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Imperative Security

Imperative security syntax issues a security call by creating a new instance of the permission object you want to invoke. You can use imperative syntax to perform demands and overrides, but not requests.

Before you make the security call, you must initialize the state data of the permission object so that it represents the particular form of the permission you need. For example, when creating a FileIOPermission object, you can use the constructor to initialize the FileIOPermission object so that it represents either unrestricted access to all files or no access to files. Or, you can use a different FileIOPermission object, passing parameters that indicate the type of access you want the object to represent (that is, read, append, or write) and what files you want the object to protect.

In addition to using imperative security syntax to invoke a single security object, you can use it to initialize a group of permissions in a permission set. For example, this technique is the only way to reliably perform assert calls on multiple permissions in one method. Use the PermissionSet and NamedPermissionSet classes to create a group of permissions and then call the appropriate method to invoke the desired security call.

You can use imperative syntax to perform demands and overrides, but not requests. You might use imperative syntax for demands and overrides instead of declarative syntax when information that you need in order to initialize the permission state becomes known only at run time. For example, if you want to ensure that callers have permission to read a certain file, but you do not know the name of that file until run time, use an imperative demand. You might also choose to use imperative checks instead of declarative checks when you need to determine at run time whether a condition holds and, based on the result of the test, make a security demand (or not).

The following code fragment shows imperative syntax for requesting that your code's callers have a custom permission called MyPermission. This permission is a hypothetical custom permission and does not exist in the .NET Framework. A new instance of MyPermision is created in MyMethod, guarding only this method with the security call.

Public Class MyClass1
   
   Public Sub New()

   End Sub
   
   Public Sub MyMethod()
      'MyPermission is demanded using imperative syntax.
      Dim Perm As New MyPermission()
      Perm.Demand()
      'This method is protected by the security call.
   End Sub
   
   Public Sub YourMethod()
      'YourMethod 'This method is not protected by the security call.
   End Sub
End Class
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