Unsupported expression: Non-overridable members (here: ) may not be used in setup / verification expressions

Question

I am using xUnit and Moq for writing test cases. I am using the code below to create a Mock but am getting the highlighted issue.

CloudBlockBlob source = null;
AccessCondition sourceAccessCondition = null;
AccessCondition destAccessCondition = null;
BlobRequestOptions options = null;
OperationContext operationContext = null;
CloudBlobContainer container = new CloudBlobContainer(uri);
Task task = null;
DeleteSnapshotOption deleteSnapshotOption = new DeleteSnapshotOption();

var mockCloudBlobClient = new Mock<Microsoft.WindowsAzure.Storage.Blob.CloudBlobClient>();
mockCloudBlobClient.Setup(repo => repo.GetContainerReference("sample")).Returns(container);

var mockCloudBlobContainer = new Mock<Microsoft.WindowsAzure.Storage.Blob.CloudBlobContainer>(uri);
mockCloudBlobContainer.Setup(repo => repo.GetBlockBlobReference("sample")).Returns(new CloudBlockBlob(uri));

var mockBlobServiceProvider = new Mock<BlobServiceProvider>();
mockBlobServiceProvider.Setup(repo => repo.GetBlockBlobContainer("sample")).Returns(new CloudBloblContainer(new Uri("http://mytest")));

The line mockBlobServiceProvider.Setup(repo => repo.GetBlockBlobContainer("sample")).Returns(new CloudBloblContainer(new Uri("http://mytest"))); is giving me this error:

System.NotSupportedException: 'Unsupported expression: repo => repo.GetBlockBlobContainer("sample")

Non-overridable members (here: BlobServiceProvider.GetBlockBlobContainer) may not be used in setup / verification expressions.'

Class BlobServiceProvider:

Public Class BlobServiceProvider
{
    public CloudBlobContainer GetBlockBlobContainer(string containerName)
    {
        CloudBlobContainer Container = blobClient.GetContainerReference(containerName);
        Container.CreateIfNotExistsAsync();
        return Container;
    }
}

How can I resolve this issue?

Solution

The moq use dynamic proxy to create wrapper around mocked type in order to be able to intercept calls and change the behaviour of the invoked methods.

How proxy is built?

• If you mocking interface proxy is just an implementation of the interface
• If you have class the proxy is override of the class

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Question: Could you mock sealed class or non virtual(abstract) method?

You guess, no. There is no way to create wrapper around it using dynamic proxy. moq itself suggesting the same, if you look at the error:

Non-overridable members (here: BlobServiceProvider.GetBlockBlobContainer) may not be used in setup

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To answer your question, either you need to extract the interface out of your provider and mock it or mark the method as virtual. Personaly, I do prefer option with the interface but it might requre some refactoring and DI.