C# 5.0 Programmer's Reference

Exercises ❘ 599
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For each such class, the program should add an item to the Tools menu. The name of
that item should be specified by the class’s DisplayName attribute. (If the class doesn’t
have that attribute, use the class’s name.)
Each Draw method should draw something on the Graphics object it is passed.
Hint: To get a class’s DisplayName attribute value, get the class’s type and use the type’s
GetCustomAttribute method. Cast the returned Attribute into a DisplayNameAttribute
and use its DisplayName property.
The following code shows an example add-in method.
[DisplayName("Rectangle")]
public static class RectangleDrawer
{
public static void Draw(Graphics gr)
{
// Draw something...
}
}
Hint: When you find an appropriate Draw method, store its MethodInfo object in the Tag
property of its menu item. That way when the user clicks that item, you can find the corresponding
MethodInfo to invoke the method.
4. The program you built for Exercise 3 looks for DLLs that hold classes that define a single Draw
method. Alternatively, you could allow the classes to hold any number of add-in methods.
Modify the program you wrote for Exercise 3 so it does that. It should use any method that
takes a Graphics object as a parameter and that has a DisplayName attribute.
5. The program you built for Exercise 4 assumes that any method that takes a Graphics object
as a parameter and that has a DisplayName attribute is a drawing add-in. That works but is
somewhat restrictive because it prevents you from having any other similar methods that are
not add-ins. Another approach is to define your own custom attribute class and apply it to
methods that are add-ins.
Modify the program and DLL(s) you wrote for Exercise 4 to use this approach. In the DLL(s),
create a DrawingAddInAttribute attribute class. Place it inside the DrawingAddIn namespace.
(The rest of the DLL doesn’t need to be in that namespace.) The following code shows how you
can create this attribute.
namespace DrawingAddIn
{
// An attribute that flags a method as a drawing add-in tool.
[AttributeUsage(AttributeTargets.Method)]
public class DrawingAddInAttribute : Attribute
{
}
}
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