How to grant full permission to a file created by my application for ALL users?

You will need to give full control to "Everyone" group on the machine. Found this post on MSDN which talks about it.

Hope this works for you.

Note to people using this.

When using literal strings for the FileSystemAccessRule, it should be WellKnownSidType.WorldSid instead of "everyone".

The reason is because there are multiple Window languages and Everyone only applies to EN ones, so for Spanish, it might be "Todos" (or something else).

using System.Security.AccessControl;
using System.Security.Principal;
using System.IO;

private void GrantAccess(string fullPath)
{
    DirectoryInfo dInfo = new DirectoryInfo(fullPath);
    DirectorySecurity dSecurity = dInfo.GetAccessControl();
    dSecurity.AddAccessRule(new FileSystemAccessRule(new SecurityIdentifier(WellKnownSidType.WorldSid, null), FileSystemRights.FullControl, InheritanceFlags.ObjectInherit | InheritanceFlags.ContainerInherit, PropagationFlags.NoPropagateInherit, AccessControlType.Allow));
    dInfo.SetAccessControl(dSecurity);
}