Cache Common Header Values.

[harshgMSFT]

For Input and output formatters the supported media types should come from a cached pool. #868 , #896

[rynowak]

Consider just making a static class with readonly instances on it and supporting instance of MediaTypeHeader in the ProducesAttribute.

Then you can write code like this:

[HttpGet("{id}")]
[Produces(MediaTypes.ApplicationJson, MediaTypes.ApplicationXml, Type = typeof(Product))]
public Product GetProduct(int id)
{
}

As a bonus our comparison for media types will generally just be a fast-path reference equality test.

[danroth27]

Agree with @rynowak.