[HttpGet("download-report")] public IActionResult DownloadFile() { string base64String = "..."; // Get your string from a DB or service byte[] fileBytes = Convert.FromBase64String(base64String); string fileName = "report.pdf"; return File(fileBytes, "application/pdf", fileName); } Use code with caution. 4. Handling Data URIs (The "Prefix" Problem)
In a web environment, "downloading" means sending the file bytes back to the user's browser with the correct headers. In a Web API Controller: download file from base64 string c#
If you are building a Windows service, WPF, or WinForms app, you can write these bytes directly to the disk using System.IO . In a Web API Controller: If you are
However, once that string reaches your C# backend or desktop app, you need to turn it back into a physical file. This guide covers the most efficient ways to handle this in .NET. 1. The Core Logic: Converting Base64 to Bytes or WinForms app