How to solve Sharepoint CSOM HTTP Request Error 42

Question

The sharepoint often displays error 429 (Too many requests) and we have already taken all the actions described in the article below, but some requests continue to be blocked.

https://docs.microsoft.com/en-us/sharepoint/dev/general-development/how-to-avoid-getting-throttled-or-blocked-in-sharepoint-online

Our scenario involves a customized desktop application (add-in) for Word, Excel, Power Point and Outlook that accesses the Sharepoint through CSOM (with user's network credentials) and we already registered this Add-in through the "/_layouts/15/AppRegNew.aspx" page and decorated all of our requests with "NONISV|{OUR ORGANIZATION NAME}|{OUR ADDIN NAME}/1.0 ", as described in:

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/register-sharepoint-add-ins

Answer 1

Follow the instructions in this Microsoft article:

https://docs.microsoft.com/en-us/sharepoint/dev/sp-add-ins/register-sharepoint-add-ins

When you have your Add-In registered, do this when you are creating SharePoint Context (using CSOM):

    private void Initialize()
    {
        this.SPCurrentContext = new ClientContext(this.Url);

        if (string.IsNullOrWhiteSpace(this.Domain))
        {
            this.SPCurrentContext.Credentials = new SharePointOnlineCredentials(this.User, ParseToSecureString(this.Password));
        }
        else
        {
            this.SPCurrentContext.Credentials = new NetworkCredential(this.User, ParseToSecureString(this.Password), this.Domain);
        }

        this.RetryCount = Properties.Settings.Default.DefaultRetryCount;
        this.RetryDelay = Properties.Settings.Default.DefaultRetryDelay;
        this.NONISV = Properties.Settings.Default.ClientAppNONISV;

        this.SPCurrentContext.ExecutingWebRequest += delegate (object sender, WebRequestEventArgs e)
        {
            e.WebRequestExecutor.WebRequest.UserAgent = this.NONISV; // This is the TRICK!!!
        };
    }

The NONISV used as User-Agent HTTP Header should be something like:

NONISV|{Your Company Name}|{Your Add-In Name}/1.0

As described here. Good luck!