Add a diagnostic when an Activity method returns void.

[kieronlanning]

Should I add a diagnostic if an Activity method returns void?

The only way to access the Activity in the future is to use Activity.Current. If the previous call to create/ start an activity from an ActivitySource has no listeners, then the resulting activity will be null.

This also means that any Event or Context method that doesn't take an Activity as a parameter will use the Activity.Current too potentially associating information with the wrong Activity. That too should be a diagnostic.

Additionally, I could control the diagnostic behaviour through an attribute property, remembering that by default the attributes don't end up in the compiled output.

[kieronlanning]

Confirmed, I'll raise a Diagnostic by default during the following:

• Is an activity method, and no activity is returned.
• Is an event or context method, and no activity is defined as a parameter.

System.Diagnostics.ActivitySource outer = new("outer");
System.Diagnostics.ActivitySource inner = new("inner");

ActivitySource.AddActivityListener(new()
{
	ShouldListenTo = act => act.Name == "outer",
	Sample = (ref ActivityCreationOptions<ActivityContext> activityOptions) => ActivitySamplingResult.AllData
});

var outerAct = outer.StartActivity("an-outer-activity", ActivityKind.Consumer);
if (outerAct == null) 
{
	throw new Exception("`outer` should not be null.");
}

if (Activity.Current?.DisplayName != "an-outer-activity")
{
	throw new Exception("`outer` should be the current activity.");
}

var innerAct = inner.StartActivity("an-innter-activity", ActivityKind.Producer, parentId: outerAct?.Id);
if (innerAct != null)
{
	throw new Exception("`inner` should be null.");
}

if (Activity.Current?.DisplayName != "an-outer-activity")
{
	throw new Exception	("`outer` should be the current activity.");
}

Console.WriteLine("the wrong activity was found.");

[kieronlanning]

Completed in v1.0.8.