Imperative Security

// ImperativeSecurity.cs
using System;
using System.Security;
using System.Security.Permissions;
using System.Runtime.InteropServices;

class NativeMethods
{
// This is a call to unmanaged code. Executing this method requires
// the UnmanagedCode security permission. Without this permission
// an attempt to call this method will throw a SecurityException:
[DllImport("msvcrt.dll")]
public static extern int puts(string str);
[DllImport("msvcrt.dll")]
internal static extern int _flushall();
}

class MainClass
{
private static void CallUnmanagedCodeWithoutPermission()
{
// Create a security permission object to describe the
// UnmanagedCode permission:
SecurityPermission perm =
new SecurityPermission(SecurityPermissionFlag.UnmanagedCode);

// Deny the UnmanagedCode from our current set of permissions.
// Any method that is called on this thread until this method
// returns will be denied access to unmanaged code.
// Even though the CallUnmanagedCodeWithPermission method
// is called from a stack frame that already
// calls Assert for unmanaged code, you still cannot call native
// code. Because you use Deny here, the permission gets
// overwritten.
perm.Deny();

try
{
Console.WriteLine("Attempting to call unmanaged code without permission.");
NativeMethods.puts("Hello World!");
NativeMethods._flushall();
Console.WriteLine("Called unmanaged code without permission. Whoops!");
}
catch (SecurityException)
{
Console.WriteLine("Caught Security Exception attempting to call unmanaged code.");
}
}

private static void CallUnmanagedCodeWithPermission()
{
// Create a security permission object to describe the
// UnmanagedCode permission:
SecurityPermission perm =
new SecurityPermission(SecurityPermissionFlag.UnmanagedCode);

// Check that you have permission to access unmanaged code.
// If you don't have permission to access unmanaged code, then
// this call will throw a SecurityException.
// Even though the CallUnmanagedCodeWithPermission method
// is called from a stack frame that already
// calls Assert for unmanaged code, you still cannot call native
// code. Because you use Deny here, the permission gets
// overwritten.
perm.Assert();

try
{
Console.WriteLine("Attempting to call unmanaged code with permission.");
NativeMethods.puts("Hello World!");
NativeMethods._flushall();
Console.WriteLine("Called unmanaged code with permission.");
}
catch (SecurityException)
{
Console.WriteLine("Caught Security Exception attempting to call unmanaged code. Whoops!");
}
}

public static void Main()
{
// The method itself will call the security permission Deny
// for unmanaged code, which will override the Assert permission
// in this stack frame.
SecurityPermission perm = new
SecurityPermission(SecurityPermissionFlag.UnmanagedCode);
perm.Assert();
CallUnmanagedCodeWithoutPermission();

// The method itself will call the security permission Assert
// for unmanaged code, which will override the Deny permission in
// this stack frame.
perm.Deny();
CallUnmanagedCodeWithPermission();
}
}