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The stack walk mechanism ensures that no untrusted callers can use your assembly to perform an operation for which they have no permission. In some circumstances, however, this mechanism might prevent operations that would be legitimate otherwise. For example, let s say you ve written a financial math library that reads some critical val ues for example, currency exchange ratios from an .ini file. This library does noth ing dangerous, yet calling its methods throws an exception if the caller has no permission to access the file system. Let s use Figure 33-8 as a guideline and assume you are the author of Assembly B. Your assembly has the right to read files, and it passes this right along to Assembly C. However, the stack walk fails when Assembly C creates a FileReader object if your caller (Assembly A) has no permission to read files. To work around this problem, you can use the Assert method of a permission object. This method tells the .NET runtime not to go past your assembly when walking up the stack while checking for a given permission object:

Distribute, and then click Relative to Margin Guides to turn on that option. to Align or Distribute, and click Align Right.

Dim filePerm As New FileIOPermission(PermissionState.Unrestricted)

filePerm.Assert()

protected override void Draw(GameTime gameTime) { Color textureColor; textureColor = new Color(redIntensity,greenIntensity,blueIntensity); spriteBatch.Begin(); spriteBatch.Draw(jakeTexture, jakeRect, textureColor); spriteBatch.End(); base.Draw(gameTime); }

Dim sr As New StreamReader( c:\myapp\vars.ini )

33:

As for the PermitOnly and Deny methods, there can be only one active Assert in the current procedure at any given time. You must use the CodeAccessPermission.RevertAssert shared method before you can assert a different permission inside the same procedure. By default, only assemblies in the My_Computer_Zone and LocalIntranet_Zone code groups can call the Assert method because the permission set associated with these zones includes a SecurityPermission object with the Assertion flag enabled. As you might expect, an assembly can only assert a permission object that the assembly owns for example, an assembly from the local intranet can assert the permission for printing to the default printer and displaying UI elements, but it can t assert the right to enumerate types and methods via reflection. If your assembly calls unmanaged code via either PInvoke or COM Interop, you should assert an appropriate SecurityPermission object, as follows:

10. On the Arrange menu, point to Align or Distribute, and click Align Top. Then point 11. Now drag the lower-left corner handle down and to the left to stretch the object

Dim secPerm As New SecurityPermission(SecurityPermissionFlags.UnmanagedCode)

secPerm.Assert()

Rather than using white as the drawing color, this version of Draw uses the color it creates based on the red, green, and blue intensity values.

If you fail to do this assertion, your assembly can t be used by untrusted code. In addi tion, if your assembly has a strong name, it must be marked with the AllowPartial lyTrustedCallers attribute, otherwise untrusted code won t be able to even load it. (See the Partially Trusted Assemblies section earlier in this chapter.) It s crucial that you use the Assert method judiciously. By asserting a permission you are claiming that callers can t trick your assembly into doing anything dangerous. Let s suppose you need to provide users with the ability to read and write files only in the C:\MyApp directory. You might be tempted to use the following approach:

Public Sub DeleteFile(ByVal filename As String) Ensure that filename begins with C:\MyApp\. If Not filename.ToLower().StartsWith( c:\myapp\ ) Then Throw New ArgumentException( Path not allowed ) End If Assert the right to access the file system. Dim filePerm As New FileIOPermission(PermissionState.Unrestricted) filePerm.Assert() Delete the file. File.Delete(filename) End Sub

Stop dragging when the object snaps to align with the bottom and right margin guides. The object snaps to the guides because To Guides is turned on under Snap on the Arrange menu. Tip If you want the freedom to position an object without snapping, hold down the Alt key as you drag the object.

Alas, a smart (and wicked) caller can delete a system file by providing a string argu ment that passes your validation test:

DeleteFile( C:\MyApp\..\Windows\Soap Bubbles.bmp )

You can use the same principle to make a picture mood light; this works especially well if you use a black-and-white image or one with really strong colors in it. You can also make a picture recognition game here, where the aim of the game is to be the first one to recognize a picture as you slowly make it brighter. Sample Code: Image MoodLight The samp e project n the 04 Image MoodL ght d rectory

Of course, you might suffer from problems far more serious than just losing your favor ite wallpaper. Here s a better approach, which is also more concise:

Part VII:

Public Sub DeleteFile(ByVal filename As String) Assert the right to access the C:\MyApp. Dim filePerm As New FileIOPermission(FileIOPermissionAccess.AllAccess, C:\MyApp ) filePerm.Assert() Delete the file. File.Delete(filename) End Sub

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