HttpClient does not serialize XML correctly

Question

When calling HttpClient\'s extension method PostAsXmlAsync, it ignores the XmlRootAttribute on the class. Is this behaviour a bug?

Answer 1

Looking at the source code of PostAsXmlAsync, we can see that it uses XmlMediaTypeFormatter which internally uses DataContractSerializer and not XmlSerializer. The former doesn't respect the XmlRootAttribute:

public static Task PostAsXmlAsync(this HttpClient client, Uri requestUri, T value, CancellationToken cancellationToken)
{
      return client.PostAsync(requestUri, value, new XmlMediaTypeFormatter(),
                    cancellationToken);
}

In order to achieve what you need, you can create a your own custom extension method which explicitly specifies to use XmlSerializer:

public static class HttpExtensions
{
    public static Task PostAsXmlWithSerializerAsync(this HttpClient client, Uri requestUri, T value, CancellationToken cancellationToken)
    {
        return client.PostAsync(requestUri, value,
                      new XmlMediaTypeFormatter { UseXmlSerializer = true },
                      cancellationToken);
    }
}