We ve changed the signature of that method, so all our clients are sadly broken. They need to be rewritten to use the new method. That s not great. A better alternative is to provide both signatures keep the old single-parameter contract around, but add an overload with the extra argument. And to ensure that the overloads behave consistently (and to avoid duplicating code) we can make the simpler method call the new method as its actual implementation. The old method was just the equivalent of calling the new method with a delay of zero, so we could replace it with the method shown in Example 3-33. This lets us provide the newly enhanced SendMessage, while continuing to support the old, simpler version.

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public void SendMessage(string messageName) { SendMessage(messageName, TimeSpan.Zero); }

(TimeSpan.Zero is a static field that returns a duration of zero.) Until C# 4.0 that s as far as we could go. However, the C# designers noticed that a lot of member overloads were just like this one: facades over an ber-implementation, with a bunch of parameters defaulted out to particular values. So they decided to make it easier for us to support multiple variations on the same method. Rather than writing lots of overloads, we can now just specify default values for a method s arguments, which saves us typing a lot of boilerplate, and helps make our default choices more transparent. Let s take out the single-parameter method overload we just added, and instead change the declaration of our multiparameter implementation, as shown in Example 3-34.

) );

public void SendMessage( string messageName, TimeSpan delay = default(TimeSpan))

The ScriptManager control provides an error handling mechanism whereby you can specify the HTML to render when an error condition is met. This is particularly useful for the client experience because you can then help your users gracefully handle errors. Within the ScriptManager control, you can use the <ErrorTemplate> tag to define your error message in HTML. So, for example, the following script will produce a user-friendly response to an error: <atlas:ScriptManager ID="ScriptManager1" runat="server"> <ErrorTemplate> There was an error processing your action.<br /> <span id="errorMessageLabel"></span> <hr /> <button type="button" id="okButton">OK</button> </ErrorTemplate> </atlas:ScriptManager> The HTML defined within the <ErrorTemplate> element will render in a simulation of a modal dialog box. This is achieved by the rest of the page being partially obscured using a semitransparent overlay. The HTML within the <ErrorTemplate> element is enabled and active. This allows you to design a pretty rich error display according to your preferences.

Even though we ve only got one method, which supports two arguments, code that tries to call it with a single argument will still work. That s because default values can fill in for missing arguments. (If we tried to call SendMessage with no arguments at all, we d get a compiler error, because there s no default for the first argument here.) But it doesn t end there. Say we had a method with four parameters, like this one:

public void MyMethod( int firstOne, double secondInLine = 3.1416, string thirdHere = "The third parameter", TimeSpan lastButNotLeast = default(TimeSpan)) { // ... }

You can draw rectangles with square or rounded corners, as shown in Figure 7-8. The methods accept either a QRect or four values representing an (x,y) pair for the top-left corner, then the width, followed by the height of the rectangle. The methods are named drawRect and drawRoundRect.

If we want to call it and specify the first parameter (which we have to, because it has no default), and the third, but not the second or the fourth, we can do so by using the names of the parameters, like this:

MyMethod(127, thirdHere: "New third parameter");

With just one method, we now have many different ways to call it we can provide all the arguments, or just the first and second, or perhaps the first, second, and third. There are many combinations. Before named arguments and defaults were added in C# 4.0, the only way to get this kind of flexibility was to write an overload for each distinct combination.

Figure 7-8. Rectangles with square and rounded corners Listing 7-5 shows how rectangles with rounded and square corners are drawn. The first two rectangles are drawn using coordinates specified directly in the method calls. The coordinates are specified as x, y, w, h; where x and y specify the top-left corner, and w, h specify the width of the rectangle.

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