Selenium C＃中两个类的调用方法

这可能是一个愚蠢的问题，但我还没有找到可以将我引向解决方案的答案

说我有一个测试方法来验证登录门户的功能。它在TestClassA中。我想在TestClassB的TestInitialize方法中运行该方法，这样我就可以可靠地让硒在空白板上启动，以测试通过该登录门户的功能。

这是有问题的测试代码

using Login_Elements;
using Dashboard;

namespace Test_Dashboard_Elements
{
[TestClass]
public class DashboardTests
{
    IWebDriver _driver;
    DashboardElements dash;

    [TestInitialize]
    public void Test_Setup()
    {
        dash = new DashboardElements(_driver);
        LoginPage login = new LoginPage(_driver);
        _driver = new FirefoxDriver();
        _driver.Navigate().GoToUrl("exampleurl/login");
        login.Login();
    }
}

它调用DashboardElements的一个实例并传递硒Webdriver，然后调用LoginPage的实例并传递硒WebDriver（我认为是问题），并从LoginPage调用login方法。

    IWebDriver _driver;

    //Username field
    [FindsBy(How = How.Id, Using = "username")]
    private IWebElement userName;

    //Password field
    [FindsBy(How = How.Id, Using = "password")]
    private IWebElement password;

    //Submit Button
    [FindsBy(How = How.ClassName, Using = "btn")]
    private  IWebElement submit_button;

    //Constructor
    public LoginPage(IWebDriver driver)
    {
        this._driver = driver;
        PageFactory.InitElements(driver, this);
    }

    //Sends passed string to username field
    public void sendUserName(string strUsername)
    {
        userName.SendKeys(strUsername);
    }

    //Sends passed string to password field
    public void sendPassword(string strPassword)
    {
        password.SendKeys(strPassword);
    }

    //Clicks submit button
    public void submit()
    {
        submit_button.Click();
    }

    public void Login()
    {
        sendUserName("username");
        sendPassword("password!");
        submit();
    }

这返回

>Message: Initialization method Test_Dashboard_Elements.DashboardTests.Test_Setup threw exception.  System.ArgumentNullException: System.ArgumentNullException: searchContext may not be null Parameter name: searchContext

我觉得这与两次传递_driver有关，但我不确定该怎么做

堆栈跟踪：

>Test Name: Test_Link_Reports
Test FullName:  Test_Dashboard_Elements.DashboardTests.Test_Link_Reports
Test Source:    c:\Users\%USER%\Documents\Visual Studio 2013\Projects\%PATH TO DIR%\Page Tests\Dashboard Tests.cs : line 29
Test Outcome:   Failed
Test Duration:  0:00:06.0255821

结果消息：

>Initialization method Test_Dashboard_Elements.DashboardTests.Test_Setup threw exception. System.ArgumentNullException: System.ArgumentNullException: searchContext may not be null
Parameter name: searchContext.
Result StackTrace:  
at OpenQA.Selenium.Support.PageObjects.DefaultElementLocatorFactory.LocateElement(ISearchContext searchContext, IEnumerable`1 bys)
   at OpenQA.Selenium.Support.PageObjects.WebElementProxy.get_WrappedElement()
   at OpenQA.Selenium.Support.PageObjects.WebElementProxy.SendKeys(String text)
   at Login_Elements.LoginPage.sendUserName(String strUsername) in c:\Users\%USER%\Documents\Visual Studio 2013\Projects\%PATH TO DIR%\Page Elements\Login Elements.cs:line 39
   at Login_Elements.LoginPage.Login() in c:\Users\%USER%\Documents\Visual Studio 2013\Projects\%PATH TO DIR%\Page Elements\Login Elements.cs:line 56
   at Test_Dashboard_Elements.DashboardTests.Test_Setup() in c:\Users\%USER%\Documents\Visual Studio 2013\Projects\%PATH TO DIR%\Page Tests\Dashboard Tests.cs:line 24
DefaultElementLocatorFactory.LocateElement(ISearchContext searchContext, IEnumerable 1 bys)
WebElementProxy.get_WrappedElement()
WebElementProxy.SendKeys(String text)
LoginPage.sendUserName(String strUsername)
LoginPage.Login()
DashboardTests.Test_Setup()