azure-functions-durable-extension: Unable to load hubName from appsettings for durable function

Description

According to the documentation found at https://docs.microsoft.com/en-us/azure/azure-functions/durable/durable-functions-task-hubs#hostjson-functions-20-1, it should be possible to use a value from the appsettings as the hubname in the host.json file. However, after updating Microsoft.Azure.WebJobs.Extensions.DurableTask to v2.1.0, an exception is now thrown during startup.

Expected behavior

When using “%MyTaskHub%” as the value of the “hubName” property in the host.json file, I’m expecting the runtime to look for a key matching “MyTaskHub” in the appsettings and use its value as the hub.

Actual behavior

When the function app starts up, I receive an exception:

System.Private.CoreLib: Exception has been thrown by the target of an invocation. Microsoft.Azure.WebJobs.Extensions.DurableTask: Task hub name ‘%MyTaskHub%’ should contain only alphanumeric characters excluding ‘-’ and have length up to 45. Microsoft.WindowsAzure.Storage: Invalid container name. Check MSDN for more information about valid container naming. Value cannot be null. (Parameter ‘provider’)

Relevant source code snippets

Instead of pasting the source code of my actual functions app which is a bit more complex, I’ve managed to replicate the issue in a simpler form using the Visual Studio functions app template. Below are steps to replicate:

1. Open Visual Studio 2019.
2. Create a new empty functions app using Azure Functions v3 (.NET Core) and select Storage Emulator as the storage account.
3. Once the project has been scaffolded, right-click on the project and add New Azure Function. Select Durable Functions Orchestration as the type.
4. Update the Microsoft.NET.Sdk.Functions NuGet package to 3.0.2.
5. Update the Microsoft.Azure.WebJobs.Extensions.DurableTask NuGet package to 2.1.0.
6. If you try and build the project at this point, you’ll get errors since the types have changed. Update the function class with the correct types to resolve the build errors.
7. Open host.json and replace the contents with { "version": "2.0", "extensions": { "durableTask": { "hubName": "%MyTaskHub%" } } }. This snippet is taken directly from the documentation (link at the top of this issue).
8. Open local.settings.json and add "MyTaskHub" : "samplehubname" to the “Values” section. Again, this snippet is taken directly from the documentation.
9. Open the function class and update [DurableClient] to [DurableClient(TaskHub = "%MyTaskHub%")].
10. Debug the functions app and during startup, the exception described above is thrown.

Known workarounds

The only way I’ve managed to get it working is to manually make sure the host.json “hubName” matches the value of “MyTaskHub” in local.settings.json. You can verify this by changing the value of “hubName” in host.json to “samplehubname” after following the replication steps above. After doing so, debug the project and it should at least run correctly.

This workaround isn’t ideal as it means I would need to update the value everytime instead of the value coming from the appsettings.

App Details

• AzureFunctionsVersion: v3
• TargetFramework: netcoreapp3.0
• Microsoft.NET.Sdk.Functions: v3.0.2
• Microsoft.Azure.WebJobs.Extensions.DurableTask: v2.1.0