Untitled

Sharing a set of XmlElementAttribute-decorated fields between types
public class RequestHeader
{
    [XmlElement]
    public string UserId { get; set; }

    [XmlElement]
    public string RequestId { get; set; }
}

[XmlRoot]
public class RequestType
{
    public RequestHeader Header { get; set; }

    // ... other fields ...
}

[XmlRoot]
public class AnotherRequestType
{
    public RequestHeader Header { get; set; }

    // ... other fields ...
}

<RequestHeader>
    <Header>                         <!-- omit this -->
        <UserId>user</UserId>
        <RequestId>123</RequestId>
    </Header>                        <!-- omit this -->
</RequestHeader>