using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Firefox;
using OpenQA.Selenium.Support.UI;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace ToolsQA
{
    class FirstTestCase
    {
        static void Main(string[] args)
        {
            IWebDriver driver = new FirefoxDriver();
            driver.Navigate().GoToUrl(" http://toolsqa.wpengine.com/automation-practice-form/");
            IWebElement element = driver.FindElement(By.Id("continents"));
            SelectElement dropdown = new SelectElement(element);
            dropdown.SelectByText("Europe");
       }
    }
}

这是错误的:

严重级代码描述项目文件行抑制状态错误CS1061'SelectElement'不包含'SelectByText'的定义,并且没有可以找到接受类型'SelectElement'的第一个参数的扩展方法'SelectByText'(您是否缺少using指令或汇编引用?)ToolsQA

怎么解决呢?