restsharp/RestSharp · error · ArgumentException

Class cannot have two properties marked with SerializeAs(Con

Error message

Class cannot have two properties marked with SerializeAs(Content = true) attribute.

What it means

Thrown by the XML serializer (Serialize path) when two or more properties on the class are marked [SerializeAs(Content = true)]. Content=true designates one property as the element's text content; XML permits a single text node, so a second one is rejected during serialization.

Source

Thrown at src/RestSharp.Serializers.Xml/XmlSerializer.cs:144

            var propType       = prop.PropertyType;
            var useAttribute   = false;
            var setTextContent = false;
            var options        = prop.GetAttribute<SerializeAsAttribute>();

            if (options != null) {
                name = options.Name.IsNotEmpty()
                    ? options.Name
                    : name;

                name = options.TransformName(name!);

                useAttribute = options.Attribute;

                setTextContent = options.Content;

                if (textContentAttributeAlreadyUsed && setTextContent)
                    throw new ArgumentException("Class cannot have two properties marked with SerializeAs(Content = true) attribute.");

                textContentAttributeAlreadyUsed |= setTextContent;
            }
            else if (globalOptions != null) {
                name = globalOptions.TransformName(name);
            }

            var nsName  = name.AsNamespaced(Namespace);
            var element = new XElement(nsName!);

            if (propType.GetTypeInfo().IsPrimitive ||
                propType.GetTypeInfo().IsValueType ||
                propType == typeof(string)) {
                var value = GetSerializedValue(rawValue);

                if (useAttribute) {
                    root.Add(new XAttribute(name, value));
                    continue;

View on GitHub (pinned to 6a50821692)

Solutions

1. Remove [SerializeAs(Content = true)] from all but one property on the class.
2. Keep a single property as the text-content representative of the element.
3. Add a unit test that serializes the type to catch duplicate Content attributes early.

Example fix

// before
[SerializeAs(Content = true)] public string Text { get; set; }
[SerializeAs(Content = true)] public string Note { get; set; }

// after
[SerializeAs(Content = true)] public string Text { get; set; }
public string Note { get; set; }

var count = typeof(T).GetProperties().Select(p => p.GetCustomAttributes(typeof(SerializeAsAttribute), false).Cast<SerializeAsAttribute>().FirstOrDefault(a => a?.Content == true)).Count(a => a != null);
if (count > 1) throw new InvalidOperationException("Multiple [SerializeAs(Content=true)] properties detected");

try { request.AddXmlBody(obj); } catch (ArgumentException ex) when (ex.Message.Contains("two properties marked with SerializeAs(Content = true)")) { /* remove duplicate Content attribute on the DTO */ }

When it happens

Trigger: Serializing an object to XML (e.g. AddXmlBody or request body serialization) where the class has two properties decorated with [SerializeAs(Content = true)].

Common situations: Decorating multiple string properties as content intentionally (misunderstanding); copy-paste error; refactoring that duplicated the attribute.

Related errors

• Class cannot have two properties marked with SerializeAs(Con
• Supplied parameter is not an XML parameter
• Parameter value is null
• Couldn't parse the value of '{value}' into the '{prop.Name}'
• Request body serialized to null

AI-assisted analysis of restsharp/RestSharp@6a50821692 (2026-08-13). Data as JSON: /api/errors/cb06807886ca2ff7. Report an issue: GitHub.