I'm trying to figure out .NET serialization stuff and experiencing a problem. I've made a simple program to test it and got stuck with using attributes. Here's the code:

[Serializable]
public class SampleClass
{
    [SoapIgnore]
    public Guid InstanceId
    {
        get;
        set;
    }
}

class Program
{
    static void Main()
    {

        SampleClass cl = new SampleClass { InstanceId = Guid.NewGuid() };
        SoapFormatter fm = new SoapFormatter();
        using (FileStream stream = new FileStream(string.Format("C:\\Temp\\{0}.inv", Guid.NewGuid().ToString().Replace("-", "")), FileMode.Create))
        {
            fm.Serialize(stream, cl);
        }
    }
}

The problem is that InstanceId is not ignored while serialization is done. What I get in .inv file is something like this:

<SOAP-ENV:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:clr="http://schemas.microsoft.com/soap/encoding/clr/1.0" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
<SOAP-ENV:Body>
<a1:SampleClass id="ref-1" xmlns:a1="http://schemas.microsoft.com/clr/nsassem/TestConsoleApp/TestConsoleApp%2C%20Version%3D1.0.0.0%2C%20Culture%3Dneutral%2C%20PublicKeyToken%3Dnull">
<_x003C_InstanceId_x003E_k__BackingField>
<_a>769807168</_a>
<_b>27055</_b>
<_c>16408</_c>
<_d>141</_d>
<_e>210</_e>
<_f>171</_f>
<_g>30</_g>
<_h>252</_h>
<_i>196</_i>
<_j>246</_j>
<_k>159</_k>
</_x003C_InstanceId_x003E_k__BackingField>
</a1:SampleClass>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

As I can understand from documentation InstanceId property is not supposed to be serialized since SoapIgnore attribute is applied to it. Am I missing something?