reactiveui/refit · error · ArgumentException

Multiple complex types found. Specify one parameter as the b

Error message

Multiple complex types found. Specify one parameter as the body using BodyAttribute

What it means

Thrown during implicit body detection for POST/PUT/PATCH when, after excluding params explicitly bound by other attributes (Body, Query, Header, Property, Url, etc.), more than one reference-type parameter remains. Refit would implicitly serialize the first such parameter as the body, but with two ambiguous complex parameters it cannot choose, so it demands you disambiguate with [Body].

Source

Thrown at src/Refit.Reflection/RestMethodInfoInternal.cs:676

        var refParamIndex = -1;
        for (var i = 0; i < parameterArray.Length; i++)
        {
            var parameter = parameterArray[i];
            if (parameter.ParameterType.GetTypeInfo().IsValueType
                || parameter.ParameterType == typeof(string)
                || sets[i].Query is not null
                || sets[i].HeaderCollection is not null
                || sets[i].Property is not null

                // A [Url] Uri parameter supplies the request URI, never the implicit body.
                || sets[i].Url is not null)
            {
                continue;
            }

            if (refParam is not null)
            {
                throw new ArgumentException(
                    "Multiple complex types found. Specify one parameter as the body using BodyAttribute");
            }

            refParam = parameter;
            refParamIndex = i;
        }

        return refParam is null
            ? null
            : Tuple.Create(
                BodySerializationMethod.Serialized,
                RefitSettings.Buffered,
                refParamIndex);
    }

    /// <summary>Holds the parsed forms of a route parameter name extracted from a URL template.</summary>
    /// <param name="RawName">The raw parameter name from the URL template.</param>
    /// <param name="Name">The normalized parameter name with any round-tripping prefix removed.</param>

View on GitHub (pinned to b455f65ecc)

Solutions

1. Mark exactly one complex parameter with [Body] to make the body explicit.
2. Annotate the non-body complex parameter with [Query] (or [AliasAs]/[HeaderCollection]) so it is excluded from body detection.
3. If both truly belong in the body, merge them into a single composite DTO.

Example fix

// before (POST with two unannotated reference types)
Task CreateAsync(Order order, Filter filter);

// after — explicit body, filter goes to query
Task CreateAsync([Body] Order order, [Query] Filter filter);

// For POST/PUT/PATCH, flag ambiguous unannotated reference-type parameters.
using System.Reflection;
var bodyVerbs = new[] { "POST", "PUT", "PATCH" };
foreach (var m in typeof(IMyApi).GetMethods())
{
    var attr = m.GetCustomAttribute<HttpMethodAttribute>();
    if (attr is null || !bodyVerbs.Contains(attr.Method)) continue;
    var complex = m.GetParameters()
        .Where(p => !p.ParameterType.IsValueType && p.ParameterType != typeof(string)
                    && p.GetCustomAttribute<BodyAttribute>() is null
                    && p.GetCustomAttribute<QueryAttribute>() is null)
        .ToList();
    if (complex.Count > 1)
        throw new InvalidOperationException($"{m.Name}: multiple complex params — mark one [Body] or the others [Query].");
}

When it happens

Trigger: A POST/PUT/PATCH method with two unannotated reference-type parameters that aren't strings and aren't bound to query/header/url, e.g. `Task PostAsync(Order order, Customer customer)`. The loop finds a second refParam and throws.

Common situations: Sending a DTO plus another object (e.g. a file or nested model) without specifying which is the body; forgetting [Query] on a filter object that should go in the query string alongside a body DTO.

Related errors

• Multipart requests may not contain a Body parameter
• Only one parameter can be a Body parameter
• URL path {relativePath} must not contain CR or LF characters
• Method "{methodInfo.Name}" is invalid. All REST Methods must

AI-assisted analysis of reactiveui/refit@b455f65ecc (2026-08-13). Data as JSON: /api/errors/d0572a9ac96582d0. Report an issue: GitHub.