generate 2d barcode vb.net Part III Essential Types in C#

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Part III Essential Types
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Finally, if you write the code shown above and compile it, it s as if the C# compiler rewrote your code to look like this (comments inserted by me):
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internal sealed class AClass { // This private field is created to cache the delegate object. // Pro: CallbackWithoutNewingADelegateObject will not create // a new object each time it is called. // Con: The cached object never gets garbage collected [CompilerGenerated] private static WaitCallback <>9__CachedAnonymousMethodDelegate1; public static void CallbackWithoutNewingADelegateObject() { if (<>9__CachedAnonymousMethodDelegate1 == null) { // First time called, create the delegate object and cache it. <>9__CachedAnonymousMethodDelegate1 = new WaitCallback(<CallbackWithoutNewingADelegateObject>b__0); } ThreadPool.QueueUserWorkItem(<>9__CachedAnonymousMethodDelegate1, 5); } [CompilerGenerated] private static void <CallbackWithoutNewingADelegateObject>b__0(Object obj) { Console.WriteLine(obj); } }
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The lambda expression must match that of the WaitCallback delegate: it returns void and takes an Object parameter . However, I specified the name of the parameter by simply putting obj to the left of the => operator . On the right of the => operator, Console.WriteLine happens to return void . However, if I had placed an expression that did not return void, the compiler-generated code would just ignore the return value because the method that the compiler generates must have a void return type to satisfy the WaitCallback delegate . It is also worth noting that the anonymous function is marked as private; this forbids any code not defined within the type from accessing the method (although reflection will reveal that the method does exist) . Also, note that the anonymous method is marked as static; this is because the code doesn t access any instance members (which it can t since CallbackWithoutNewingADelegateObject is itself a static method . However, the code can reference any static fields or static methods defined within the class . Here is an example:
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internal sealed class AClass { private static String sm_name; // A static field
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public static void CallbackWithoutNewingADelegateObject() { ThreadPool.QueueUserWorkItem( // The callback code can reference static members. obj =>Console.WriteLine(sm_name+ ": " + obj), 5); } }
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17 Delegates
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If the CallbackWithoutNewingADelegateObject method had not been static, the anonymous method s code could contain references to instance members . If it doesn t contain references to instance members, the compiler will still produce a static anonymous method since this is more efficient than an instance method because the additional this parameter is not necessary . But, if the anonymous method s code does reference an instance member, the compiler will produce a nonstatic anonymous method:
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internal sealed class AClass { private String m_name; // An instance field // An instance method public void CallbackWithoutNewingADelegateObject() { ThreadPool.QueueUserWorkItem( // The callback code can reference instance members. obj => Console.WriteLine(m_name+ ": " + obj), 5); } }
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On the left-hand side of the => operator is where you specify the names of any arguments that are to be passed to the lambda expression . There are some rules you must follow here . See the examples below:
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// If the delegate takes no arguments, use () Func<String> f = () => "Jeff"; // If the delegate takes 1+ arguments, you can explicitly specify the types Func<Int32, String> f2 = (Int32 n) => n.ToString(); Func<Int32, Int32, String> f3 = (Int32 n1, Int32 n2) => (n1 + n2).ToString(); // If the delegate takes 1+ arguments, the compiler can infer the types Func<Int32, String> f4 = (n) => n.ToString(); Func<Int32, Int32, String> f5 = (n1, n2) => (n1 + n2).ToString(); // If the delegate takes 1 argument, you can omit the ()s Func<Int32, String> f6 = n => n.ToString(); // If the delegate has ref/out arguments, you must explicitly specify ref/out and the type Bar b = (out Int32 n) => n = 5;
For the last example, assume that Bar is defined as follows:
delegate void Bar(out Int32 z);
On the right-hand side of the => operator is where you specify the anonymous function body . It is very common for the body to consist of a simple or complex expression that ultimately returns a non-void value . In the code just above, I was assigning lambda expressions that returned Strings to all the Func delegate variables . It is also quite common for the body to consist of a single statement . An example of this is when I called ThreadPool.QueueUserWorkItem, passing it a lambda expression that called Console.WriteLine (which returns void) .
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