C# 4 and .NET 4

Code access security ❘ 565
If you just use the .NET classes for file I/O you don’t have to demand the FileIOPermission yourself as
this is demanded by the .NET classes doing file I/O. However, you need to make the demand yourself if you
wrap native API calls such as CreateFileTransacted(). Also, you can use this mechanism to demand
custom permissions from the caller.
Using the sandbox aPi to Host Unprivileged Code
By default, with a desktop application, the application has full trust. Using the Sandbox API, you can create
an app-domain that doesn’t have full trust.
To see the Sandbox API in action, first create a C# library project named RequireFileIOPermissionsDemo. This
library contains the class RequirePermissionsDemo with the method RequireFilePermissions(). This
method returns true or false, depending on whether the code has file permissions. With the implementation
of this code, the File class creates a file where the path is passed with the argument variable path. In
case writing the file fails, an exception of type SecurityException is thrown. The File class checks for
the FileIOSecurity as you saw earlier with the DemandPermissonDemo sample. If the security check
fails, a SecurityException is thrown by the Demand() method of the FileIOSecurity class. Here, the
SecurityException is caught to return false from the RequireFilePermissions() method.
using System;
using System.IO;
using System.Security;
[assembly: AllowPartiallyTrustedCallers()]
namespace Wrox.ProCSharp.Security
{
[SecuritySafeCritical]
public class RequirePermissionsDemo : MarshalByRefObject
{
public bool RequireFilePermissions(string path)
{
bool accessAllowed = true;
try
{
StreamWriter writer = File.CreateText(path);
writer.WriteLine("written successfully");
writer.Close();
}
catch (SecurityException)
{
accessAllowed = false;
}
}
}
}
return accessAllowed;
code snippet RequireFileIOPermissionsDemo/RequirePermissionsDemo.cs
The hosting application where the Sandbox API is used is the project AppDomainHost that is a simple
C# console application. The Sandbox API is an overload of the AppDomain.CreateDomain() method that
creates a new app-domain in a sandbox. This method requires four parameters including the name of the
app-domain, the evidence that is taken from the current app-domain, the AppDomainSetup information,
and a permission set. The permission set that is created only contains SecurityPermission with the flag
SecurityPermissionFlag.Execution so that the code is allowed to execute — nothing more. In the
new sandboxed app-domain, the object of type DemandPermissions in the assembly DemandPermission
is instantiated.
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