不一致的可访问性 - 开发 Web 服务

我正在尝试创建一个用于更新简单数据库表的 Web 服务。我有一个更新方法，该方法将参数作为员工类型的对象。我已经包含了 Employee 类所属命名空间的引用。由于我不明白的原因，我收到以下错误：可访问性不一致：参数类型"EmployeeDBApplication.Employee"比方法"EmployeeStoreWS.EmployeeStoreService.update（EmployeeDBApplication.Employee）"更难访问

class Employee
{
    private int id;
    public int Id
    {
        get { return id; }
        set { id = value; }
    }
    private double salary;
    public double Salary
    {
        get { return salary; }
        set { salary = value; }
    }
    private string address;
    public string Address
    {
        get { return address; }
        set { address = value; }
    }
    private string firstname;
    public string Firstname
    {
        get { return firstname; }
        set { firstname = value; }
    }
    private string lastname;
    public string Lastname
    {
        get { return lastname; }
        set { lastname = value; }
    }

    public override string ToString() {
        string x;
        x = "Employee ID:" + this.id + "'tName:" + this.firstname + "'t" + this.lastname + "'n'tSalary:" + this.salary + "'t Address:" + this.address; 
        return x;
    }
}

和网络服务：

public class EmployeeStoreService : System.Web.Services.WebService
{
    //id    int(11) NO  PRI 0   
    //firstname varchar(255)    YES         
    //lastname  varchar(255)    YES         
    //address   varchar(255)    YES         
    //salary    double  YES         
    [WebMethod]
    public MySqlConnection getConnection()
    {
        return new MySqlConnection("Database=sakila;Data Source=localhost;User Id=root;Password=george 01");
    }
    [WebMethod]
    public void update(Employee employee)
    {
        MySqlConnection connection = null;
        try
        {
            connection = getConnection();
            connection.Open();
            MySqlCommand myCommand = new MySqlCommand("UPDATE employee SET (?id,?firstname,?lastname,?address,?salary) WHERE employee.id = ?id");
            myCommand.Prepare();
            myCommand.Parameters.AddWithValue("?id", employee.Id);
            myCommand.Parameters.AddWithValue("?firstname", employee.Firstname);
            myCommand.Parameters.AddWithValue("?lastname", employee.Lastname);
            myCommand.Parameters.AddWithValue("?address", employee.Address);
            myCommand.Parameters.AddWithValue("?salary", employee.Salary);
            myCommand.ExecuteNonQuery();
            connection.Close();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
        finally
        {
            if (connection != null)
                connection.Close();
        }
    }
}

public class Employee

class Employee
{
    private int id;
    //...

public class Employee
{
    private int id;
    //...