TextEncoding

IsSupported says the runtime has TextDecoder at all. IsEncodingSupported asks about one label - there is no list of encodings anywhere in the platform, so it is answered by constructing a decoder and seeing whether it throws. GetCanonicalName normalizes an alias: 'sjis' and 'shift-jis' both come back as 'shift_jis'.

@inject Bit.Butil.TextEncoding textEncoding

var supported = await textEncoding.IsSupported();
var canDecode = await textEncoding.IsEncodingSupported("shift_jis");
var name = await textEncoding.GetCanonicalName("sjis");   // "shift_jis"

The whole point of the class. Each sample below is the same greeting written in a different code page - bytes that are meaningless as UTF-8. Decode them with the right label and they read correctly; decode them as UTF-8 and you get the mojibake every legacy import starts with.

var text = await textEncoding.Decode(bytes, "shift_jis");

// Guessing at the encoding? Turn on fatal and a wrong guess
// comes back as null instead of as replacement characters:
var strict = await textEncoding.Decode(bytes, "shift_jis", fatal: true);

TextEncoder produces UTF-8 and nothing else - there is no legacy encoder in the platform, by design. GetUtf8ByteLength answers the length question without moving the bytes across the interop boundary, which is what a check against a server-side limit actually needs.

var bytes = await textEncoding.Encode("héllo");     // byte[]
var length = await textEncoding.GetUtf8ByteLength("héllo");   // 6, not 5

Content that arrives a chunk at a time - a download, a fetch body, a file read piece by piece - can split a multi-byte character across two chunks. Decoding each chunk on its own turns that character into replacement characters; a streaming decoder holds the incomplete sequence until the next chunk completes it. The demo splits the Shift_JIS sample straight through the middle of a character to show the difference.

await using var decoder = await textEncoding.CreateDecoder("shift_jis");

foreach (var chunk in chunks)
{
    text += await decoder.Decode(chunk);   // more: true is the default
}

text += await decoder.Flush();             // emits anything still pending