Passing container name as a parameter to BlobTrigger

Question

I have a little C# app that does some stuff when a new blob is added to a container in a storage account.

I'm working locally using Visual Studio 2017.

The VS template gives me:

using System.IO;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Host;
using Microsoft.Extensions.Logging;

namespace LogicAppTriggerFunction
{
    public static class BlobTrigger
    {
        [FunctionName("BlobTrigger")]
        public static void Run([BlobTrigger("samples-workitems/{name}", Connection = "")]Stream myBlob, string name, ILogger log)
        {
            log.LogInformation($"C# Blob trigger function Processed blob\n Name:{name} \n Size: {myBlob.Length} Bytes");
        }
    }
}

I'm trying to replace "samples-workitems" with a variable that relates to the name of my container. I only want to work with one container, but I don't want to "hard code" it.

If I am on the right lines, this setting is stored in function.json that is built at compile-time. When working locally, I think / thought that the contents of function.json is read by local.settings.json, however I now believe that to be for appSettings.json.

Reading the MS doco, particularly on trigger bindings, it looks like myContainer/{name} should be read from function.json. I understand that function.json is built dynamically and shouldn't be modified.

I have tried various methods that I know to essentially replace the string myContainer/{name}, but they all result in the error:

[20/08/2018 14:56:36] Run: Microsoft.Azure.WebJobs.Host: Error indexing method 'TriggerLogicApp.Run'. Microsoft.Azure.WebJobs.Host: Invalid blob trigger path '{container}/{name}'. Container paths cannot contain {resolve} tokens.

I believe what I need to do is "create" (or update) the path value in function.json, programatically, but I can't find out how to do this.

Can anyone shed any light on what I need to do and possibly how?

Answer 1

Specify your container name in local.settings.json locally or in Application settings on Azure.

{
    "IsEncrypted": false,
    "Values": {
        ....
        "MyBlobContainer":"samples-workitems"
    }
}

In your function signature, use App setting binding expressions wrapped in percent signs.

public static void Run([BlobTrigger("%MyBlobContainer%/{name}", Connection = "")]Stream myBlob, string name, ILogger log)