reactiveui/refit · error · NotSupportedException

SystemTextJsonQueryConverter requires RefitSettings.ContentS

Error message

SystemTextJsonQueryConverter requires RefitSettings.ContentSerializer to be a SystemTextJsonContentSerializer.

What it means

SystemTextJsonQueryConverter<T>.Flatten throws NotSupportedException when RefitSettings.ContentSerializer is not a SystemTextJsonContentSerializer. The converter reuses the serializer's JsonSerializerOptions to flatten the value, so a different serializer implementation cannot supply those options.

Source

Thrown at src/Refit/SystemTextJsonQueryConverter.cs:35

/// The value's runtime type is walked, so a polymorphic value contributes its actual properties. When the configured
/// serializer uses a source-generated <c>TypeInfoResolver</c> the walk is reflection- and AOT-free; otherwise it falls
/// back to System.Text.Json's reflection resolver. Nested objects are flattened under a dotted key; collections use the
/// configured <see cref="RefitSettings.CollectionFormat"/>.
/// </remarks>
[System.Diagnostics.DebuggerDisplay("{ToString(),nq}")]
public sealed class SystemTextJsonQueryConverter<T> : IQueryConverter<T>
{
    /// <inheritdoc/>
    public void Flatten(T value, string keyPrefix, ref GeneratedQueryStringBuilder builder, RefitSettings settings)
    {
        if (value is null)
        {
            return;
        }

        if (settings.ContentSerializer is not SystemTextJsonContentSerializer serializer)
        {
            throw new NotSupportedException(
                $"SystemTextJsonQueryConverter requires {nameof(RefitSettings)}.{nameof(RefitSettings.ContentSerializer)} to be a {nameof(SystemTextJsonContentSerializer)}.");
        }

        SystemTextJsonQueryFlattener.FlattenObject(value, keyPrefix, ref builder, settings, serializer.SerializerOptions, 0);
    }
}

View on GitHub (pinned to b455f65ecc)

Solutions

1. Set RefitSettings.ContentSerializer to a SystemTextJsonContentSerializer so the options are available
2. If you must use a different content serializer, write a custom IQueryConverter that does not depend on System.Text.Json options
3. Align both the content serializer and the query converter to the same JSON stack
4. Register the query converter only for interfaces whose settings use SystemTextJsonContentSerializer

Example fix

// before
var settings = new RefitSettings(new NewtonsoftJsonContentSerializer());
settings.QueryConverter = new SystemTextJsonQueryConverter<MyFilter>();
await api.SearchAsync(filter); // throws - serializer mismatch

// after
var jsonOpts = new JsonSerializerOptions();
var settings = new RefitSettings(new SystemTextJsonContentSerializer(jsonOpts));
settings.QueryConverter = new SystemTextJsonQueryConverter<MyFilter>();

if (settings.ContentSerializer is not SystemTextJsonContentSerializer)
    throw new NotSupportedException(
        "SystemTextJsonQueryConverter requires a SystemTextJsonContentSerializer.");
settings.QueryConverter = new SystemTextJsonQueryConverter<MyFilter>();

static bool IsSystemTextJson(RefitSettings s) =>
    s.ContentSerializer is SystemTextJsonContentSerializer;

try { await api.SearchAsync(filter); }
catch (NotSupportedException ex) when (ex.Message.Contains("SystemTextJsonContentSerializer"))
{
    // align the content serializer to System.Text.Json or use a custom query converter
}

When it happens

Trigger: Registering SystemTextJsonQueryConverter<T> as the query converter for a type while RefitSettings.ContentSerializer is a non-System.Text.Json serializer (e.g. Newtonsoft.Json or a custom IHttpContentSerializer). The mismatch is detected on the first call to Flatten for a non-null value.

Common situations: Mixing a Newtonsoft-based content serializer with the System.Text.Json query converter; registering the query converter globally without aligning the content serializer; upgrading Refit and inheriting the converter while the app still uses a legacy serializer.

Related errors

• This interface needs the reflection request builder, which i
• The configured IHttpContentSerializer does not support strea
• Sequence contains more than one matching element
• Sequence contains no matching element
• Unexpected parameter type in a Multipart request. Parameter

AI-assisted analysis of reactiveui/refit@b455f65ecc (2026-08-13). Data as JSON: /api/errors/dc6a42308c9ea854. Report an issue: GitHub.