Is CheckOut/Checkin via C# CSOM guaranteed to be atomic?

Question

I have a C# CSOM code that updates a page based on its current context, code sample below.

Now, I want to run several instances of this code. I'm concerned that the instances would overwrite each other: If 2 instances read the page and update their stuff, then one instance's update would overwrite the other's.

Do the checkout/checkin protect me again that? What would happen if both instances do it in the same time?

void UploadToSharepoint(string SharepointFolder, string pageName)
{
    using (var context = new ClientContext(SharepointFolder))
    {
        // handle context.Credentials
        var pagesList = context.Web.Lists.GetByTitle("Pages");
        context.Load(pagesList);
        context.ExecuteQuery();

        var pageQuery = new CamlQuery()
        {
            ViewXml = $@"<View><Query><Where>
            <Eq>
                <FieldRef Name='FileLeafRef'/>
                <Value Type='Text'>{pageName}</Value>
            </Eq>
        </Where></Query><RowLimit>100</RowLimit></View>",
        };
        var pages = pagesList.GetItems(pageQuery);
        context.Load(pages);
        context.ExecuteQuery();

        ListItem page = pages.Single();

        page.File.CheckOut();
        string pageContent = page["PublishingPageContent"];
        page["PublishingPageContent"] = pageContent;
        page.Update();
        page.File.CheckIn(comment: "", checkInType: CheckinType.MajorCheckIn);
        page.File.Publish(comment: "");

        context.Load(page);
        context.ExecuteQuery();    
    }
}

I use: <package id="Microsoft.SharePointOnline.CSOM" version="16.1.6518.1200" />

Answer 1

Regarding atomicity, the documentation does not explicitly say so but the CheckOut() method throws an SPException if "An error occurred when checking out the file." I would therefore assume this method executes synchronously and the file is locked after the method returns.

The CheckOut() method throws an SPException if check-out is not possible (e.g. the file is already checked out). You must handle this situation if you want to run this code in parallel.

You can either:

• Catch the SPException and try again later.
• Not catch (or log and re-throw) the exception and deal with it in the calling code.

Do not try to find out if the file can be checked out first. There is no guarantee that the file is checked out just before calling CheckOut().