Visual Studio Azure Function Publishing function.json

Question

I am new to Azure Functions, and attempting to put them to good use in conjunction with Blob Storage.

I have downloaded Visual Studio 2017 Preview which has given me access to creating an Azure Function project template (https://marketplace.visualstudio.com/items?itemName=AndrewBHall-MSFT.AzureFunctionToolsforVisualStudio2017)

Then following this example https://blogs.msdn.microsoft.com/webdev/2017/05/10/azure-function-tools-for-visual-studio-2017/ I am able to create a new Function in VS, then publish to Azure.

When I then look on Azure, I can see my new Function, but it just shows json from within a function.json file.

So this works fine when my Function code is just

public static void Run([BlobTrigger("images/{name}", 
    Connection = "fakename_STORAGE")]Stream myBlob, string name, TraceWriter log)

However, when I want to start playing around with outputs and I change my code to include out string myOutputBlob

public static void Run([BlobTrigger("images/{name}", 
    Connection = "fakename_STORAGE")]Stream myBlob, string name, out string myOutputBlob, TraceWriter log)

Then Publish to Azure, I see an error:

Microsoft.Azure.WebJobs.Host: Cannot bind parameter 'myOutputBlob' to type String&. Make sure the parameter Type is supported by the binding.

I can get round this buy updating the function.json file on Azure to include additional information,

but every time I publish to Azure, this file is overwritten.

I have tried adding a function.json file to my project which matches the config required, but this does not work. Am I missing something?

How can I ensure the function.json file is not overwritten every time? Do I need to configure my project to allow me to manage the file?

Answer 1

You have to decorate your output parameter with an attribute to configure what kind of output binding you need:

public static void Run(
    [BlobTrigger("images/{name}", Connection = "fakename_STORAGE")]Stream myBlob, 
    string name, 
    [Blob("images/copy-{name}", Connection = "fakename_STORAGE")]out string myOutputBlob, 
    TraceWriter log)

which will add the required lines in your function.json during the publish.