Is a Null Header Value a Set header value?

[benaadams]

Setting a Header to null

• doesn't change its count (stays at zero)
• doesn't change the Response output (header is not output)

However it will cause an error if a real value is added after:

app.Run(async (context) =>
{
    var resposne = context.Response;

    // Prints zero
    Console.WriteLine(resposne.Headers["warning"].Count);

    // Does not change response
    resposne.Headers.Add("warning", new StringValues((string)null));

    // Prints zero
    Console.WriteLine(resposne.Headers["warning"].Count);

    // Throws: System.ArgumentException: An item with the same key has already been added.
    resposne.Headers.Add("warning", new StringValues("Hello"));

    await context.Response.WriteAsync("Hello World!");
});

If you can't smell it, see it, taste it, does it exist?

Should it not throw, but set the value? (as no value is set)

From aspnet/KestrelHttpServer#2347 (which was treating it as not set for perf reasons; i.e. don't expend extra cycles tracking nothing for no effect other than to just to throw an error on a second add)

/cc @halter73 @davidfowl

[benaadams]

Clearly the correct answer is null value is not set 😉
As it acts in every measurable way as not set other than throwing exception on a second .Add

So not throwing would be be restoring sanity (bug fix); rather than a behavior change (break)

[benaadams]

Setting to empty string by contrast, will return 1 for count and output a header (with no value); so throwing in that case makes sense.

The only wrinkle/corner case in the logic is

resposne.Headers.Add("warning", new StringValues(new string[3]));

Will return 3 as count and output no headers; but if you do that you are just being very strange.

So should StringValues remove nulls when setting to a string[] so that would also return count 0?

[benaadams]

Added change to remove nulls from arrays to cover the corner case.

[benaadams]

/cc @Tratcher

[halter73]

It think we've decided we don't care about the array-of-nulls corner case here.