Best way to make a file writeable in c#

Two ways:

System.IO.FileInfo fileInfo = new System.IO.FileInfo(filePath);
fileInfo.IsReadOnly = true/false;

or

// Careful! This will clear other file flags e.g. `FileAttributes.Hidden`
File.SetAttributes(filePath, FileAttributes.ReadOnly/FileAttributes.Normal);

The IsReadOnly property on FileInfo essentially does the bit-flipping you would have to do manually in the second method.

To set the read-only flag, in effect making the file non-writeable:

File.SetAttributes(filePath,
    File.GetAttributes(filePath) | FileAttributes.ReadOnly);

To remove the read-only flag, in effect making the file writeable:

File.SetAttributes(filePath,
    File.GetAttributes(filePath) & ~FileAttributes.ReadOnly);

To toggle the read-only flag, making it the opposite of whatever it is right now:

File.SetAttributes(filePath,
    File.GetAttributes(filePath) ^ FileAttributes.ReadOnly);

This is basically bitmasks in effect. You set a specific bit to set the read-only flag, you clear it to remove the flag.

Note that the above code will not change any other properties of the file. In other words, if the file was hidden before you executed the above code, it will stay hidden afterwards as well. If you simply set the file attributes to .Normal or .ReadOnly you might end up losing other flags in the process.