I have this webservice;

public class InfoSpreaderServices : System.Web.Services.WebService
    {

        [WebMethod]
        public void AddUser(int listRecno, string firstname, string lastname, string email) {
            if (!DataManager.EmailExists(listRecno, email)) {
                DataManager.CreateNewRecipient(listRecno, firstname, lastname, email);
            }
            
        }

        [WebMethod]
        public void RemoveUser(int recipientRecno, string email) {
            DataRow r = DataManager.GetListRecipient(recipientRecno);

            if (r["Email"].ToString() == email) {
                DataManager.DeleteRecipient(recipientRecno);
            }
        }
    }

and I try to use the AddUser method like this;

InfoSpreadServices.InfoSpreaderServicesSoapClient client = new InfoSpreadServices.InfoSpreaderServicesSoapClient();
client.AddUser(int.Parse(TextBoxListRecno.Text), TextBoxFirstname.Text, TextBoxLastname.Text, TextBoxEmail.Text);

But I always get this error;

The operation 'AddUserAsync' could not be loaded because it has a parameter or return type of type System.ServiceModel.Channels.Message or a type that has MessageContractAttribute and other parameters of different types. When using System.ServiceModel.Channels.Message or types with MessageContractAttribute, the method must not use any other types of parameters.

any ideas on how to fix this ??

You have decorated one or more objects with [MessageContract] instead of [OperationContract]. When you use this attribute you no longer pass individual parameters but the whole object instead. Please replace MessageContract with OperationContract or only pass in the class that has the MessageContract attribute

  
public void AddUser(Recipient recipient)
{

}

[MessageContract]
public class Recipient
{

}