Reading the http post using WCF

[ServiceContract]
public interface ISampleService
{
    [OperationContract]
    [WebInvoke(UriTemplate = "invoke")]
    void DoWork(Stream input);
}

<form method="post" action="Service.svc/invoke">
    <label for="firstName">First Name</label>: <input type="text" name="firstName" value="" />
    <br /><br />
    <label for="lastName">Last Name</label>: <input type="text" name="lastName" value="" />
    <p><input type="submit" /></p>
</form>

public void DoWork(Stream input)
{
    StreamReader sr = new StreamReader(input);
    string s = sr.ReadToEnd();
    sr.Dispose();
    NameValueCollection qs = HttpUtility.ParseQueryString(s);
    string firstName = qs["firstName"];
    string lastName = qs["lastName"];
 

}