How to programmatically control access to files in C# using FileSecurity objects

A quick demonstration on how to control access to files in real-time. As described in the Microsoft documentation, the control access to a file can be added or removed by obtaining the FileSecurity object from that file, then modified, and then applied back to the file. In this example I choose to change the file access properties of a simple icon file "icon.ico": So that before modifying the properties it can be opened straightforwardly as follows: To demonstrate how we can alter the access properties in real-time, create a C# console application in Visual Studio and add the following code. Note the use of WindowsIdentity.GetCurrent() api used to obtain the necessary username/domain details of the current Windows user: [code language="csharp"] using System; using System.IO; using System.Security.AccessControl; using System.Security.Principal; namespace FileControlAccess { class Program { static void Main(string[] args) { try { const string fileName = "c:\\data\\icon.ico"; var domain = WindowsIdentity.GetCurrent().Name; Console.WriteLine("Adding access control entry for " + fileName); // Add the access control entry to the file. AddFileSecurity(fileName, domain, FileSystemRights.ReadData, AccessControlType.Deny); Console.WriteLine("Removing access control entry from " + fileName); // Remove the access control entry from the file. RemoveFileSecurity(fileName, domain, FileSystemRights.ReadData, AccessControlType.Deny); Console.WriteLine("Done."); } catch (Exception e) { Console.WriteLine(e); } } // Adds an ACL entry on the specified file for the specified account. public static void AddFileSecurity(string fileName, string account, FileSystemRights rights, AccessControlType controlType) { // Get a FileSecurity object that represents the current security settings. FileSecurity fSecurity = File.GetAccessControl(fileName); // Add the FileSystemAccessRule to the security settings. fSecurity.AddAccessRule(new FileSystemAccessRule(account, rights, controlType)); File.SetAccessControl(fileName, fSecurity); } // Removes an ACL entry on the specified file for the specified account. public static void RemoveFileSecurity(string fileName, string account, FileSystemRights rights, AccessControlType controlType) { // Get a FileSecurity object that represents the current security settings. FileSecurity fSecurity = File.GetAccessControl(fileName); // Remove the FileSystemAccessRule from the security settings. fSecurity.RemoveAccessRule(new FileSystemAccessRule(account, rights, controlType)); File.SetAccessControl(fileName, fSecurity); } } } [/code] On stepping through the code, we first add an access control entry to DENY the users 'Read' access to the 'icon.ico' file: On inspecting the file's Security properties we observe that this Deny access property has indeed been added: And when we try to open the file we observe that we can't: We then step further through the code and remove the access control property we just added: This is also observed in the file's security properties as shown: So that we can now open the file unopposed as before:

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