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Windows Azure – BlockBlob PutBlock Method

Posted by Paul on November 30, 2011

In this article we are discussing about uploading blob using PutBlock method. I would like to give a note that there are two types of blob in Windows Azure:

· Block Blob

· Page Blob

The Block Blob uploading is discussed here as Block Blobs are used for large file uploads. The associated code was developed to upload large files to Windows Azure Blob. The large file will be splitted into 4 MB chunks and then uploaded.


Following are the important methods used:

· CloudBlockBlob.PutBlock()

· CloudBlockBlob.PutBlockList()

The PutBlock method is called for each 4MB file chunk and PutBlockList method is called at the end passing the file chunk ids.


Following is the code of AzureBlobUtil.cs

public class AzureBlobUtil


private const int MaxBlockSize = 4000000; // Approx. 4MB chunk size

public Uri UploadBlob(string filePath, CloudStorageAccount account, string containerName)


byte[] fileContent = File.ReadAllBytes(filePath);

string blobName = Path.GetFileName(filePath);

return UploadBlob(fileContent, account, containerName, blobName);


public Uri UploadBlob(byte[] fileContent, CloudStorageAccount account, string containerName, string blobName)


CloudBlobClient blobclient = account.CreateCloudBlobClient();

CloudBlobContainer container = blobclient.GetContainerReference(containerName);


CloudBlockBlob blob = container.GetBlockBlobReference(blobName);

HashSet<string> blocklist = new HashSet<string>();

foreach (FileBlock block in GetFileBlocks(fileContent))




new MemoryStream(block.Content, true),






return blob.Uri;


private IEnumerable<FileBlock> GetFileBlocks(byte[] fileContent)


HashSet<FileBlock> hashSet = new HashSet<FileBlock>();

if (fileContent.Length == 0)

return new HashSet<FileBlock>();

int blockId = 0;

int ix = 0;

int currentBlockSize = MaxBlockSize;

while (currentBlockSize == MaxBlockSize)


if ((ix + currentBlockSize) > fileContent.Length)

currentBlockSize = fileContent.Length - ix;

byte[] chunk = new byte[currentBlockSize];

Array.Copy(fileContent, ix, chunk, 0, currentBlockSize);


new FileBlock()


Content = chunk,

Id = Convert.ToBase64String(System.BitConverter.GetBytes(blockId))


ix += currentBlockSize;



return hashSet;


public Stream DownloadBlobAsStream(CloudStorageAccount account, string blobUri)


Stream stream = new MemoryStream();

CloudBlobClient blobclient = account.CreateCloudBlobClient();

CloudBlockBlob blob = blobclient.GetBlockBlobReference(blobUri);

if (blob != null)


return stream;


public string DownloadBlobAsFile(CloudStorageAccount account, string blobUri)


Stream stream = DownloadBlobAsStream(account, blobUri);

string fileName = "Blob.file"; // Please change file name based on your need

if (stream != null)


FileStream fileStream = File.Create(fileName);

stream.Position = 0;



return fileName;


return string.Empty;



internal class FileBlock


public string Id





public byte[] Content






Testing Code

For uploading blob into the container use the following code:

// Upload

Uri blobUri = _blobUtil.UploadBlob(




// Download

Stream stream = _blobUtil.DownloadBlobAsStream(




Associated Example

On running the example, after opening and uploading an image file, you can see the following result. The first box shows the source image. The second box shows the uploaded image from Azure.



Please ensure the Storage Emulator is running while testing the example


You can find more information on Block and Page blob using following link:


In this article we have seen the usage of Block Blob upload as chunks. The source code can be used to upload large files to Azure Blob. The associated example can be downloaded along with this article. The utility method was verified with 100MB file upload and download.


2 Responses to “Windows Azure – BlockBlob PutBlock Method”

  1. devesh said

    hey, after uploading a large file using your code…it works fine…but when we try to download the file it gives error…if it is an image then the image doesn’t error as invalid format..please help

    • Jean Paul said

      Hello Devesh,

      Have you tried downloading using both the methods?
      If possible please send the image file, upload, download code to me
      I will check it myself and reply you..

      Jean Paul

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