Part III Essential Types in C#.NET

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Part III Essential Types
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// Construct a 'type' delegate wrapping the specified instance method. public static Delegate CreateDelegate(Type type, Object firstArgument, MethodInfo method); // firstArgument means 'this' public static Delegate CreateDelegate(Type type, Object firstArgument, MethodInfo method, Boolean throwOnBindFailure); // Invoke a delegate passing it parameters public Object DynamicInvoke(params Object[] args); }
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All of the CreateDelegate methods here construct a new object of a Delegate-derived type identified by the first parameter, type . The MethodInfo parameter indicates the method that should be called back; you d use reflection APIs (discussed in 23, Assembly Loading and Reflection ) to obtain this value . If you want the delegate to wrap an instance method, you will also pass to CreateDelegate a firstArgument parameter indicating the object that should be passed as the this parameter (first argument) to the instance method . Finally, CreateDelegate normally throws an ArgumentException if the delegate cannot bind to the method specified by the method parameter . This can happen if the signature of the method identified by method doesn t match the signature required by the delegate identified by the type parameter . However, if you pass false for the throwOnBindFailure parameter, an ArgumentException will not be thrown; null will be returned instead . Important The System.Delegate class has many more overloads of the CreateDelegate
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method that I do not show here . You should never call any of these other methods . As a matter of fact, Microsoft regrets even defining them in the first place . The reason is because these other methods identify the method to bind to by using a String instead of a MethodInfo . This means that an ambiguous bind is possible causing your application to behave unpredictably . System.Delegate s DynamicInvoke method allows you to invoke a delegate object s call-
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back method, passing a set of parameters that you determine at runtime . When you call DynamicInvoke, it internally ensures that the parameters you pass are compatible with the parameters the callback method expects . If they re compatible, the callback method is called . If they re not, an ArgumentException is thrown . DynamicInvoke returns the object the callback method returned . The following code shows how to use the CreateDelegate and DynamicInvoke methods:
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using System; using System.Reflection; using System.IO;
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// Here are some different delegate definitions internal delegate Object TwoInt32s(Int32 n1, Int32 n2); internal delegate Object OneString(String s1);
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17 Delegates
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public static class Program { public static void Main(String[] args) { if (args.Length < 2) { String fileName = Path.GetFileNameWithoutExtension( Assembly.GetEntryAssembly().Location); String usage = @"Usage:" + "{0}{1} delType methodName [Arg1] [Arg2]" + "{0} where delType must be TwoInt32s or OneString"+ "{0} if delType is TwoInt32s, methodName must be Add or Subtract" + "{0} if delType is OneString, methodName must be NumChars or Reverse" + "{0}" + "{0}Examples:" + "{0} {1} TwoInt32s Add 123 321" + "{0} {1} TwoInt32s Subtract 123 321" + "{0} {1} OneString NumChars \"Hello there\"" + "{0} {1} OneString Reverse \"Hello there\""; Console.WriteLine(usage, Environment.NewLine, fileName); return; } // Convert the delType argument to a delegate type Type delType = Type.GetType(args[0]); if (delType == null) { Console.WriteLine("Invalid delType argument: " + args[0]); return; } Delegate d; try { // Convert the Arg1 argument to a method MethodInfo mi = typeof(Program).GetMethod(args[1], BindingFlags.NonPublic | BindingFlags.Static); // Create a delegate object that wraps the static method d = Delegate.CreateDelegate(delType, mi); } catch (ArgumentException) { Console.WriteLine("Invalid methodName argument: " + args[1]); return; } // Create an array that will contain just the arguments // to pass to the method via the delegate object Object[] callbackArgs = new Object[args.Length - 2]; if (d.GetType() == typeof(TwoInt32s)) { try { // Convert the String arguments to Int32 arguments for (Int32 a = 2; a < args.Length; a++) callbackArgs[a - 2] = Int32.Parse(args[a]); } catch (FormatException) { Console.WriteLine("Parameters must be integers."); return; } }
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