Using a mocked IAmazonDynamoDB instance in DynamoDBContext

[kpenergy]

The Question

Is there any way to inject a mocked IAmazonDynamoDB instance into AmazonDynamoDB , in order to catch the request that hits the mocked client so the request details can be verified?

I'm trying to do so by injecting a mocked IAmazonDynamoDB instance into DynamoDBContext, constructing a new instance of my class to be passed into the DynamoDBContext, and calling SaveAsync, but I'm hitting a NullReferenceException.

[Fact]
public void Test()
{
	var clientMock = new Mock<IAmazonDynamoDB>();

	var context = new DynamoDBContext(clientMock.Object);

	Dictionary<string, AttributeValue> attributes = null;

	clientMock
		.Setup(m => m.PutItemAsync(It.IsAny<string>(), It.IsAny<Dictionary<string, AttributeValue>>(), default))
		.Callback((string s, Dictionary<string, AttributeValue> a, CancellationToken c) => attributes = a);

	var model = new MyModel { HashKey = Guid.NewGuid().ToString() };

	context.SaveAsync(model).Wait();
}

public class MyModel
{
	[DynamoDBHashKey]
	public string HashKey { get; set; }
}

The exception I'm hitting is:

System.NullReferenceException: 'Object reference not set to an instance of an object.'
Amazon.DynamoDBv2.DocumentModel.Table.DDBClient.get returned null.

This seems to be a result of the SDK casting the injected IAmazonDynamoDB instance to AmazonDynamoDBClient when using NETSTANDARD. The Amazon.DynamoDBv2.DocumentModel.Table class's private constructor casts the mocked interface to AmazonDynamoDBClient, but the cast fails and results in null. This is the code which seems to be causing the null.

private Table(IAmazonDynamoDB ddbClient, TableConfig config)
{
	if (config == null)
		throw new ArgumentNullException("config");

	if (ddbClient == null)
		throw new ArgumentNullException("ddbClient");

#if NETSTANDARD
	DDBClient = ddbClient as AmazonDynamoDBClient; // THIS CAUSES NULL
#else
	DDBClient = ddbClient;
#endif
	Config = config;
}

Perhaps I'm using the wrong approach to achieve my goal. If there's any way to do this, I'd appreciate the support. Thanks!

Environment

• SDK Version: 3
• Package Version: 3.5.4.29
• OS Info: Windows 10
• Build Environment Visual Studio
• Targeted .NET Platform: .Net Core 3.1

---

This is a ❓ general question

[Kralizek]

This is an old issue that we have also been discussing here: #1589
Nice to see it's not just me feeling the pain.

[ashishdhingra]

Hi @kpenergy,

Good morning.

Thanks for posting guidance question. There are already 2 issues in the backlog:

• Cannot mock DynamoDBContext.CreateBatchGet due to lack of interfaces #1396
• DynamoDB Table misrepresents dependency on IAmazonDynamoDB #1589

This is a breaking change and the fix would most likely be incorporated in the next major version of SDK. Could you please add 👍 reaction to both issues so that it gets some traction from developer?

I'm not sure what guidance could be provided here since it is a breaking change. Hope this provides some information.

Thanks,
Ashish

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