In this example, we’ll take a Block Blob and an example of a class named Assignment. The Pessimistic Concurrency approach takes a Lease on a Blob Client and allows overwrite only if the Lease is not expired else it’ll give HttpStatusCode.PreconditionFailed error. For more details, check the following document.

The code below is a .Net 6 Console App.

The Assignment Class has the following Properties:

public class Assignment
{
    public int Id { get; set; }
    public string Code { get; set; }
    public string Kind { get; set; }
    public double pehe { get; set; }
}

The Console App’s Program.cs code will fetch blob content every time and manually add another Assignment. In the 4th step, it’ll fetch content from another Blob and append the Deserialized object to the original list of Assignments being built in previous steps and finally overwrite the first Blob with all Assignments.

using Azure;
using Azure.Storage.Blobs;
using Azure.Storage.Blobs.Models;
using Azure.Storage.Blobs.Specialized;
using Newtonsoft.Json;
using System.Net;
using System.Text;

await PessimisticConcurrencyBlob();

Console.WriteLine("done");
Console.ReadLine();

async Task PessimisticConcurrencyBlob()
{
    Console.WriteLine("Demonstrate pessimistic concurrency");
    string connectionString = "xxxx"; //ConfigurationManager.ConnectionStrings["storage"].Con;
    string filename = "testAssignment.json";
    string containerName = "mycontainer";
    BlobServiceClient _blobServiceClient = new BlobServiceClient(connectionString);
    BlobContainerClient containerClient = _blobServiceClient.GetBlobContainerClient(containerName);


    BlobClient blobClient = containerClient.GetBlobClient(filename);
    BlobLeaseClient blobLeaseClient = blobClient.GetBlobLeaseClient();


    string filename2 = "assignments.json";
    BlobClient blobClient2 = containerClient.GetBlobClient(filename2);

    try
    {
        // Create the container if it does not exist.
        await containerClient.CreateIfNotExistsAsync();
        var blobAssList = await RetrieveBlobContentAsync(blobClient);
        // Upload json to a blob.
        Assignment assignment1 = new Assignment()
        {
            Id = 8,
            Code = "ABC",
            Kind =  "Lead",
            pehe = 10.0
        };
        blobAssList.Add(assignment1);

        var blobContents1 = JsonConvert.SerializeObject(blobAssList);
        byte[] byteArray = Encoding.ASCII.GetBytes(blobContents1);
        using (MemoryStream stream = new MemoryStream(byteArray))
        {
            BlobContentInfo blobContentInfo = await blobClient.UploadAsync(stream, overwrite: true);
        }

        // Acquire a lease on the blob.
        BlobLease blobLease = await blobLeaseClient.AcquireAsync(TimeSpan.FromSeconds(60));
        Console.WriteLine("Blob lease acquired. LeaseId = {0}", blobLease.LeaseId);

        // Set the request condition to include the lease ID.
        BlobUploadOptions blobUploadOptions = new BlobUploadOptions()
        {
            Conditions = new BlobRequestConditions()
            {
                LeaseId = blobLease.LeaseId
            }
        };

        // Write to the blob again, providing the lease ID on the request.
        // The lease ID was provided, so this call should succeed.
        // Upload json to a blob.
        blobAssList = await RetrieveBlobContentAsync(blobClient);
        Assignment assignment2 = new Assignment()
        {
            Id = 9,
            Code = "DEF",
            Kind = "Assignment",
            pehe = 20.0
        };
        blobAssList.Add(assignment2);
        var blobContents2 = JsonConvert.SerializeObject(blobAssList);
        byteArray = Encoding.ASCII.GetBytes(blobContents2);

        using (MemoryStream stream = new MemoryStream(byteArray))
        {
            BlobContentInfo blobContentInfo = await blobClient.UploadAsync(stream, blobUploadOptions);
        }

        // This code simulates an update by another client.
        // The lease ID is not provided, so this call fails.

        // Acquire a lease on the blob.
        BlobLease blobLease2 = await blobLeaseClient.AcquireAsync(TimeSpan.FromSeconds(60));
        Console.WriteLine("Blob lease acquired. LeaseId = {0}", blobLease2.LeaseId);

        // Set the request condition to include the lease ID.
        BlobUploadOptions blobUploadOptions2 = new BlobUploadOptions()
        {
            Conditions = new BlobRequestConditions()
            {
                LeaseId = blobLease2.LeaseId
            }
        };

        blobAssList = await RetrieveBlobContentAsync(blobClient);
        Assignment assignment3 = new Assignment()
        {
            Id = 10,
            Code = "GHI",
            Kind = "Assignment",
            pehe = 30.0
        };
        blobAssList.Add(assignment3);
        var blobContents3 = JsonConvert.SerializeObject(blobAssList);
        byteArray = Encoding.ASCII.GetBytes(blobContents3);

        using (MemoryStream stream = new MemoryStream(byteArray))
        {
            // This call should fail with error code 412 (Precondition Failed).
            BlobContentInfo blobContentInfo = await blobClient.UploadAsync(stream, blobUploadOptions2);
        }

        // Calling another blob and add to first blob.
        BlobLease blobLease3 = await blobLeaseClient.AcquireAsync(TimeSpan.FromSeconds(60));
        Console.WriteLine("Blob lease acquired. LeaseId = {0}", blobLease3.LeaseId);

        // Set the request condition to include the lease ID.
        BlobUploadOptions blobUploadOptions3 = new BlobUploadOptions()
        {
            Conditions = new BlobRequestConditions()
            {
                LeaseId = blobLease3.LeaseId
            }
        };

        var blobAssList2 = await RetrieveBlobContentAsync(blobClient2);
        blobAssList.AddRange(blobAssList2);
        var blobContents4 = JsonConvert.SerializeObject(blobAssList);
        byteArray = Encoding.ASCII.GetBytes(blobContents4);

        using (MemoryStream stream = new MemoryStream(byteArray))
        {
            // This call should fail with error code 412 (Precondition Failed).
            BlobContentInfo blobContentInfo = await blobClient.UploadAsync(stream, blobUploadOptions3);
        }

    }
    catch (RequestFailedException e)
    {
        if (e.Status == (int)HttpStatusCode.PreconditionFailed)
        {
            Console.WriteLine(
                @"Precondition failure as expected. The lease ID was not provided.");
        }
        else
        {
            Console.WriteLine(e.Message);
            throw;
        }
    }
    finally
    {
        await blobLeaseClient.ReleaseAsync();
    }
}

The code for fetching the Blob Content is as follows:

async Task<List<Assignment>> RetrieveBlobContentAsync(BlobClient blobClient)
{
    //List<Assignment> assignments = new List<Assignment>();

    var response = await blobClient.DownloadAsync();
    string content = string.Empty;
    using (var streamReader = new StreamReader(response.Value.Content))
    {
        while (!streamReader.EndOfStream)
        {
            content = await streamReader.ReadToEndAsync();
        }
    }

    var assignments = JsonConvert.DeserializeObject<List<Assignment>>(content);

    return assignments;
}