restsharp/RestSharp · error · ArgumentNullException

Parameter value is null

Error message

Parameter value is null

What it means

XmlRestSerializer.Serialize throws ArgumentNullException if the supplied parameter's Value is null. XML serialization of a null object is undefined (System.Xml.Serialization produces no meaningful output), so RestSharp rejects it explicitly rather than emitting an empty document.

Solutions

1. Ensure the object passed to XML serialization is non-null before adding it as a body.
2. Guard AddXmlBody calls with a null check; skip the body or throw a clearer upstream error.
3. If null bodies are legitimately possible, decide on a sentinel/empty object instead of null.

Example fix

// before
request.AddXmlBody(maybeNullObject); // null -> throws on serialize

// after
if (maybeNullObject is not null)
    request.AddXmlBody(maybeNullObject);

if (parameter is XmlParameter xml && xml.Value is null)
    throw new ArgumentNullException(nameof(parameter), "XmlParameter value is null");
xmlSerializer.Serialize(parameter);

static bool HasXmlValue(Parameter p) => p is XmlParameter x && x.Value is not null;

When it happens

Trigger: Calling XmlRestSerializer.Serialize on an XmlParameter whose Value property is null; using AddXmlBody with a null object that gets wrapped in an XmlParameter with a null value.

Common situations: Passing a null domain object to AddXmlBody; a conditional code path that builds an XmlParameter from a variable that can be null; default(T) for a reference type yielding null.

Related errors

• Request body serialized to null
• Non-string body found with unsupported content type
• Supplied parameter is not an XML parameter
• Class cannot have two properties marked with…
• Failed to put file as content because body parameters were…

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/1e4998e107af69b5. Report an issue: GitHub.

Appendix: source

Thrown at src/RestSharp/Serializers/Xml/XmlRestSerializer.cs:35

public class XmlRestSerializer(IXmlSerializer serializer, IXmlDeserializer deserializer) : IRestSerializer {
    IXmlDeserializer _deserializer = deserializer;
    IXmlSerializer   _serializer   = serializer;

    public XmlRestSerializer() : this(new DotNetXmlSerializer(), new DotNetXmlDeserializer()) { }

    public ISerializer         Serializer           => _serializer;
    public IDeserializer       Deserializer         => _deserializer;
    public string[]            AcceptedContentTypes => ContentType.XmlAccept;
    public SupportsContentType SupportsContentType  => contentType => contentType.Value.EndsWith("xml", StringComparison.InvariantCultureIgnoreCase);

    public DataFormat DataFormat => DataFormat.Xml;

    public string? Serialize(Parameter parameter) {
        if (parameter is not XmlParameter xmlParameter)
            throw new ArgumentException("Supplied parameter is not an XML parameter", nameof(parameter));

        if (parameter.Value == null)
            throw new ArgumentNullException(nameof(parameter), "Parameter value is null");

        var savedNamespace = _serializer.Namespace;
        _serializer.Namespace = xmlParameter.XmlNamespace ?? savedNamespace;

        var result = _serializer.Serialize(parameter.Value);

        _serializer.Namespace = savedNamespace;

        return result;
    }

    public XmlRestSerializer WithXmlSerializer(IXmlSerializer xmlSerializer) {
        _serializer = xmlSerializer;
        return this;
    }

    public XmlRestSerializer WithXmlDeserializer(IXmlDeserializer xmlDeserializer) {
        _deserializer = xmlDeserializer;

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