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書名： Microsoft Windows Azure Development Cookbook
作者名： Neil Mackenzie
本章字數： 551字
更新時間： 2021-04-02 18:51:59

Downloading a blob asynchronously

The Windows Azure Storage Client library provides synchronous and asynchronous versions of nearly all the methods that access the Windows Azure Storage Service.

The asynchronous methods follow the common language runtime (CLR) Asynchronous Programming Model (APM). In this model, asynchronous methods for an action are defined as a pair named BeginAction and EndAction. The asynchronous operation is initiated through a call to BeginAction and is cleaned up by a call to EndAction. BeginAction has a parameter that is a callback delegate and EndAction must be invoked in that delegate.

This apparent complexity can be greatly simplified through the use of a lambda expression to represent the callback delegate. Furthermore, local variables defined in the method containing the lambda expression are available inside the lambda expression. This removes any difficulty caused by a need to pass variables into the delegate. Using a lambda expression, instead of a callback delegate, makes using the asynchronous methods almost as simple as using the synchronous methods.

In this recipe, we will learn how to use asynchronous methods to download a blob into a file.

How to do it...

We are going to download a blob asynchronously. We do this as follows:

Add a new class named DownloadBlobExample to the project.
Set the Target Framework for the project to .NET Framework 4.

Add the following assembly references to the project:

Microsoft.WindowsAzure.StorageClient
System.Configuration

Add the following using statements to the top of the class file:

using Microsoft.WindowsAzure;
using Microsoft.WindowsAzure.StorageClient;
using System.Configuration;
using System.Net;
using System.IO;

Add the following method, creating a container and blob, to the class:

private static void CreateContainerAndBlob(String containerName, String blobName)
{
  CloudStorageAccount cloudStorageAccount =CloudStorageAccount.Parse(ConfigurationManager.AppSettings[
    "DataConnectionString"]);
  CloudBlobClient cloudBlobClient =cloudStorageAccount.CreateCloudBlobClient();

  CloudBlobContainer cloudBlobContainer =cloudBlobClient.GetContainerReference(containerName);
  cloudBlobContainer.CreateIfNotExist();

  CloudBlockBlob cloudBlockBlob =cloudBlobContainer.GetBlockBlobReference(blobName);
  cloudBlockBlob.UploadText("To be, or not to be");
}

Add the following method, downloading a blob, to the class:

private static void DownloadBlob(String containerName,String blobName, String fileName)
{
  CloudStorageAccount cloudStorageAccount =CloudStorageAccount.Parse(ConfigurationManager.AppSettings["DataConnectionString"]);
  CloudBlobClient cloudBlobClient =cloudStorageAccount.CreateCloudBlobClient();

  CloudBlobContainer cloudBlobContainer =cloudBlobClient.GetContainerReference(containerName);
  CloudBlockBlob cloudBlockBlob =cloudBlobContainer.GetBlockBlobReference(blobName);

  FileStream fileStream = new FileStream(fileName,FileMode.Append);
  IAsyncResult iAsyncResult =cloudBlockBlob.BeginDownloadToStream(fileStream,(result) =>
    {
      cloudBlockBlob.EndDownloadToStream(result);
      fileStream.Close();
    },
    null);
  return;
}

Add the following method, using the methods added earlier, to the class:

public static void UseDownloadBlobExample()
{
  String containerName = "{CONTAINER_NAME}";
  String blobName = "{BLOB_NAME}";
  String fileName = Path.GetTempPath() + @"\{FILE_NAME}";

  CreateContainerAndBlob(containerName, blobName);
  DownloadBlob(containerName, blobName, fileName);
}

Add the following to the configuration section of app.config:

<appSettings>
  <add key="DataConnectionString"value="UseDevelopmentStorage=true"/>
</appSettings>

How it works...

In steps 1 through 4, we set up the class. In step 5, we initialize a CloudStorageAccount from app.config and use it to create a CloudBlobClient instance. We use this to create a container and a blob.

In step 6, we follow the same route to get a CloudBlockBlob reference to the blob. We use it to invoke the BeginDownloadToStream() method. We pass this a lambda expression, instead of a callback delegate, and invoke EndDownloadToStream() in the lambda expression to clean up the asynchronous call. We use the ability to refer to local objects from inside the lambda expression to close the FileStream used in the download.

In step 7, we add a method that sets the parameters for the call to DownloadBlob(). We download the blob to a temp directory. We must replace {CONTAINER_NAME} and {BLOB_NAME} with appropriate container and blob names, and {FILE_NAME} with the file name.

In step 8, we add the connection string to the app.config configuration file.

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Microsoft Windows Azure Development Cookbook

Downloading a blob asynchronously

How to do it...

How it works...