.net web api不以json格式返回

config.Formatters.XmlFormatter.SupportedMediaTypes.Clear();
config.Formatters.XmlFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/xml"));

public class CsvMediaTypeFormatter : BufferedMediaTypeFormatter
{
    public CsvMediaTypeFormatter()
    {
        SupportedMediaTypes.Add(new MediaTypeHeaderValue("text/csv"));
    }

    public override bool CanWriteType(Type type)
    {
        return typeof(IEnumerable).IsAssignableFrom(type);
    }

    public override void WriteToStream(Type type, object value, Stream writeStream, HttpContent content)
    {
        using (var writer = new StreamWriter(writeStream))
        {
            var csv = new CsvWriter(writer);
            csv.WriteRecords((IEnumerable)value);
        }
    }
}

config.Formatters.Add(new CsvMediaTypeFormatter());