c# mocking IFormFile CopyToAsync() method

Question

I am working on a unit test for an async function that converts a list of IFormFile to a list of my own arbitrary database file classes.

The method that converts the file data to a byte array is:

internal async Task<List<File>> ConvertFormFilesToFiles(ICollection<IFormFile> formFiles)
{
    var file = new File
    {
        InsertDateTime = DateTime.Now,
        LastChangeDateTime = DateTime.Now
    };
    if (formFile.Length > 0)
    {
        using (var memoryStream = new MemoryStream())
        {
            await formFile.CopyToAsync(memoryStream, CancellationToken.None);
            file.FileData = memoryStream.ToArray();
        }
    }
    // ...
}

The function receives an ICollection of IFormFiles so it is mockable.

For now I have test code like this:

//Arrange
var fileConverter = new FilesConverter();
using (var fileStream = new FileStream("Files/uploadme.txt", FileMode.Open, FileAccess.Read))
{
    using (var memoryStream = new MemoryStream())
    {
        var newMemoryStream = new MemoryStream();
        fileStream.CopyTo(memoryStream);
        FormFileMock.Setup(f => f.CopyToAsync(newMemoryStream, CancellationToken.None)).Returns(Task.CompletedTask);
        // some other setups

        //Act
        var result = await fileConverter.ConvertFormFilesToFiles(new List<IFormFile> { FormFileMock.Object });
        //Assert
        Assert.IsTrue(result.Any());
    }
}

The newMemoryStream variable is created because the function calls the CopyToAsync method with a new and empty memorystream (I'm not sure if this is necessary).

The problem is that the await formFile.CopyToAsync(memoryStream, CancellationToken.None) doesn't copy any data to the memoryStream.

Answer 1

I know this might be unpopular because using a mock framework is totally "in" these days, but why not simply leave the framework be framework and go the simple, easy way? You can create a FormFile without any mocking. Just the real deal:

var fileConverter = new FilesConverter(FilesConverterLoggerMock.Object, FileDataMock.Object);

// access to a real file should really not be in a "unit" test, but anyway: 
using (var stream = new MemoryStream(File.ReadAllBytes("Files/uploadme.txt"))
{
  // create a REAL form file
  var formFile = new FormFile(stream , 0, stream.Length, "name", "filename");

  //Act
  var result = await fileConverter.ConvertFormFilesToFiles(new List<IFormFile> { formFile });

  //Assert
  Assert.IsTrue(result.Any());
}

Answer 2

The problem is that the await formFile.CopyToAsync(memoryStream, CancellationToken.None) doesn't copy any data to the memoryStream.

According to your setup. Nothing will actually be copied.

You just setup the call to return as completed. No actual functionality was implemented.

You'll need to add a Callback to perform some desired functionality before returning the task.

FormFileMock
    .Setup(_ => _.CopyToAsync(It.IsAny<Stream>(), CancellationToken.None))
    .Callback<Stream, CancellationToken>((stream, token) => {
        //memory stream in this scope is the one that was populated
        //when you called **fileStream.CopyTo(memoryStream);** in the test
        memoryStream.CopyTo(stream);
    }) 
    .Returns(Task.CompletedTask);

Answer 3

I combined the answers of @Nkosi and @nvoigt. As @nvoigt pointed out: access to a real file should really not be in a "unit" test. So I replaced the file with a byte array like so:

using (var memoryStream = new MemoryStream(new byte[]{1,2,3,4}))

Instead of a complete file.

And I implemented the .callback on the mocked object as suggested by @Nkosi

FormFileMock.Setup(f => f.CopyToAsync(It.IsAny<Stream>(), CancellationToken.None))
    .Callback<Stream, CancellationToken>((stream, token) =>
    {
        // with memoryStream being the stream from inside the using statement
        memoryStream.CopyTo(stream);
    }).Returns(Task.CompletedTask);

And now it works.