Simple function in C# to download a file from the internet, supports to resume the download.

DownloadFile.cs

public static void downloadFile(string sourceURL, string destinationPath)
{
    long fileSize = 0;
    int bufferSize = 1024;
    bufferSize *= 1000;
    long existLen = 0;
    
    System.IO.FileStream saveFileStream;
    if (System.IO.File.Exists(destinationPath))
    {
        System.IO.FileInfo destinationFileInfo = new System.IO.FileInfo(destinationPath);
        existLen = destinationFileInfo.Length;
    }

    if (existLen > 0)
        saveFileStream = new System.IO.FileStream(destinationPath,
                                                  System.IO.FileMode.Append,
                                                  System.IO.FileAccess.Write,
                                                  System.IO.FileShare.ReadWrite);
    else
        saveFileStream = new System.IO.FileStream(destinationPath,
                                                  System.IO.FileMode.Create,
                                                  System.IO.FileAccess.Write,
                                                  System.IO.FileShare.ReadWrite);
 
    System.Net.HttpWebRequest httpReq;
    System.Net.HttpWebResponse httpRes;
    httpReq = (System.Net.HttpWebRequest) System.Net.HttpWebRequest.Create(sourceURL);
    httpReq.AddRange((int) existLen);
    System.IO.Stream resStream;
    httpRes = (System.Net.HttpWebResponse) httpReq.GetResponse();
    resStream = httpRes.GetResponseStream();
 
    fileSize = httpRes.ContentLength;
 
    int byteSize;
    byte[] downBuffer = new byte[bufferSize];
 
    while ((byteSize = resStream.Read(downBuffer, 0, downBuffer.Length)) > 0)
    {
        saveFileStream.Write(downBuffer, 0, byteSize);
    }
} 

Called the UnauthorizedAccessException in 21 line
How to fix it?

Called the UnauthorizedAccessException in 21 line
How to fix it?

I guess you pass folder name without filename.

This give this kind of error => C:\myfolder\

This works fine => C:\myfolder\myfile.ext

You need to close the stream with saveFileStream.Close() at the end of your function. Else if you try to use the file immediately after downloading, you will get an exception.

Hi, I'm really new to C# (this is like my first time using it) I'm trying to make a C# commandline app and one of the commands I'm trying to make is a "checkforupdates" command which then checks one of my private repositories for a filename similar to my commandline app's filename but with a higher version number (so say for example Alpha 1 detects Alpha 2 and it's ready to download it, but if Alpha 1 detects Alpha 1 it doesn't download it). Can you please help me with this?

Hi, I'm really new to C# (this is like my first time using it) I'm trying to make a C# commandline app and one of the commands I'm trying to make is a "checkforupdates" command which then checks one of my private repositories for a filename similar to my commandline app's filename but with a higher version number (so say for example Alpha 1 detects Alpha 2 and it's ready to download it, but if Alpha 1 detects Alpha 1 it doesn't download it). Can you please help me with this?

please spend some time learning c# and you will find a solution.

Called the UnauthorizedAccessException in 21 line
How to fix it?

I guess you pass folder name without filename.

This give this kind of error => C:\myfolder\

This works fine => C:\myfolder\myfile.ext

Thank you! It's worked!

System.UnauthorizedAccessException: 'Access to the path 'F:\asd' is denied.'

Please help me!

System.UnauthorizedAccessException: 'Access to the path 'F:\asd' is denied.'

Please help me!

This is not related to code, the code telling you that you do not have permission on F drive with the given folder name

System.UnauthorizedAccessException: 'Access to the path 'F:\asd' is denied.'

Please help me!

Check if drive F: exists (open file explorer and make sure you can access the drive with the files/folders in it etc.)
then try running the code