Using a Constraint with a Generic Method in Visual C#

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Using a Constraint with a Generic Method
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You can add constraints to the type arguments of a generic method by specifying them after the parameter list For example, the following version of CopyInsert( ) will work only with reference types:
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public static bool CopyInsert<T>(T e, int idx, T[] src, T[] target) where T : class {
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If you were to try this version in the program shown earlier, the following call to CopyInsert( ) would not compile because int is a value type, not a reference type:
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// Now wrong because T must be a reference type! ArrayUtilsCopyInsert(99, 2, nums, nums2); // Now illegal!
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You mentioned that there are cases in which the compiler cannot infer the type to use for a type parameter when a generic method is called and the type will need to be explicitly specified Can you give an example Yes Among others, this situation can occur when a generic method has no parameters For example, consider this generic method:
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class SomeClass { public static T SomeMeth<T>() where T: new() { return new T(); } //
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C# 30: A Beginner s Guide
When this method is invoked, there are no arguments from which the type of T can be inferred The return type of T is not sufficient for the inference to take place Therefore, this won t work:
someObj = SomeClassSomeMeth(); // won't work
Instead, it must be invoked with an explicit type specified For example:
someObj = SomeClassSomeMeth<MyClass>(); // fixed
Generic Delegates
Like methods, delegates can also be generic To declare a generic delegate, use this general form: delegate ret-type delegate-name<type-parameter-list>(arg-list); Notice the placement of the type parameter list It immediately follows the delegate s name The advantage of generic delegates is that they let you define, in a type-safe manner, a generalized form that can then be matched to any compatible method The following program demonstrates a generic delegate called Invert that has one type parameter called T It returns type T and takes an argument of type T
// Demonstrate a generic delegate using System; // Declare a generic delegate delegate T Invert<T>(T v); class GenDelegateDemo { // Return the reciprocal of a double static double Recip(double v) { return 1 / v; } // Reverse a string and return the result static string ReverseStr(string str) { string result = ""; foreach(char ch in str) result = ch + result; return result; }
A generic delegate
13: Generics
static void Main() { // Construct two Invert delegates Invert<double> invDel = Recip; Invert<string> invDel2 = ReverseStr;
Create double and string instances of Invert
ConsoleWriteLine("The reciprocal of 4 is " + invDel(40)); ConsoleWriteLine(); string str = "ABCDEFG"; ConsoleWriteLine("Original string: " + str); str = invDel2(str); ConsoleWriteLine("Reversed string: " + str); } }
The output is shown here
The reciprocal of 4 is 025 Original string: ABCDEFG Reversed string: GFEDCBA
Let s look closely at this program First, notice how the Invert delegate is declared:
delegate T Invert<T>(T v);
Notice that T can be used as the return type even though the type parameter T is specified after the name Invert Inside GenDelegateDemo, the methods Recip( ) and ReverseStr( ) are declared, as shown here:
static double Recip(double v) { static string ReverseStr(string str) {
The Recip( ) method returns the reciprocal of the double value passed as an argument The ReverseStr( ) method, which is adapted from an earlier example, reverses a string and returns the result Inside Main( ), a delegate called invDel is instantiated and assigned a reference to Recip( )
Invert<double> invDel = Recip;
Because Recip( ) takes a double argument and returns a double value, Recip( ) is compatible with a double instance of Invert In a similar fashion, the delegate invDel2 is created and assigned a reference to ReverseStr( )
Invert<string> invDel2 = ReverseStr;
C# 30: A Beginner s Guide
Because ReverseStr( ) takes a string argument and returns a string result, it is compatible with the string version of Invert Because of the type-safety inherent in generics, you cannot assign incompatible methods to delegates For example, assuming the preceding program, the following statement would be in error:
Invert<int> invDel = ReverseStr; // Error!
Because ReverseStr( ) takes a string argument and returns a string result, it cannot be assigned to an int version of Invert
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