How to mock a BlobBatchClient to be used in unit tests

Question

Hello,

In my project, we need to change the tier of multiple blobs and for this we use: await blobBatchClient.SetBlobsAccessTierAsync

The problem is how to add unit tests for this code:

First attempt:

        var blobBatchClient = new Mock<BlobBatchClient>();
        var blobServiceClient = new Mock<BlobServiceClient>();
        blobServiceClient.Setup(m => m.GetBlobContainerClient(It.IsAny<string>())).Returns(blobContainerClient.Object);
        blobServiceClient.Setup(m => m.GetBlobBatchClient()).Returns(blobBatchClient.Object);

Returns this error:

Unsupported expression: m => m.GetBlobBatchClient()

Extension methods (here: SpecializedBlobExtensions.GetBlobBatchClient) may not be used in setup / verification expressions.

Second attempt: mock the BlobContaineClient and pass it to the constructor of BlobBatchClient, but it does not work either.

       var blobContainerClient = new Mock<BlobContainerClient>();
       blobContainerClient.Setup(m => m.Name).Returns(Guid.NewGuid().ToString);
       blobContainerClient.Setup(m => m.Uri).Returns(new Uri("https://test.com"));
       var blobBatchClient = new BlobBatchClient(blobContainerClient.Object);

Answer 1

Hey there Angela Calborean

Thats a good question and thanks for using QandA platform.

So Since Moq doesn't support mocking extension methods directly, it's best to abstract away the creation of BlobBatchClient in your production code. Create an interface that represents the functionality you need from BlobBatchClient, and then implement that interface in a class that internally uses BlobBatchClient.

Inject the interface representing the BlobBatchClient functionality into the classes.

public interface IBlobBatchClientWrapper
{
    Task SetBlobsAccessTierAsync(IEnumerable<Uri> blobUris, AccessTier tier, CancellationToken cancellationToken = default);
}
public class BlobBatchClientWrapper : IBlobBatchClientWrapper
{
    private readonly BlobBatchClient _blobBatchClient;
    public BlobBatchClientWrapper(BlobServiceClient blobServiceClient)
    {
        _blobBatchClient = blobServiceClient.GetBlobBatchClient();
    }
    public async Task SetBlobsAccessTierAsync(IEnumerable<Uri> blobUris, AccessTier tier, CancellationToken cancellationToken = default)
    {
        // Delegate the actual functionality to the BlobBatchClient instance
        await _blobBatchClient.SetBlobsAccessTierAsync(blobUris, tier, cancellationToken);
    }
}

Now, in your production code, use IBlobBatchClientWrapper instead of directly using BlobBatchClient.

For your unit tests, you can now easily mock IBlobBatchClientWrapper using Moq

var blobBatchClientWrapperMock = new Mock<IBlobBatchClientWrapper>();
blobBatchClientWrapperMock.Setup(m => m.SetBlobsAccessTierAsync(It.IsAny<IEnumerable<Uri>>(), It.IsAny<AccessTier>(), It.IsAny<CancellationToken>()))
    .Returns(Task.CompletedTask);

var myService = new MyService(blobBatchClientWrapperMock.Object);

If this helps kindly accept the answer thanks much.