[API Proposal]: System.Net.Mime.MediaTypeMap

[stephentoub]

Background and motivation

It's reasonably common to need to get a MIME / media type for a file extension, or vice versa. The core .NET libraries don't include any exposed mapping, so developers end up needing to re-create the subset of the mapping that's relevant to their current needs. Java supports this with URLConnection.getFileNameMap, Go supports this with its built-in mime.TypeByExtension, Swift has UTType... .NET should have this available as well. It would support all IANA mappings plus very common ones like those from Apache mime.types or mime-db.

API Proposal

namespace System.Net.Mime; // existing namespace for MediaTypeNames

public static class MediaTypeMap // members could alternatively be added to MediaTypeNames if we wanted to avoid the extra type
{
    public static bool TryGetMediaType(ReadOnlySpan<char> extension, [NotNullWhen(true)] out string? mediaType);
    public static bool TryGetExtension(ReadOnlySpan<char> mediaType, [NotNullWhen(true)] out string? extension);
}

API Usage

UriContent image = new(uri, MediaTypeMap.TryGetMediaType(Path.GetExtension(uri.AbsolutePath), out var mediaType) ? mediaType : "image/*"));

Alternative Designs

namespace System.Net.Mime; // existing namespace for MediaTypeNames

public static class MediaTypeMap // members could alternatively be added to MediaTypeNames if we wanted to avoid the extra type
{
    public static string? GetMediaType(ReadOnlySpan<char> extension);
    public static string? GetExtension(ReadOnlySpan<char> mediaType);
}

namespace System.Net.Mime; // existing namespace for MediaTypeNames

public static class MediaTypeMap // members could alternatively be added to MediaTypeNames if we wanted to avoid the extra type
{
    // optional, on top of the functions
    public static IReadOnlyDictionary<string, string> MediaTypeToExtension { get; }
    public static IReadOnlyDictionary<string, string> ExtensionToMediaType { get; }
}

Risks

• We may not be "complete" in the opinion of some consumers and may have a trickle of requests for adding new entries. Those new entries would not require public API, though; they'd just be updating the internal map.

EDIT: updated after internal discussion

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I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• System.Net.Mime.MediaTypeNames missing types #1489

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[MichalPetryka]

// optional
public static IReadOnlyDictionary<string, string> MediaTypeToExtension { get; }
public static IReadOnlyDictionary<string, string> ExtensionToMediaType { get; }

Why not FrozenDictionary?

[stephentoub]

// optional
public static IReadOnlyDictionary<string, string> MediaTypeToExtension { get; }
public static IReadOnlyDictionary<string, string> ExtensionToMediaType { get; }

Why not FrozenDictionary?

The implementation could still be one. If we were to add such properties (I suggested they'd be optional), having them be interfaces gives us flexibility for their implementations.