.NET/Java PDF, Tiff, Barcode SDK Library

Custom attributes add information to your code that will be compiled into an assembly and stored alongside your values and types. This information can then be read programmatically via reflection or by the runtime itself. Attributes can be associated with types, members of types, and top-level values. They can also be associated with do statements. An attribute is specified in brackets, with the attribute name in angle brackets. For example: [<Obsolete>] Attribute names, by convention, end with the string Attribute, so the actual name of the Obsolete attribute is ObsoleteAttribute. An attribute must immediately precede what it modifies. The following code marks the function, functionOne, as obsolete: #light open System [<Obsolete>] let functionOne () = () An attribute is essentially just a class, and when you use an attribute, you are really just making a call to its constructor. In the previous example, Obsolete has a parameterless constructor, and it can be called with or without parentheses. In this case, we called it without parentheses. If you want to pass arguments to an attribute s constructor, then you must use parentheses and separate arguments with commas. For example: #light open System [<Obsolete("it is a pointless function anyway!")>] let functionTwo () = () Sometimes an attribute s constructor does not expose all the properties of the attribute. If you want to set them, you need to specify the property and a value for it. You specify the property name, an equals sign, and the value after the other arguments to the constructor. The next example sets the Unrestricted property of the PrintingPermission attribute to true:

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#light open System.Drawing.Printing open System.Security.Permissions [<PrintingPermission(SecurityAction.Demand, Unrestricted = true)>] let functionThree () = () You can use two or more attributes by separating the attributes with semicolons: #light open System open System.Drawing.Printing open System.Security.Permissions [<Obsolete; PrintingPermission(SecurityAction.Demand)>] let functionFive () = () So far, we ve used attributes only with values, but using them with type or type members is just as straightforward. The following example marks a type and all its members as obsolete: #light open System [<Obsolete>] type OOThing = class [<Obsolete>] val stringThing : string [<Obsolete>] new() = {stringThing = ""} [<Obsolete>] member x.GetTheString () = x.string_thing end If you intend to use WinForms or Windows Presentation Foundation (WPF) graphics within your program, you must ensure that the program is a single-thread apartment. This is because the libraries that provide the graphical components use unmanaged (not compiled by the CLR) code under the covers. The easiest way to do this is by using the STAThread attribute. This must modify the first do statement in the last file passed to the compiler, that is, the first statement that will execute when the program runs. For example: #light open System open System.Windows.Forms let form = new Form() [<STAThread>] do Application.Run(form)

You must also be careful regarding the order of the document.createElement and setAttribute statements with regard to where you add the newly created input element to its parent. In some browsers, you can add the newly created element only to its parent element after the element has been created and the type attribute has been correctly set. For instance, the following code snippet may behave unexpectedly in some browsers: document.getElementById("formElement").appendChild(button); button.setAttribute("type", "button"); To avoid potentially erratic behavior, be sure to create the input element and set all its attributes, especially the type attribute, before adding it to the parent, like so: var button = document.createElement("input"); button.setAttribute("type", "button"); document.getElementById("formElement").appendChild(button); Following this simple rule can help eliminate difficult-to-diagnose problems that may arise later.

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Once attributes have been added to types and values, it s possible to use reflection to find which values and types are marked with which attributes. This is usually done with the IsDefined or GetCustomAttributes methods of the System.Reflection.MemberInfo class, meaning they are available on most objects used for reflection including System.Type. The next example shows how to look for all types that are marked with the Obsolete attribute: #light let obsolete = |> |> |> |> |>

System.AppDomain.CurrentDomain.GetAssemblies() List.of_array List.map ( fun assm -> assm.GetTypes() ) Array.concat List.of_array List.filter ( fun m -> m.IsDefined((type System.ObsoleteAttribute), true))

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