Performing a Conditioned Download

If you have any questions during the development, post them on the Issues page of GitHub. For details about parameters and usage of each API, see the API Reference.

When downloading an object, you can specify one or more conditions. Only when the conditions are met, the object will be downloaded. Otherwise, an exception indicating a download failure will be thrown.

You can set the following conditions:

• The ETag of an object is the MD5 check value of the object.
• If the download request includes IfUnmodifiedSince or IfMatch and the specified condition is not met, an exception will be thrown with HTTP status code 412 Precondition Failed returned.
• If the download request includes IfModifiedSince or IfNoneMatch and the specified condition is not met, an exception will be thrown with HTTP status code 304 Not Modified returned.

Sample code:

// Initialize configuration parameters.
ObsConfig config = new ObsConfig();
config.Endpoint = "https://your-endpoint";
// Hard-coded or plaintext AK/SK are risky. For security purposes, encrypt your AK/SK and store them in the configuration file or environment variables. In this example, the AK/SK are stored in environment variables for identity authentication. Before running this example, configure environment variables AccessKeyID and SecretAccessKey.
// Obtain an AK/SK pair on the management console. For details, see https://support.huaweicloud.com/intl/en-us/usermanual-ca/ca_01_0003.html.
string accessKey= Environment.GetEnvironmentVariable("AccessKeyID", EnvironmentVariableTarget.Machine);
string secretKey= Environment.GetEnvironmentVariable("SecretAccessKey", EnvironmentVariableTarget.Machine);
// Create an instance of ObsClient.
ObsClient client = new ObsClient(accessKey, secretKey, config);
// Download an object in conditioned mode.
try
{
    DateTime datetime = new DateTime(2018, 3, 10, 12, 00, 00);
    GetObjectRequest request = new GetObjectRequest()
    {
        BucketName = "bucketname",
        ObjectKey = "objectname",
        IfModifiedSince = datetime,
    };
    using (GetObjectResponse response = client.GetObject(request))
    {
        string dest = "savepath";
        if (!File.Exists(dest))
        {
            response.WriteResponseStreamToFile(dest);
        }
        Console.WriteLine("Get object response: {0}", response.StatusCode);
    }
}
catch (ObsException ex)
{
   Console.WriteLine("ErrorCode: {0}", ex.ErrorCode);
   Console.WriteLine("ErrorMessage: {0}", ex.ErrorMessage);
}