PermissionSet Class In C#

Introduction

This article has been excerpted from the book "The Complete Visual C# Programmer's Guide" by the Authors of C# Corner.

The PermissionSet class represents a collection of different permissions and supports the methods that use and modify those permissions. You can add, remove, assert, deny, and copy permissions.

The example in Listing 22.20 uses the PermissionSet class to obtain the permissions necessary to gain full access to a file and to read a TEMP environment variable. Before changing the permissions of a file, the file will have a default permission set.

The Deny method of the PermissionSet prevents callers from accessing the protected resource even if they have been granted permission to access it. The PermitOnly method of the PermissionSet ensures that only the resources specified by this permission object can be accessed, even if the code has been granted permission to other resources. FileIOPermissionAccess specifies the actions that can be performed on the file or folder. The EnvironmentPermission class instance, added to the PermissionSet in Listing 22.20, manages access to user and system environment variables by granting permissions on how a user queries and modifies user and system environment variables.

Listing 22.20: PermissionSet Sample1

using System;
using System.IO;
using System.Security.Permissions;
using System.Security;
public class TestClass
{
    public static void Main()
    {
        // Try to access resources using the permissions currently available.
        AttemptAccess("Default permissions");
        // Create a permission set that allows read access to the TEMP environment variable 
        // and read, write, and append access to C:\myfile.txt
        PermissionSet ps = new PermissionSet(PermissionState.None);
        ps.AddPermission(new EnvironmentPermission(EnvironmentPermissionAccess.Read, "TEMP"));
        ps.AddPermission(new FileIOPermission(FileIOPermissionAccess.Read |
                          FileIOPermissionAccess.Write | FileIOPermissionAccess.Append, @"c:\myfile.txt"));

        // Test if we can access the resource contained in ps
        ps.Assert();
        // Try to access resources using the permissions we've just asserted.
        AttemptAccess("Assert permissions.");
        // Remove this stack frame's Assert
        CodeAccessPermission.RevertAssert();
        // Deny access to the resources we specify
        ps.Deny();
        // Try to access resources using the permissions we've just denied.
        AttemptAccess("Deny permissions.");
        // Remove this stack frame's Deny so we're back to default permissions.
        CodeAccessPermission.RevertDeny();
        // Make the permissions indicate the only things that we're allowed to do.
        ps.PermitOnly();
        // Try to access resources using only the permissions we've just permitted.
        AttemptAccess("PermitOnly permissions.");
        // Remove this stack frame's PermitOnly so we're back to default permissions.
        CodeAccessPermission.RevertPermitOnly();
        // Remove the FileIOPermissions from the permission set
        ps.RemovePermission(typeof(FileIOPermission));
        // Try to access resources using only the Environment permissions.
        ps.PermitOnly();
        AttemptAccess("PermitOnly without FileIOPermission permissions.");
        // Remove this stack frame's PermitOnly so we're back to default permissions.
        CodeAccessPermission.RevertPermitOnly();
        // Remove the EnvironmentPermission from the permission set
        ps.RemovePermission(typeof(EnvironmentPermission));
        // Try to access resources using no permissions.
        ps.PermitOnly();
        AttemptAccess("PermitOnly without any permissions");
        // Remove this stack frame's PermitOnly so we're back to default permissions.
        CodeAccessPermission.RevertPermitOnly();
        Console.ReadLine();
    }
    public static void AttemptAccess(String strMsg)
    {
        Console.WriteLine(strMsg);
        FileStream fs = null;
        String env = null;
        // Try to access a file
        try
        {
            fs = new FileStream("SomeFile", FileMode.OpenOrCreate);
        }
        catch (Exception)
        {
            // ignore catch
        }
        // Try to read an environment variable
        try
        {
            env = Environment.GetEnvironmentVariable("TEMP");
        }
        catch (Exception)
        {
            // ignore catch
        }
        // Display what we successfully did and what we failed.
        Console.WriteLine("test results: " + ((fs != null) ?
                          "File opened." : "File cannot be opened!") + " " +
                          ((env != null) ? "Environment read." : "Environment cannot be read!"));

        // close file stream
        fs?.Close();
    }
}