SharePoint Online 401 Error.

Question

Hello,

I am trying to use the Microsoft.SharePoint.Client (CSOM) API in a C# application, but I keep receiving a 401 Unauthorized error.

Within Azure, I created an App Registration and a secret key with the following API permissions:

I registered the app within SharePoint using the URL: https://*****admin.sharepoint.com/_layouts/15/AppRegNew.aspx.

After registration, I configured the app with the permission XML and selected the "Trust It" option.

Here is the permission XML I used:

<AppPermissionRequests AllowAppOnlyPolicy="true">
    <AppPermissionRequest Scope="https://****.sharepoint.com/content/sitecollection" Right="FullControl" />
</AppPermissionRequests>

Despite ensuring all IDs and secrets are correct, I am still encountering the 401 error. I would greatly appreciate any assistance or suggestions to resolve this.

Below is the code I'm using:

static void Main(string[] args)
{
    string siteUrl = "https://****.sharepoint.com/sites/Casemanagement";
    string clientId = "*************";
    string clientSecret = "**************";
    string tenantId = "**************";
    string documentLibraryName = "Documents";
    string folderPath = "457/457805/457805-002";
    string accessToken = GetAccessToken(tenantId, clientId, clientSecret, siteUrl);
    try
    {
        var context = new ClientContext(siteUrl);
        context.ExecutingWebRequest += (sender, e) =>
        {
            e.WebRequestExecutor.WebRequest.Headers["Authorization"] = "Bearer " + accessToken;
        };
        var web = context.Web;
        var library = web.Lists.GetByTitle(documentLibraryName);
        var folder = library.RootFolder.Folders.GetByUrl(folderPath);
        var files = folder.Files;
        context.Load(files);
        context.ExecuteQuery();
        Console.WriteLine("Files in folder:");
        foreach (var file in files)
        {
            Console.WriteLine(file.Name);
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Failed to retrieve files: {ex.Message}");
        Console.ReadLine();
        throw;
    }
}
static string GetAccessToken(string tenantId, string clientId, string clientSecret, string resource)
{
    try
    {
        IConfidentialClientApplication app = ConfidentialClientApplicationBuilder.Create(clientId)
            .WithClientSecret(clientSecret)
            .WithAuthority(new Uri($"https://login.microsoftonline.com/{tenantId}"))
            .Build();
        var result = app.AcquireTokenForClient(new string[] { "https://mddus.sharepoint.com/.default" }).ExecuteAsync().Result;
        Console.WriteLine($"Token Expires On: {result.ExpiresOn}");
        return result.AccessToken;
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Failed to acquire token: {ex.Message}");
        throw;
    }
}