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Delegates in Properties
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The delegates we ve seen so far have taken one or more parameters, and returned either void (an Action<>) or a bool (a Predicate<T>). But we can define a delegate to any sort of function we like. What if we want to provide a mechanism that allows the client to be notified when each processing step has been completed, and provide the processor with some text to insert into a process log Our callback delegate might look like this:
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delegate string LogTextProvider(Document doc);
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We could add a property to our DocumentProcessor so that we can get and set the callback function (see Example 5-16).
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class DocumentProcessor { public LogTextProvider LogTextProvider { get; set; } // ... }
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And then we could make use of it in our Process method, as shown in Example 5-17.
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public void Process(Document doc) { // First time, do the quick check foreach (ActionCheckPair process in processes) { if (process.QuickCheck != null && !process.QuickCheck(doc)) { Console.WriteLine("The process will not succeed."); if (LogTextProvider != null)
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{
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}
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Console.WriteLine(LogTextProvider(doc)); } return;
}
// Then perform the action foreach (ActionCheckPair process in processes) { process.Action(doc); if (LogTextProvider != null) { Console.WriteLine(LogTextProvider(doc)); } }
Notice that we re checking that our property is not null, and then we use standard delegate syntax to call the function that it references. Let s set a callback in our client (see Example 5-18).
static void Main(string[] args) { // ... DocumentProcessor processor = Configure(); processor.LogTextProvider = (doc => "Some text for the log..."); } // ...
Here we used a lambda expression to provide a delegate that takes a Document parameter called doc, and returns a string. In this case, it is just a constant string. Later, we ll do some work to emit a more useful message. Take a moment to notice again how compact the lambda syntax is, and how the compiler infers all those parameter types for us. Remember how much code we had to write to do this sort of thing back in the world of abstract base classes Compile and run, and we see the following output:
Processing document 1 The processing will not succeed. Some text for the log... Processing document 2 Document traduit. Some text for the log... Spellchecked document. Some text for the log...
Repaginated document. Some text for the log... Highlighting 'millennium' Some text for the log...
That s an example of a delegate for a function that returns something other than void or a bool. As you might have already guessed, the .NET Framework provides us with a generic type so that we don t have to declare those delegates by hand, either.
Generic Delegates for Functions
The .NET Framework exposes a generic class called Func<T, TResult>, which you can read as Func-of T and TResult. As with Predicate<T> and Action<T> the first type parameter determines the type of the first parameter of the function referenced by the delegate. Unlike Predicate<T> or Action<T> we also get to specify the type of the return value, using the last type parameter: TResult.
Just like Action<T>, there is a whole family of Func<> types which take one, two, three, and more parameters. Before .NET 4, Func<> went up to four parameters, but now goes all the way up to 16.
So we could replace our custom delegate type with a Func<>. We can delete the delegate declaration:
delegate string LogTextProvider(Document doc);
and update the property:
public Func<Document,string> LogTextProvider { get; set; }
We can build and run without any changes to our client code because the new property declaration still expects a delegate for a function with the same signature. And we still get a bit of log text:
Processing document 1 The processing will not succeed. Some text for the log... Processing document 2 Document traduit. Some text for the log... Spellchecked document. Some text for the log... Repaginated document.
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