Send !DOCTYPE inside XML content using Refit

Question

How could I be sure that every time Refit sends an API call to an endpoint the !DOCTYPE declaration is defined.

<?xml version="1.0"?>
<!DOCTYPE greeting SYSTEM "hello.dtd">
<greeting>Hello, world!</greeting> 

I've found that when it's missing I got a 400 error response with this content:

No message type defined for xml root node 'greeting, line 45:16
Technical ID 14f2a71af84#2fef8552034a12

Inside the Program file I've defined this code:

services
    .AddRefitClient<IEndpointApi>(new()
    {
        ContentSerializer = new XmlContentSerializer()
    });

The models look like these:

[XmlSchemaProvider("hello.dtd")]
[XmlType("greeting")]
[XmlRoot("greeting")]
public class Greeting 
{
    [XmlText]
    public string Text { get; set; }
}

And the Refit client looks like this:

public interface IEndpointApi
{
    [Post("/endpoint")]
    Task<Response> SendData([Body] Greeting greeting);
}

Answer 1

Found a solution IHttpContentSerializer, maybe not the best one but it's working.

public class XmlDocTypeContentSerializer : IHttpContentSerializer
{
    /// <summary>Original XmlContentSerializer from Refit but without a possibility to add a !DOCTYPE.</summary>
    private readonly XmlContentSerializer _xmlContentSerializer;

    public XmlContentSerializerSettings ContentSerializerSettings { get; init; }

    public XmlDocTypeContentSerializer() : this(new())
    { }

    public XmlDocTypeContentSerializer(XmlContentSerializerSettings settings)
    {
        // No settings to process DTD or !DOCTYPE on writing... :-(
        settings.XmlReaderWriterSettings.ReaderSettings.DtdProcessing = DtdProcessing.Parse;

        ContentSerializerSettings = settings;
        _xmlContentSerializer = new(settings);
    }

    public async Task<T?> FromHttpContentAsync<T>(HttpContent content, CancellationToken cancellationToken = default) =>
        await _xmlContentSerializer.FromHttpContentAsync<T>(content, cancellationToken);

    public string? GetFieldNameForProperty(PropertyInfo propertyInfo) =>
        _xmlContentSerializer.GetFieldNameForProperty(propertyInfo);

    public HttpContent ToHttpContent<T>(T item)
    {
        ArgumentNullException.ThrowIfNull(item);

        Type type = item.GetType();
        StringWriter stringWriter = new();
        XmlSerializer xmlSerializer = new(type);
        XmlWriter xmlWriter = XmlWriter.Create(stringWriter, ContentSerializerSettings.XmlReaderWriterSettings.WriterSettings);

        xmlWriter.WriteDocType(name: "greeting", pubid: null, sysid: "hello.dtd", subset: null);
        xmlSerializer.Serialize(xmlWriter, item);

        return new StringContent(stringWriter.ToString(), new MediaTypeHeaderValue("application/xml"));
    }
}