IHttpContentSerializer make ToHttpContent async

[KurinnyiVadym]

It would be useful if the ToHttpContent method could be asynchronous.
In my case, the request body type T needs to be available before I can perform an asynchronous encryption step. I initially considered using a DelegatingHandler, but at that stage, the request body has already been serialised into raw JSON, and the original type T is no longer accessible.
Having an async ToHttpContent would make it possible to work with the typed request model before it’s serialised, enabling scenarios like encryption or other async preprocessing steps.

[glennawatson]

You do not actually need ToHttpContent itself to be async to do async work before the bytes go on the wire. HttpContent is already an async-serialization abstraction: its SerializeToStreamAsync runs at send time. So you can return a custom HttpContent from a custom IHttpContentSerializer.ToHttpContent<T> that captures the typed T and performs your async encryption when it is serialized.

sealed class EncryptingContent<T>(T item, IHttpContentSerializer inner) : HttpContent
{
    protected override async Task SerializeToStreamAsync(Stream stream, TransportContext? context)
    {
        // you still have the typed model here
        using var plain = inner.ToHttpContent(item);
        var bytes = await plain.ReadAsByteArrayAsync().ConfigureAwait(false);
        var encrypted = await EncryptAsync(bytes).ConfigureAwait(false);
        await stream.WriteAsync(encrypted).ConfigureAwait(false);
    }

    protected override bool TryComputeLength(out long length) { length = -1; return false; }
}

public sealed class EncryptingSerializer(IHttpContentSerializer inner) : IHttpContentSerializer
{
    public HttpContent ToHttpContent<T>(T item) => new EncryptingContent<T>(item, inner);
    public Task<T?> FromHttpContentAsync<T>(HttpContent content, CancellationToken ct = default)
        => inner.FromHttpContentAsync<T>(content, ct);
    public string? GetFieldNameForProperty(PropertyInfo p) => inner.GetFieldNameForProperty(p);
}

Wire it via new RefitSettings { ContentSerializer = new EncryptingSerializer(new SystemTextJsonContentSerializer()) }. This keeps the typed T available and moves the async step to serialization time, which is exactly the deferral you want - no interface change needed.

Leaving this open as an idea is fine, but the pattern above unblocks the encryption scenario today without an async ToHttpContent.

[vadymkurinnyi]

Glenn, thank you for the solution.