Streaming responses from a minimal api

Daily Knowledge Drop

Using "usual techniques", it is not possible to stream a response from a minimal api, as the yield keyword cannot be used inside an anonymous method or lambda expression. To work around this limitation, a local function can be created which returns IAsyncEnumerable, and the local function result in turn, is returned from the minimal api.

Limitation

Suppose we need to define an endpoint, which when called will stream a bool response indicating if current date time minute is 13 or not (not very practical or useful, but will work for demo purposes):

app.MapGet("/isminute13", async () =>
{
    // when the endpoint is called, loop until
    // the user cancels
    while (true)
    {
        // return a bool indicating if the current minute is 13
        yield return (DateTime.Now.Minute == 13);

        // wait 1 second, and repeat
        await Task.Delay(1000);
    }
});

The issue here, is that it does not compile, as:

The yield statement cannot be used inside an anonymous method or lambda expression

But, there is a way around this constraint of the lambda expression!

Local function

The error itself gives a clue on how to solve the problem - yield cannot be used inside a anonymous method or lambda expression, so why don't not instead create and use a non-anonymous method!:

app.MapGet("/isminute13", () =>
{
    // move all previous logic into this local method
    async IAsyncEnumerable<bool> DateTimeStream()
    {
        while (true)
        {
            yield return (DateTime.Now.Minute == 13);
            await Task.Delay(1000);
        }
    }

    // return the result of the method
    return DateTimeStream();
});

Executing the above code, and calling the endpoint will result in the following output:

[false,false,false,false,false,false,false,false,false,true,true,true

Every second a new bool will be appended to the response, indicating if the minute is 13 or not.

Simple, easy solution with minimal additional coding!

Notes

Streaming responses from an api is not something I've seen leveraged much before - however with the simplicity of configuring a minimal api for streaming, it is quick and easy to get an application up and running to determine if streaming can add value to the application or business.

References

David Fowler Tweet

Daily Drop 161: 15-09-2022

At the start of 2022 I set myself the goal of learning one new coding related piece of knowledge a day.
It could be anything - some.NET / C# functionality I wasn't aware of, a design practice, a cool new coding technique, or just something I find interesting. It could be something I knew at one point but had forgotten, or something completely new, which I may or may never actually use.

The Daily Drop is a record of these pieces of knowledge - writing about and summarizing them helps re-enforce the information for myself, as well as potentially helps others learn something new as well.