public class ValuesController : ApiController
{
  private readonly CloudStorageAccount account;
  private readonly string blobContainer;
  ...
  /// 

  /// Return a limited access key that allows the caller to upload a file 
  /// to this specific destination for a defined period of time.
  /// 
  private StorageEntitySas GetSharedAccessReferenceForUpload(string blobName)
  {
    var blobClient = this.account.CreateCloudBlobClient();
    var container = blobClient.GetContainerReference(this.blobContainer);

    var blob = container.GetBlockBlobReference(blobName);

    var policy = new SharedAccessBlobPolicy
    {
      Permissions = SharedAccessBlobPermissions.Write,

      // Specify a start time five minutes earlier to allow for client clock skew.
      SharedAccessStartTime = DateTime.UtcNow.AddMinutes(-5),

      // Specify a validity period of five minutes starting from now. 
      SharedAccessExpiryTime = DateTime.UtcNow.AddMinutes(5)
    };

    // Create the signature. 
    var sas = blob.GetSharedAccessSignature(policy);

    return new StorageEntitySas
    {
      BlobUri = blob.Uri,
      Credentials = sas,
      Name = blobName
    };
  }

  public struct StorageEntitySas
  {
    public string Credentials;
    public Uri BlobUri;
    public string Name;
  }
}