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For one reason or another, somewhere down the line, you're probably going to want to modify a file's access permissions from your code. Maybe your users have a nasty habit of overwriting them, or you want to ensure that newly-created files are given a specific permission mask. Whatever the reason, the following C# code example shows how to modify a file's access permissions using the System.Security.Principal and System.Security.AccessControl namespaces.

C# code:

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

// …

// deny WRITE permission to DOMAIN\user
string fileLocation = "c:\\temp.txt";
NTAccount acct = new NTAccount("DOMAIN", "user");
FileSecurity sec = System.IO.File.GetAccessControl(fileLocation);
sec.AddAccessRule(new FileSystemAccessRule(acct, FileSystemRights.Write, AccessControlType.Deny));
System.IO.File.SetAccessControl(fileLocation, sec);

In the example, the Write permission is being denied to the DOMAIN\user. Different combinations of the FileSystemRights and AccessControlType options available could be leveraged for different results (i.e., allowing Read access).