Unit Testing Cosmos DB .NET Applications

Mocking Cosmos DB for Unit Testing

When writing unit tests for .NET applications that use Cosmos DB, it's important to avoid actually hitting the database during testing. This can be achieved using a mocking library to simulate the behavior of Cosmos DB in a controlled way. In this tutorial, we'll explore how to mock Cosmos DB for unit testing .NET applications.

Syntax

Mock<Microsoft.Azure.Cosmos.Container> mockContainer = new Mock<Microsoft.Azure.Cosmos.Container>();
Mock<Microsoft.Azure.Cosmos.CosmosClient> mockClient = new Mock<Microsoft.Azure.Cosmos.CosmosClient>();

Example

[TestClass]
public class CosmosDbServiceTest
{
    private static readonly string databaseName = "TestDatabase";
    private static readonly string containerName = "TestContainer";
    private static readonly string partitionKeyName = "TestPartitionKey";
    private static readonly string partitionKeyValue = "TestPartitionKeyValue";

    [TestMethod]
    public async Task TestQueryItems()
    {
        List<Item> items = new List<Item>();
        items.Add(new Item { Id = "1", PartitionKey = partitionKeyValue });
        items.Add(new Item { Id = "2", PartitionKey = partitionKeyValue });

        var container = new Mock<Container>();
        var queryable = items.AsQueryable();

        var mockIterator = new Mock<FeedIterator<Item>>();
        mockIterator.SetupSequence(i => i.HasMoreResults)
            .Returns(true)
            .Returns(false);

        mockIterator.Setup(i => i.ReadNextAsync(default))
            .ReturnsAsync(queryable);

        container.Setup(c => c.GetItemQueryIterator<Item>(It.IsAny<QueryDefinition>(), null, null))
            .Returns(mockIterator.Object);

        var cosmosClient = new Mock<CosmosClient>();
        cosmosClient.Setup(c => c.GetContainer(databaseName, containerName))
            .Returns(container.Object);

        var cosmosDbService = new CosmosDbService(cosmosClient.Object, databaseName, containerName, partitionKeyName);

        var result = await cosmosDbService.QueryItemsAsync<Item>(new QueryDefinition("SELECT * FROM c"));

        Assert.IsNotNull(result);
        Assert.AreEqual(result.Count, 2);
    }
}

Output

Unit test result: Passed

Explanation

In this example, we're using the Moq mocking library to create mock Cosmos DB objects. First, we create a mock container and a mock client. We then set up the behavior of the mock container to return a mock iterator that returns a set of test items. We also set up the behavior of the client to return the mock container when asked for our test database and container.

We then use these mock objects in our unit test for the CosmosDbService class, which is responsible for executing queries against the Cosmos DB database. We create an instance of this class with our mock client and test parameters, and execute a query using our mock container. Finally, we test the results of the query to ensure that it returned the expected number of items.

Use

Use this approach to unit test .NET applications that use Cosmos DB, without actually hitting the database during testing.

Important Points

• Unit testing with Cosmos DB can be achieved using mocking libraries like Moq.
• Using mocks in your unit tests allows you to simulate the behavior of Cosmos DB in a controlled way, without actually hitting the database during testing.
• By simulating the response of a Cosmos DB container, you can test your application's behavior under various scenarios and ensure its correctness.

Summary

In this tutorial, we explored how to mock Cosmos DB for unit testing .NET applications. We learned how to use the Moq mocking library to create mock Cosmos DB objects and simulate the behavior of the database during testing. With this approach, you can test your application's behavior under various scenarios, without actually hitting the database during testing.