现在可能太晚了，但是只有用户定义的命名空间是序列化的:

public static string XmlSerialize<T>(this T obj) where T : class
        {
            Type serialType = typeof(T);
            var xsSubmit = new XmlSerializer(serialType);

            XmlWriterSettings xws = new XmlWriterSettings() { OmitXmlDeclaration = true, Indent = true };
            

            var Namespaces = new XmlSerializerNamespaces(new XmlQualifiedName[] {           
            new XmlQualifiedName(string.Empty,  GetXmlNameSpace(serialType) ?? string.Empty )


        });
 private static string GetXmlNameSpace(Type target)
        {
            XmlRootAttribute attribute = (XmlRootAttribute)Attribute.GetCustomAttribute(target, typeof(XmlRootAttribute));
            return attribute == null ? null : attribute.Namespace;
        }

并在自定义类中定义命名空间

 [XmlRoot("IdentityTerminal",Namespace = "http://my-name-space/XMLSchema")]
   
    public class IdentityTerminal
    {
    }

这段代码允许只使用用户定义的名称空间，而忽略默认的名称空间。

可以将下面的函数复制到任何对象，以使用System.Xml名称空间添加XML保存函数。

/// <summary>
/// Saves to an xml file
/// </summary>
/// <param name="FileName">File path of the new xml file</param>
public void Save(string FileName)
{
    using (var writer = new System.IO.StreamWriter(FileName))
    {
        var serializer = new XmlSerializer(this.GetType());
        serializer.Serialize(writer, this);
        writer.Flush();
    }
}

要从保存的文件创建对象，添加以下函数并将[ObjectType]替换为要创建的对象类型。

/// <summary>
/// Load an object from an xml file
/// </summary>
/// <param name="FileName">Xml file name</param>
/// <returns>The object created from the xml file</returns>
public static [ObjectType] Load(string FileName)
{
    using (var stream = System.IO.File.OpenRead(FileName))
    {
        var serializer = new XmlSerializer(typeof([ObjectType]));
        return serializer.Deserialize(stream) as [ObjectType];
    }
}

这比调用类的ToString方法要复杂一些，但也不是很复杂。

下面是一个简单的下拉函数，可以用来序列化任何类型的对象。它返回一个包含序列化XML内容的字符串:

public string SerializeObject(object obj)
{
    System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument();
    System.Xml.Serialization.XmlSerializer serializer = new System.Xml.Serialization.XmlSerializer(obj.GetType());
    using (System.IO.MemoryStream ms = new System.IO.MemoryStream()) {
        serializer.Serialize(ms, obj);
        ms.Position = 0;
        xmlDoc.Load(ms);
        return xmlDoc.InnerXml;
    }
}

我的工作代码。返回utf8 xml启用空命名空间。

// override StringWriter
public class Utf8StringWriter : StringWriter
{
    public override Encoding Encoding => Encoding.UTF8;
}

private string GenerateXmlResponse(Object obj)
{    
    Type t = obj.GetType();

    var xml = "";

    using (StringWriter sww = new Utf8StringWriter())
    {
        using (XmlWriter writer = XmlWriter.Create(sww))
        {
            var ns = new XmlSerializerNamespaces();
            // add empty namespace
            ns.Add("", "");
            XmlSerializer xsSubmit = new XmlSerializer(t);
            xsSubmit.Serialize(writer, obj, ns);
            xml = sww.ToString(); // Your XML
        }
    }
    return xml;
}

示例返回响应Yandex api支付Aviso url:

<?xml version="1.0" encoding="utf-8"?><paymentAvisoResponse xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" performedDatetime="2017-09-01T16:22:08.9747654+07:00" code="0" shopId="54321" invoiceId="12345" orderSumAmount="10643" />

基于上述解决方案，这里有一个扩展类，您可以使用它序列化和反序列化任何对象。任何其他XML属性都由您决定。

就像这样使用它:

        string s = new MyObject().Serialize(); // to serialize into a string
        MyObject b = s.Deserialize<MyObject>();// deserialize from a string



internal static class Extensions
{
    public static T Deserialize<T>(this string value)
    {
        var xmlSerializer = new XmlSerializer(typeof(T));

        return (T)xmlSerializer.Deserialize(new StringReader(value));
    }

    public static string Serialize<T>(this T value)
    {
        if (value == null)
            return string.Empty;

        var xmlSerializer = new XmlSerializer(typeof(T));

        using (var stringWriter = new StringWriter())
        {
            using (var xmlWriter = XmlWriter.Create(stringWriter, new XmlWriterSettings { Indent = true }))
            {
                xmlSerializer.Serialize(xmlWriter, value);
                return stringWriter.ToString();
            }
        }
    }
}

以上所有点赞的答案都是正确的。这是最简单的版本:

private string Serialize(Object o)
{
    using (var writer = new StringWriter())
    {
        new XmlSerializer(o.GetType()).Serialize(writer, o);
        return writer.ToString();
    }
}