Azure Blob storage output binding for Azure Functions

The output binding allows you to modify and delete blob storage data in an Azure Function.

For information on setup and configuration details, see the overview.


The following example is a C# function that uses a blob trigger and two output blob bindings. The function is triggered by the creation of an image blob in the sample-images container. It creates small and medium size copies of the image blob.

using System.Collections.Generic;
using System.IO;
using Microsoft.Azure.WebJobs;
using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Formats;
using SixLabors.ImageSharp.PixelFormats;
using SixLabors.ImageSharp.Processing;

public class ResizeImages
    public static void Run([BlobTrigger("sample-images/{name}")] Stream image,
        [Blob("sample-images-sm/{name}", FileAccess.Write)] Stream imageSmall,
        [Blob("sample-images-md/{name}", FileAccess.Write)] Stream imageMedium)
        IImageFormat format;

        using (Image<Rgba32> input = Image.Load<Rgba32>(image, out format))
            ResizeImage(input, imageSmall, ImageSize.Small, format);

        image.Position = 0;
        using (Image<Rgba32> input = Image.Load<Rgba32>(image, out format))
            ResizeImage(input, imageMedium, ImageSize.Medium, format);

    public static void ResizeImage(Image<Rgba32> input, Stream output, ImageSize size, IImageFormat format)
        var dimensions = imageDimensionsTable[size];

        input.Mutate(x => x.Resize(dimensions.Item1, dimensions.Item2));
        input.Save(output, format);

    public enum ImageSize { ExtraSmall, Small, Medium }

    private static Dictionary<ImageSize, (int, int)> imageDimensionsTable = new Dictionary<ImageSize, (int, int)>() {
        { ImageSize.ExtraSmall, (320, 200) },
        { ImageSize.Small,      (640, 400) },
        { ImageSize.Medium,     (800, 600) }


Attributes and annotations

In C# class libraries, use the BlobAttribute.

The attribute's constructor takes the path to the blob and a FileAccess parameter indicating read or write, as shown in the following example:

public static void Run(
    [BlobTrigger("sample-images/{name}")] Stream image,
    [Blob("sample-images-md/{name}", FileAccess.Write)] Stream imageSmall)

You can set the Connection property to specify the storage account to use, as shown in the following example:

public static void Run(
    [BlobTrigger("sample-images/{name}")] Stream image,
    [Blob("sample-images-md/{name}", FileAccess.Write, Connection = "StorageConnectionAppSetting")] Stream imageSmall)

For a complete example, see Output example.

You can use the StorageAccount attribute to specify the storage account at class, method, or parameter level. For more information, see Trigger - attributes.


The following table explains the binding configuration properties that you set in the function.json file and the Blob attribute.

function.json property Attribute property Description
type n/a Must be set to blob.
direction n/a Must be set to out for an output binding. Exceptions are noted in the usage section.
name n/a The name of the variable that represents the blob in function code. Set to $return to reference the function return value.
path BlobPath The path to the blob container.
connection Connection The name of an app setting that contains the Storage connection string to use for this binding. If the app setting name begins with "AzureWebJobs", you can specify only the remainder of the name here. For example, if you set connection to "MyStorage", the Functions runtime looks for an app setting that is named "AzureWebJobsMyStorage." If you leave connection empty, the Functions runtime uses the default Storage connection string in the app setting that is named AzureWebJobsStorage.

The connection string must be for a general-purpose storage account, not a blob-only storage account.
n/a Access Indicates whether you will be reading or writing.

When you're developing locally, app settings go into the local.settings.json file.


You can bind to the following types to write blobs:

  • TextWriter
  • out string
  • out Byte[]
  • CloudBlobStream
  • Stream
  • CloudBlobContainer1
  • CloudBlobDirectory
  • ICloudBlob2
  • CloudBlockBlob2
  • CloudPageBlob2
  • CloudAppendBlob2

1 Requires "in" binding direction in function.json or FileAccess.Read in a C# class library. However, you can use the container object that the runtime provides to do write operations, such as uploading blobs to the container.

2 Requires "inout" binding direction in function.json or FileAccess.ReadWrite in a C# class library.

If you try to bind to one of the Storage SDK types and get an error message, make sure that you have a reference to the correct Storage SDK version.

In async functions, use the return value or IAsyncCollector instead of an out parameter.

Binding to string or Byte[] is only recommended if the blob size is small, as the entire blob contents are loaded into memory. Generally, it is preferable to use a Stream or CloudBlockBlob type. For more information, see Concurrency and memory usage earlier in this article.

Exceptions and return codes

Binding Reference
Blob Blob Error Codes
Blob, Table, Queue Storage Error Codes
Blob, Table, Queue Troubleshooting

Next steps