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consideration can behave unexpectedly: security exceptions may be thrown, and the applications will have no means of recovering. CAS permissions are defined by a set of attribute classes that implement the abstract System.Security.CodeAccessPermissionClass. Table 17-1 details the most commonly used attributes.
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Table 17-1. CAS Permission Classes
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Attribute Class Name System.Net.DnsPermission System.Security.Permissions.EnvironmentPermission
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Permission Represented Accessing the DNS Reading and writing environment variables Reading and writing to event log System.Diagnostics.EventLogPermission services Accessing files that are selected System.Security.Permissions.FileDialogPermission by the user via a file dialog window Reading, appending, or writing System.Security.Permissions.FileIOPermission files or directories System.Security.Permissions.IsolatedStorageFilePermission Accessing private virtual file systems Accessing printing services System.Drawing.Printing.PrintingPermission Reading or modifying the System.Security.Permissions.RegistryPermission Windows registry Opening socket connections to System.Net.SocketPermission remote machines System.Data.SqlClient.SqlClientPermission Accessing SQL servers Creating user interfaces System.Security.Permissions.UIPermission Accessing HTTP Internet System.Net.WebPermission resources
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Declarative CAS Statements
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Declarative statements also known as soft statements are a means of indicating the permissions that an assembly requires. The CLR reads the declarative statements from an assembly prior to execution; if the required permissions cannot be granted, a System.Security.SecurityException will be thrown and the code will not be executed. Declarative CAS statements fall into three categories minimum permissions, optional permissions, and denied permissions as described here:
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Minimum permissions are those that an application needs in order to provide core services and cannot function without. Optional permissions are those that the application would like to have (perhaps in order to offer additional features) but that are not essential. Denied permissions are those that the application should never be granted, even if they're permitted by the CAS security policies.
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Statement Scope
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Declarative statements can be applied to an assembly, a class, or a method; this approach allows the programmer to limit the scope of permissions. Declarative CAS statements cannot be applied to other member types, such as properties or indexers. The following example demonstrates the three statement scopes, which appear in boldface:
using System; using System.Security.Permissions; [assembly:FileIOPermissionAttribute(SecurityAction.RequestMinimum, Unrestricted = true)] [EnvironmentPermission(SecurityAction.Demand, Unrestricted=true)] class MyClass { [System.Net.WebPermission(SecurityAction.Demand, Unrestricted=true)] public void myMethod() { // method statements } public void myOtherMethod() { // methods statements } public static void Main() { } } class MyOtherClass { // class definition statements }
The syntax of these statements is explained here:
The first statement, prefixed with assembly, requests access to the file system for the entire assembly; any method contained in MyClass and MyOtherClass can make calls that access the file system. The second statement requests permission for all methods of MyClass to be able to read and write environment variables. This permission will not be available to methods of MyOtherClass. The third statement requests access to HTTP Internet servers for the MyClass.myMethod method; this permission will not be available to MyClass.myOtherMethod or to any method of MyOtherClass.
Declarative Statement Syntax
The first argument for a declarative attribute is a value from the System.Security.Permissions.SecurityAction enumeration; this value allows the programmer to specify the kind of CAS request that the statement represents. Not all values can be applied for all permission scopes. Table 17-2 details these values.
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Table 17-2. SecurityAction Values
Value Scope Action RequestMinimum Assembly A request for the minimum permissions required for the code to run. RequestOptional Assembly A request for additional permissions that are optional (not required to run). RequestRefuse Assembly A request that permissions that might be misused should not be granted to the calling code. Class, All callers higher in the call stack are required to have been Demand method granted the permission specified by the current permission object. Class, The calling code can access the resource identified by the Assert method current permission object, even if callers higher in the stack haven't been granted permission to access the resource. Class, The ability to access the resource specified by the current Deny method permission object is denied to callers, even if they have been granted permission to access it. The second argument represents a property that will be used to control the way that the permission restricts access to the protected resource. The statements in the preceding example all used the Unrestricted=true argument, which specifies that the declarative statement applies to all aspects of the protected resource for example, allowing access to all HTTP servers. The Unrestricted property is the only one that is common to all permission classes; each class defines a unique set of properties that reflect how the permission is applied. For example, the WebPermission class shown in the preceding example defines the Connect property that specifies the URL to which the permission applies. The following statement demonstrates how to combine the SecurityAction.Deny value with the WebPermission.Connect property to prevent access to a specific URL:
[System.Net.WebPermission(SecurityAction.Deny, Connect="http://www.mycompany.com/index.html")]
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