要生成并行测试的Extent报告，可以使用XUnit和SpecFlow来编写测试代码和生成报告。下面是一个示例解决方法：

1. 首先，确保你已经使用NuGet安装了XUnit和SpecFlow的相关包。

2. 创建一个SpecFlow项目，并添加一个.feature文件，用于定义测试场景和步骤。

3. 在项目中创建一个XUnit测试类，用于实现SpecFlow的步骤和断言。

using Xunit;
using TechTalk.SpecFlow;

[Binding]
public class MyFeatureSteps
{
    private int result;

    [Given(@"I have entered (.*) into the calculator")]
    public void GivenIHaveEnteredIntoTheCalculator(int number)
    {
        // Enter the number into the calculator
    }

    [When(@"I press add")]
    public void WhenIPressAdd()
    {
        // Press the add button
    }

    [Then(@"the result should be (.*) on the screen")]
    public void ThenTheResultShouldBeOnTheScreen(int expectedResult)
    {
        // Assert the result on the screen
        Assert.Equal(expectedResult, result);
    }
}

4. 在项目中创建一个XUnit测试类，用于并行运行SpecFlow测试和生成Extent报告。

using Xunit;
using Xunit.Abstractions;
using TechTalk.SpecFlow;
using AventStack.ExtentReports;
using AventStack.ExtentReports.Reporter;

public class ParallelTestRunner
{
    private readonly ITestOutputHelper output;
    private static ExtentReports extent;
    private static ExtentTest test;

    public ParallelTestRunner(ITestOutputHelper output)
    {
        this.output = output;
    }

    [Fact]
    public void RunTestsInParallel()
    {
        // Create the Extent report and test
        var htmlReporter = new ExtentHtmlReporter("TestReport.html");
        extent = new ExtentReports();
        extent.AttachReporter(htmlReporter);
        test = extent.CreateTest("My Feature Test");

        // Run the SpecFlow tests in parallel
        var thread = new Thread(() => RunSpecFlowTests());
        thread.Start();
        thread.Join();

        // Flush the report and close the Extent instance
        extent.Flush();
        extent.Close();
    }

    private void RunSpecFlowTests()
    {
        // Specify the SpecFlow test assembly and configuration
        var testAssembly = Assembly.GetExecutingAssembly();
        var config = new TechTalk.SpecFlow.Configuration.Configuration(testAssembly.Location);

        // Run the SpecFlow tests
        var testRunner = TechTalk.SpecFlow.TestRunnerManager.GetTestRunner(config);
        var testAssemblyPath = Path.GetDirectoryName(testAssembly.Location);
        var testResultsPath = Path.Combine(testAssemblyPath, "TestResults");
        var testResults = testRunner.RunTests(testAssembly, testResultsPath);

        // Log the SpecFlow test results to the Extent report
        foreach (var testResult in testResults)
        {
            var stepResults = testResult.TestResult.StepResults;
            foreach (var stepResult in stepResults)
            {
                if (stepResult.Exception != null)
                {
                    test.Log(Status.Fail, stepResult.Exception.ToString());
                }
                else
                {
                    test.Log(Status.Pass, stepResult.StepDefinitionType + " " + stepResult.StepInfo.Text);
                }
            }
        }
    }
}

5. 运行该测试类，你将获得一个Extent报告文件（TestReport.html），其中包含了并行运行的SpecFlow测试结果。

请注意，上述代码只是一个示例，你可能需要根据你的具体需求进行修改和调整。另外，你可能需要安装其他相关的NuGet包和配置文件来支持并行测试和生成Extent报告。