Advanced Topics in .NET framework

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Advanced Topics
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give the compiled assembly more permissions than it would normally have. Admin istrators can list the minimal, optional, and refused permission sets of an assembly with the PermView utility:
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PERMVIEW myapp.exe
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The optional /decl switch also lists all the declarative security attributes associated with classes and methods (see Figure 33-10), information that is useful to developers who want to use the types defined in an assembly:
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PERMVIEW /decl mylibrary.dll
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Figure 33-10 The PermView utility
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Identity Permission Classes
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A group of five permission classes in the System.Security.Permissions namespace and their corresponding attributes don t map to any system resource. You recognize these classes because their names end with IdentityPermission: StrongNameIdentityPermis sion, PublisherIdentityPermission, UrlIdentityPermission, SiteIdentityPermission, and ZoneIdentityPermission. You typically use these classes to ensure that your code can be called only by assemblies with a given identity for example, assemblies coming from a given zone, site, or URL.
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<ZoneIdentityPermission(SecurityAction.LinkDemand, Zone:=SecurityZone.MyComputer)> _
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Class SampleClass
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End Class
<UrlIdentityPermission(SecurityAction.LinkDemand, Url:=" )> _
Class SampleClass2
End Class
You can use identity permission objects to demand permissions by using either an imperative or a declarative coding style but this technique is rarely useful because it requires that all the assemblies in the call stack have the same identity. Instead, these permission objects are typically used with a LinkDemand action and declarative secu rity (which explains why I deferred the explanation of these objects until now). The StrongNameIdentityPermission class is especially useful to ensure that an assembly can be used only by a specific assembly. This class exposes the Name, PublicKey, and Version properties, but you often omit the last one so that it works with any version of the caller. In addition, you can omit the Name value to grant access to all the assem blies coming from the same publisher (more specifically, all the assemblies with a given public key):
<Assembly: StrongNameIdentityPermission(SecurityAction.RequestMinimum, _ PublicKey:="002400683011af5799aced238935f32ab125790aa787786343440023410 & 073958138838746ac86ef8732623f87978223aced6767169876acde74e59a6457be26ee0 045467467ce68a123cdef89648292518478536964ace47852361985636487523b7e0e25b 71ad39acef23457893574852568eac5863298de685de69124de56eded66978425008142e f723bfe602345459574568790b365852cdead454585c6 )> _ & _ & _ & _
Used in conjunction with the SecurityManager.IsGranted shared method, identity permissions objects are useful to let the assembly know which zone, site, or URL it was launched from:
If SecurityManager.IsGranted( _ New ZoneIdentityPermission(SecurityZone.Intranet)) Then This assembly has been launched from a network drive. End If
The SuppressUnmanagedCodeSecurity Attribute
When managed code calls unmanaged code through either PInvoke or COM Interop the .NET runtime fires a stack walk demanding a SecurityPermission object with the SecurityPermissionFlag.UnmanagedCode flag. You can reduce the overhead by flagging the method that calls into unmanaged code with a SuppressUnmanagedCodeSecurity attribute. This attribute forces a link demand when the method is JITcompiled but suppresses the full stack walk at each method invocation:
<SuppressUnmanagedCodeSecurity()> _ Sub DoSomething() Calls to unmanaged code through PInvoke don t cause a demand at runtime. End Sub
You can also narrow the scope of the attribute to a specific unmanaged procedure by applying it together with the DllImport attribute
Part VII:
Advanced Topics
<System.Runtime.InteropServices.DllImport( kernel32", EntryPoint:="Beep ), _ SuppressUnmanagedCodeSecurity()> _ Function WinBeep(ByVal freq As Integer, ByVal duration As Integer) As Integer End Function
You can also apply the SuppressUnmanagedCodeSecurity attribute at the class level, in which case it affects all the methods in the class. The story is completely different when you call unmanaged code via COM Interop. In this case, you must apply the attribute to the interface through which you invoke the methods in the COM class. (Applying the attribute to just the method that accesses the COM object has no effect.) This requirement forces you to explicitly access COM objects though a managed interface they implement, which is an unnatural coding style. More frequently, you ask the TlbImp utility to apply this attribute to all unmanaged interfaces being imported by specifying the /unsafe option:
TLBIMP /unsafe mycomponent.dll
Disabling CAS
This long discussion about code access security demonstrated the amount of activity that goes on under the hood whenever your code accesses a protected resource, either directly or indirectly. Stack walks are performed in all cases, even if all the assemblies involved are fully trusted. In general, most developers are willing to trade some performance in exchange for better security, but there are times when you don t really need to protect your code with CAS. This is the case, for example, when you run a stand-alone Windows Forms application that uses only local assemblies on a computer that isn t connected to the Internet or that is protected by a firewall. In cases such as these, you can decide to turn code access secu rity off. You can do this from the command prompt with the s switch of the CasPol utility:
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