SharePointFileUpload.cs

using PnP.Framework;
using PnP.Framework.Modernization.Publishing;
using PnP.Framework.Authentication;
using System;
using System.IO;
using System.Threading.Tasks;
using Microsoft.SharePoint.Client;

namespace SharePointFileUpload
{
    class Program
    {
        static async Task Main(string[] args)
        {
            string siteUrl = "https://yourtenant.sharepoint.com/sites/yoursite";
            string libraryName = "Documents"; // The name of the document library
            string filePath = @"C:\path\to\your\file.txt"; // Path to the file you want to upload
            string fileName = "file.txt"; // The name you want to give the file in SharePoint
            
            // Create the authentication manager using the client ID and secret
            var clientId = "your-client-id";
            var tenantId = "your-tenant-id";
            var clientSecret = "your-client-secret";

            // Initialize the PnP AuthenticationManager
            AuthenticationManager authManager = new AuthenticationManager(clientId, tenantId, clientSecret);

            // Connect to the SharePoint site
            using (ClientContext context = authManager.GetContext(siteUrl))
            {
                try
                {
                    // Load the web object
                    Web web = context.Web;
                    context.Load(web);
                    await context.ExecuteQueryAsync();

                    Console.WriteLine("Connected to SharePoint site: " + web.Title);

                    // Get the document library
                    List library = web.Lists.GetByTitle(libraryName);
                    context.Load(library.RootFolder);
                    await context.ExecuteQueryAsync();

                    // Get the target folder in the library
                    Folder folder = library.RootFolder;
                    string fileContent = File.ReadAllText(filePath);

                    // Upload the file
                    FileCreationInformation newFile = new FileCreationInformation
                    {
                        Content = System.Text.Encoding.UTF8.GetBytes(fileContent),
                        Url = fileName,
                        Overwrite = true
                    };

                    Microsoft.SharePoint.Client.File uploadFile = folder.Files.Add(newFile);
                    context.Load(uploadFile);
                    await context.ExecuteQueryAsync();

                    Console.WriteLine("File uploaded successfully!");
                }
                catch (Exception ex)
                {
                    Console.WriteLine("Error: " + ex.Message);
                }
            }
        }
    }
}