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Page through a collection using the Microsoft Graph SDKs

  • Artikkeli

For performance reasons, collections of entities are often split into pages and each page is returned with a URL to the next page. The PageIterator class simplifies consuming of paged collections. PageIterator handles enumerating the current page and requesting subsequent pages automatically.

Alternatively, you can use the @odata.nextLink property to manually request subsequent pages.

Request headers

If you send any additional request headers in your initial request, those headers are not included by default in subsequent page requests. If those headers need to be sent on subsequent requests, you must set them explicitly.

Iterate over all the messages

The following example shows iterating over all the messages in a user's mailbox.

Tip

This example sets a small page size using the top parameter for demonstration purposes. You can set the page size up to 999 to minimize the number of requests that are necessary.

var messages = await graphClient.Me.Messages
    .GetAsync(requestConfiguration =>
    {
        requestConfiguration.QueryParameters.Top = 10;
        requestConfiguration.QueryParameters.Select =
            ["sender", "subject", "body"];
        requestConfiguration.Headers.Add(
            "Prefer", "outlook.body-content-type=\"text\"");
    });

if (messages == null)
{
    return;
}

var pageIterator = PageIterator<Message, MessageCollectionResponse>
    .CreatePageIterator(
        graphClient,
        messages,
        // Callback executed for each item in
        // the collection
        (msg) =>
        {
            Console.WriteLine(msg.Subject);
            return true;
        },
        // Used to configure subsequent page
        // requests
        (req) =>
        {
            // Re-add the header to subsequent requests
            req.Headers.Add("Prefer", "outlook.body-content-type=\"text\"");
            return req;
        });

await pageIterator.IterateAsync();

Stopping and resuming the iteration

Some scenarios require stopping the iteration process in order to perform other actions. It is possible to pause the iteration by returning false from the iteration callback. Iteration can be resumed by calling the resume method on the PageIterator.

int count = 0;
int pauseAfter = 25;

var messages = await graphClient.Me.Messages
    .GetAsync(requestConfiguration =>
    {
        requestConfiguration.QueryParameters.Top = 10;
        requestConfiguration.QueryParameters.Select =
            ["sender", "subject"];
    });

if (messages == null)
{
    return;
}

var pageIterator = PageIterator<Message, MessageCollectionResponse>
    .CreatePageIterator(
        graphClient,
        messages,
        (msg) =>
        {
            Console.WriteLine(msg.Subject);
            count++;
            // If we've iterated over the limit,
            // stop the iteration by returning false
            return count < pauseAfter;
        });

await pageIterator.IterateAsync();

while (pageIterator.State != PagingState.Complete)
{
    Console.WriteLine("Iteration paused for 5 seconds...");
    await Task.Delay(5000);
    // Reset count
    count = 0;
    await pageIterator.ResumeAsync();
}

Manually requesting subsequent pages

As an alternative to using the PageIterator class, you can manually check the response for an @odata.nextLink property and request the next page.

var messages = await graphClient.Me.Messages
    .GetAsync(requestConfiguration =>
    {
        requestConfiguration.QueryParameters.Top = 10;
    });

while (messages?.Value != null)
{
    foreach (var message in messages.Value)
    {
        Console.WriteLine(message.Subject);
    }

    // If OdataNextLink has a value, there is another page
    if (!string.IsNullOrEmpty(messages.OdataNextLink))
    {
        // Pass the OdataNextLink to the WithUrl method
        // to request the next page
        messages = await graphClient.Me.Messages
            .WithUrl(messages.OdataNextLink)
            .GetAsync();
    }
    else
    {
        // No more results, exit loop
        break;
    }
}