskAttribute tells VSTHRD003 that a member always produces a completed task. Applying it to a mutable member is unsafe because the member can later be assigned an incomplete task.

Examples of patterns that are flagged by this analyzer

[CompletedTask]
private static Task CachedTask { get; set; } = Task.CompletedTask;

The diagnostic is reported on [CompletedTask], not at each use of CachedTask. Consumers take the attribute at face value and do not report VSTHRD003.

Solution

Apply [CompletedTask] only to methods that always return completed tasks, readonly fields, or non-ref get-only properties:

[CompletedTask]
private static readonly Task CachedTask = Task.CompletedTask;

If the member must remain mutable, remove [CompletedTask]. VSTHRD003 will then analyze each use normally.