具有 I/O 个依赖项的单元测试

Question

我想测试以下 class 但 I/O 和密封的 class 依赖项使它变得非常困难。

public class ImageDrawingCombiner
{
    /// <summary>
    ///     Save image to a specified location in path
    /// </summary>
    /// <param name="path">Location to save the image</param>
    /// <param name="surface">The image as canvas</param>
    public void CombineDrawingsIntoImage(Uri path, Canvas surface)
    {
        Size size = new Size(surface.ActualWidth, surface.ActualHeight);

        // Create a render bitmap and push the surface to it
        RenderTargetBitmap renderBitmap = new RenderTargetBitmap(
            (int)size.Width, (int)size.Height, 96d, 96d, PixelFormats.Pbgra32);
        renderBitmap.Render(surface);

        SaveBitmapAsPngImage(path, renderBitmap);
    }

    // SaveBitmapAsPngImage(path, renderBitmap);
    private void SaveBitmapAsPngImage(Uri path, RenderTargetBitmap renderBitmap)
    {
        // Create a file stream for saving image
        using (FileStream outStream = new FileStream(path.LocalPath, FileMode.OpenOrCreate))
        {
            // Use png encoder for our data
            PngBitmapEncoder encoder = new PngBitmapEncoder();
            // push the rendered bitmap to it
            encoder.Frames.Add(BitmapFrame.Create(renderBitmap));
            // save the data to the stream
            encoder.Save(outStream);
        }
    }
}

稍微重构了 SaveBitmapAsPngImage 方法：

// SaveBitmapAsPngImage(path, renderBitmap, new PngBitmapEncoder());
    public void SaveBitmapAsPngImage(Uri path, BitmapSource renderBitmap, BitmapEncoder pngBitmapEncoder)
    {
        // Create a file stream for saving image
        using (FileStream outStream = new FileStream(path.LocalPath, FileMode.OpenOrCreate))
        {
            // Use png encoder for our data
            // push the rendered bitmap to it
            pngBitmapEncoder.Frames.Add(BitmapFrame.Create(renderBitmap));
            // save the data to the stream
            pngBitmapEncoder.Save(outStream);

}

使其 public 可测试（代码味道？）。它仍在使用 FileStream。有人会建议用 MemoryStream and/or 工厂模式替换它，但最后它必须保存到某个地方的图像文件中。

即使我用包装器或接口 (SystemInterface) 替换所有基于 I/O 的调用： - 应该在哪里初始化实例？在复合根？泡沫太多了... - 如何使用 DI 避免 "up to 3 constructor parameter" 规则？ - 对于这个简单的功能来说，这一切听起来工作量很大

测试应确保生成图像文件。

编辑：尝试运行 @Nkosi Moq 测试，但需要维修。已替换：

var renderBitmap = new Canvas();

与：

Size renderSize = new Size(100, 50);
var renderBitmap = new RenderTargetBitmap(
    (int)renderSize.Width, (int)renderSize.Height, 96d, 96d, PixelFormats.Pbgra32);

测试结果：

BitmapServiceTest.BitmapService_Should_SaveBitmapAsPngImage threw exception: System.IO.IOException: Cannot read from the stream. ---> System.Runtime.InteropServices.COMException: Exception from HRESULT: 0x88982F72 at System.Windows.Media.Imaging.BitmapEncoder.Save(Stream stream)

编码器似乎对模拟的 Moq 流不满意。 PngBitmapEncoder 依赖项是否也应该通过方法注入（并在测试中模拟）？

Answer 1

这完全是设计问题。尽量避免与实现问题紧密耦合（类应该依赖于抽象而不是具体化）。

根据您当前的设计考虑以下内容

public interface IBitmapService {
    void SaveBitmapAsPngImage(Uri path, BitmapSource renderBitmap);
}

public interface IFileSystem {
    Stream OpenOrCreateFileStream(string path);
}

public class PhysicalFileSystem : IFileSystem {
    public Stream OpenOrCreateFileStream(string path) {
        return new FileStream(path, FileMode.OpenOrCreate);
    }
}

public class BitmapService : IBitmapService {
    private readonly IFileSystem fileSystem;

    public BitmapService(IFileSystem fileSystem) {
        this.fileSystem = fileSystem;
    }

    // SaveBitmapAsPngImage(path, renderBitmap);
    public void SaveBitmapAsPngImage(Uri path, BitmapSource renderBitmap) {
        // Create a file stream for saving image
        using (var outStream = fileSystem.OpenOrCreateFileStream(path.LocalPath)) {
            // Use png encoder for our data
            PngBitmapEncoder encoder = new PngBitmapEncoder();
            // push the rendered bitmap to it
            encoder.Frames.Add(BitmapFrame.Create(renderBitmap));
            // save the data to the stream
            encoder.Save(outStream);
        }
    }
}

public interface IImageDrawingCombiner {
    void CombineDrawingsIntoImage(Uri path, Canvas surface);
}

public class ImageDrawingCombiner : IImageDrawingCombiner {
    private readonly IBitmapService service;

    public ImageDrawingCombiner(IBitmapService service) {
        this.service = service;
    }

    /// <summary>
    ///  Save image to a specified location in path
    /// </summary>
    /// <param name="path">Location to save the image</param>
    /// <param name="surface">The image as canvas</param>
    public void CombineDrawingsIntoImage(Uri path, Canvas surface) {
        var size = new Size(surface.ActualWidth, surface.ActualHeight);
        // Create a render bitmap and push the surface to it
        var renderBitmap = new RenderTargetBitmap(
            (int)size.Width, (int)size.Height, 96d, 96d, PixelFormats.Pbgra32);
        renderBitmap.Render(surface);
        service.SaveBitmapAsPngImage(path, renderBitmap);
    }
}

FileStream 是一个实现问题，可以在单独进行单元测试时抽象出来。

上面的每个实现都可以单独测试，它们的依赖项可以根据需要进行模拟和注入。在生产中，可以使用 DI 容器在组合根中添加依赖项。

How to assert that encoder.Save(outStream) is called?

鉴于您控制流的创建并且 System.IO.Stream 是抽象的，您可以轻松模拟它并验证它是否被写入，因为 encode.Save 必须写入流，而履行职责。

这是一个使用 Moq 模拟框架的简单示例，目标是上一个示例中的重构代码。

[TestClass]
public class BitmapServiceTest {
    [TestMethod]
    public void BitmapService_Should_SaveBitmapAsPngImage() {
        //Arrange
        var mockedStream = Mock.Of<Stream>(_ => _.CanRead == true && _.CanWrite == true);
        Mock.Get(mockedStream).SetupAllProperties();
        var fileSystemMock = new Mock<IFileSystem>();
        fileSystemMock
            .Setup(_ => _.OpenOrCreateFileStream(It.IsAny<string>()))
            .Returns(mockedStream);

        var sut = new BitmapService(fileSystemMock.Object);
        var renderBitmap = new Canvas();
        var path = new Uri("//A_valid_path");

        //Act
        sut.SaveBitmapAsPngImage(path, renderBitmap);

        //Assert
        Mock.Get(mockedStream).Verify(_ => _.Write(It.IsAny<byte[]>(), It.IsAny<int>(), It.IsAny<int>()));
    }
}

一位评论员建议使用内存流，我会在大多数其他情况下建议这样做，但在这种情况下，流被放置在被测方法中，因为它被包装在 using 语句中。这将使流中的调用成员在处理后抛出异常。通过完全模拟流，您可以更好地控制断言所调用的内容。

具有 I/O 个依赖项的单元测试

Unit testing with I/O dependencies

c#

wpf

nunit

rhino-mocks