I've just declared a constant for the "application/json" content type in one of my classes.
public const string JsonContentType = "application/json";
I'm not sure it is a good practice.
Does .NET framework have a predefined const for "application/json"?
See Newer Answer. This answer is now inaccurate.
While there are some MIME constants defined in MediaTypeNames (see here), there no constant for "application/json".
Putting additional content types in a shared const is probably best practice, better than defining them in string literals a million times throughout your code at least.
Plus it gives you the flexibility of using new/custom MIME types, which a specific .NET version might not have.