LINQ + Webservice

Question

Hello,

I have a problem, I created a Webservice with LINQ to SQL but is giving error, someone could tell me what's wrong?

My Webservice:

[WebMethod(Description = "Linq and Webservice")]
  public List<User> returnUser()
  {
    List<User> people= null;

    dbLinkITDataContext db = new dbLinkITDataContext();
    var query = from c in db.Users
          select c;
    people = query.ToList();

    return people;
  }

ERRO:

 

System.InvalidOperationException: There was an error generating the XML document. ---> System.InvalidOperationException: A circular reference was detected while serializing an object of type Usuario.
  at System.Xml.Serialization.XmlSerializationWriter.WriteStartElement(String name, String ns, Object o, Boolean writePrefixed, XmlSerializerNamespaces xmlns)
  at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write10_Usuario(String n, String ns, Usuario o, Boolean isNullable, Boolean needType)
  at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write6_Empresa(String n, String ns, Empresa o, Boolean isNullable, Boolean needType)
  at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write10_Usuario(String n, String ns, Usuario o, Boolean isNullable, Boolean needType)
  at Microsoft.Xml.Serialization.GeneratedAssembly.XmlSerializationWriter1.Write11_ArrayOfUsuario(Object o)
  at Microsoft.Xml.Serialization.GeneratedAssembly.ListOfUsuarioSerializer.Serialize(Object objectToSerialize, XmlSerializationWriter writer)
  at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
  --- End of inner exception stack trace ---
  at System.Xml.Serialization.XmlSerializer.Serialize(XmlWriter xmlWriter, Object o, XmlSerializerNamespaces namespaces, String encodingStyle, String id)
  at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o, XmlSerializerNamespaces namespaces)
  at System.Xml.Serialization.XmlSerializer.Serialize(TextWriter textWriter, Object o)
  at System.Web.Services.Protocols.XmlReturnWriter.Write(HttpResponse response, Stream outputStream, Object returnValue)
  at System.Web.Services.Protocols.HttpServerProtocol.WriteReturns(Object[] returnValues, Stream outputStream)
  at System.Web.Services.Protocols.WebServiceHandler.WriteReturns(Object[] returnValues)
  at System.Web.Services.Protocols.WebServiceHandler.Invoke()