Fan-in fan-out

ctx.WhenAll runs several activities concurrently and resumes the workflow once all of them have completed — and like every step, the fan-out is durable: a crash mid-flight resumes cleanly without re-running finished branches.

Parallel activities with typed results

Use ctx.CallActivity to create the pending activities, then await them together. Up to three differently-typed activities come back as a tuple:

var (hello, goodbye) = await ctx.WhenAll(
    ctx.CallActivity((HelloActivities a) => a.Hello(input.Name)),
    ctx.CallActivity((HelloActivities a) => a.Goodbye(input.GoodbyeId)),
    cancellationToken
);

Fan-out over a collection

When every branch has the same result type, pass a sequence and get an array back:

var results = await ctx.WhenAll(
    customers.Select(c =>
        ctx.CallActivity((EmailActivities a) => a.SendNewsletter(c.Email))
    ),
    cancellationToken
);

What happens on failure?

How it stays durable