.net barcode generator Part III Essential Types in .NET

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Part III Essential Types
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If you want the body to consist of two or more statements, then you must enclose it in curly braces . And if the delegate expects a return value, then you must have a return statement inside the body . Here is an example:
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Func<Int32, Int32, String> f7 = (n1, n2) => { Int32 sum = n1 + n2; return sum.ToString(); };
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Important In case it s not obvious, let me explicitly point out that the main benefit of lambda
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expressions is that they remove a level of indirection from within your source code . Normally, you d have to write a separate method, give that method a name, and then pass the name of that method where a delegate is required . The name gives you a way to refer to a body of code, and if you need to refer to the same body of code from multiple locations in your source code, then writing a method and giving it a name is a great way to go . However, if you need to have a body of code that is referred to only once within your source code, then a lambda expression allows you to put that code directly inline without having to assign it a name, thus increasing programmer productivity .
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Note When C# 2 .0 came out, it introduced a feature called anonymous methods. Like lambda
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expressions (introduced in C# 3 .0), anonymous methods describes a syntax for creating anonymous functions . It is now recommended (in section 7 .14 of the C# Language Specification) that developers use the newer lambda expression syntax rather than the older anonymous method syntax because the lambda expression syntax is more terse, making code easier to write, read, and maintain . Of course, Microsoft's C# compiler continues to support parsing both syntaxes for creating anonymous functions so that developers are not forced to modify any code that was originally written for C# 2 .0 . In this book, I will explain and use only the lambda expression syntax .
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Syntactical Shortcut #3: No Need to Wrap Local Variables in a Class Manually to Pass Them to a Callback Method
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I ve already shown how the callback code can reference other members defined in the class . However, sometimes, you might like the callback code to reference local parameters or variables that exist in the defining method . Here s an interesting example:
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internal sealed class AClass { public static void UsingLocalVariablesInTheCallbackCode(Int32 numToDo) { // Some local variables Int32[] squares = new Int32[numToDo]; AutoResetEvent done = new AutoResetEvent(false); // Do a bunch of tasks on other threads for (Int32 n = 0; n < squares.Length; n++) { ThreadPool.QueueUserWorkItem( obj => { Int32 num = (Int32) obj; // This task would normally be more time consuming squares[num] = num * num;
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17 Delegates
// If last task, let main thread continue running if (Interlocked.Decrement(ref numToDo) == 0) done.Set(); }, n); } // Wait for all the other threads to finish done.WaitOne(); // Show the results for (Int32 n = 0; n < squares.Length; n++) Console.WriteLine("Index {0}, Square={1}", n, squares[n]); } }
This example really shows off how easy C# makes implementing what used to be a pretty complex task . The method above defines one parameter, numToDo, and two local variables, squares and done . And the body of the lambda expression refers to these variables . Now imagine that the code in the body of the lambda expression is placed in a separate method (as is required by the CLR) . How would the values of the variables be passed to the separate method The only way to do this is to define a new helper class that also defines a field for each value that you want passed to the callback code . In addition, the callback code would have to be defined as an instance method in this helper class . Then, the UsingLocalVariablesInTheCallbackCode method would have to construct an instance of the helper class, initialize the fields from the values in its local variables, and then construct the delegate object bound to the helper object/instance method . Note When a lambda expression causes the compiler to generate a class with parameter/local
variables turned into fields, the lifetime of the objects that the variables refer to are lengthened . Usually, a parameter/local variable goes out of scope at the last usage of the variable within a method . However, turning the variable into a field causes the field to keep the object that it refers to alive for the whole lifetime of the object containing the field . This is not a big deal in most applications, but it is something that you should be aware of .
This is very tedious and error-prone work, and, of course, the C# compiler does all this for you automatically . When you write the code shown above, it s as if the C# compiler rewrites your code so that it looks something like this (comments inserted by me):
internal sealed class AClass { public static void UsingLocalVariablesInTheCallbackCode(Int32 numToDo) { // Some local variables WaitCallback callback1 = null; // Construct an instance of the helper class <>c__DisplayClass2 class1 = new <>c__DisplayClass2();
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