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PART I
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The output from the program is shown here:
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Replacing spaces with hyphens Resulting string: This-is-a-test Removing spaces Resulting string: Thisisatest Reversing string Resulting string: tset a si sihT
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Let s examine this program closely The program declares a delegate type called StrMod, shown here:
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delegate string StrMod(string str);
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Notice that StrMod takes one string parameter and returns a string Next, in DelegateTest, three static methods are declared, each with a single parameter of type string and a return type of string Thus, they match the StrMod delegate These methods perform some type of string modification Notice that ReplaceSpaces( ) uses one of string s methods, called Replace( ), to replace spaces with hyphens In Main( ), a StrMod reference called strOp is created and assigned a reference to ReplaceSpaces( ) Pay close attention to this line:
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StrMod strOp = new StrMod(ReplaceSpaces);
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Notice how the method ReplaceSpaces( ) is passed as a parameter Only its name is used; no parameters are specified This can be generalized When instantiating a delegate, you specify only the name of the method to which you want the delegate to refer Of course, the method s signature must match that of the delegate s declaration If it doesn t, a compiletime error will result Next, ReplaceSpaces( ) is called through the delegate instance strOp, as shown here:
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str = strOp("This is a test");
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Because strOp refers to ReplaceSpaces( ), ReplaceSpaces( ) is invoked
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Part I:
The C# Language
Next, strOp is assigned a reference to RemoveSpaces( ), and then strOp is called again This time, RemoveSpaces( ) is invoked Finally, strOp is assigned a reference to Reverse( ) and strOp is called This results in Reverse( ) being called The key point of the example is that the invocation of strOp results in a call to the method referred to by strOp at the time at which the invocation occurs Thus, the method to call is resolved at runtime, not compile time
Delegate Method Group Conversion
Since version 20, C# has included an option that significantly simplifies the syntax that assigns a method to a delegate This feature is called method group conversion, and it allows you to simply assign the name of a method to a delegate, without using new or explicitly invoking the delegate s constructor For example, here is the Main( ) method of the preceding program rewritten to use method group conversions:
static void Main() { // Construct a delegate using method group conversion StrMod strOp = ReplaceSpaces; // use method group conversion string str; // Call methods through the delegate str = strOp("This is a test"); ConsoleWriteLine("Resulting string: " + str); ConsoleWriteLine(); strOp = RemoveSpaces; // use method group conversion str = strOp("This is a test"); ConsoleWriteLine("Resulting string: " + str); ConsoleWriteLine(); strOp = Reverse; // use method group conversion str = strOp("This is a test"); ConsoleWriteLine("Resulting string: " + str); }
Pay special attention to the way that strOp is created and assigned the method ReplaceSpaces( ) in this line:
StrMod strOp = ReplaceSpaces; // use method group conversion
The name of the method is assigned directly to strOp C# automatically provides a conversion from the method to the delegate type This syntax can be generalized to any situation in which a method is assigned to (or converted to) a delegate type Because the method group conversion syntax is simpler than the old approach, it is used throughout the remainder of this book
Using Instance Methods as Delegates
Although the preceding example used static methods, a delegate can also refer to instance methods It must do so, however, through an object reference For example, here is a rewrite of the previous example, which encapsulates the string operations inside a class called
15:
Delegates, Events, and Lambda Expressions
StringOps Notice that the method group conversion syntax can also be applied in this situation
PART I
// Delegates can refer to instance methods, too using System; // Declare a delegate type delegate string StrMod(string str); class StringOps { // Replaces spaces with hyphens public string ReplaceSpaces(string s) { ConsoleWriteLine("Replacing spaces with hyphens"); return sReplace(' ', '-'); } // Remove spaces public string RemoveSpaces(string s) { string temp = ""; int i; ConsoleWriteLine("Removing spaces"); for(i=0; i < sLength; i++) if(s[i] != ' ') temp += s[i]; return temp; } // Reverse a string public string Reverse(string s) { string temp = ""; int i, j; ConsoleWriteLine("Reversing string"); for(j=0, i=sLength-1; i >= 0; i--, j++) temp += s[i]; return temp; } } class DelegateTest { static void Main() { StringOps so = new StringOps(); // create an instance of StringOps // Initialize a delegate StrMod strOp = soReplaceSpaces; string str; // Call methods through delegates str = strOp("This is a test"); ConsoleWriteLine("Resulting string: " + str); ConsoleWriteLine();
Part I:
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