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If you want to give obj a default value, would you use
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obj = null; // works only for reference types
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obj = 0; // works only for numeric types and enums, but not structs
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The solution to this problem is to use another form of default, shown here: default(type) This is the operator form of default, and it produces a default value of the specified type, no matter what type is used Thus, continuing with the example, to assign obj a default value of type T, you would use this statement:
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obj = default(T);
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This will work for all type arguments, whether they are value or reference types Here is a short program that demonstrates default:
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// Demonstrate the default operator using System; class MyClass { // }
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// Construct a default value of T class Test<T> { public T obj; public Test() { // The following statement will work only for reference types // obj = null; // can't use // The following statement will work only for numeric value types obj = 0; // can't use // This statement works for both reference and value types obj = default(T); Create a default value for any T } } class DefaultDemo { static void Main() { // Construct Test using a reference type Test<MyClass> x = new Test<MyClass>();
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Q: A:
Some of the declarations that involve generic types are very long Is there a way to make them shorter The new C# 30 implicitly typed variable feature can shorten a long declaration that includes an initializer As you know from 2, in a var declaration, the type of the variable is determined by the type of the initializer Therefore, a declaration such as
SomeClass<String, bool> someObj = new SomeClass<string, bool>("testing", false);
can be more compactly written as
var someObj = new SomeClass<string, bool>("testing", false);
Although the use of var does shorten the code in this case, its primary use is with anonymous types, which are described in 14 Furthermore, because implicitly typed variables are new to C#, it s not clear (at the time of this writing) that the preceding use of var will be considered a best practice by all C# practitioners Also, the coding standards used in your workplace may preclude such use
13: Generics
if(xobj == null) ConsoleWriteLine("xobj is null"); // Construct Test using a value type Test<int> y = new Test<int>(); if(yobj == 0) ConsoleWriteLine("yobj is 0"); } }
The output is shown here:
xobj is null yobj is 0
Generic Structures
You can create a structure that takes type parameters The syntax for a generic structure is the same as for generic classes For example, in the following program, the KeyValue structure, which stores key/value pairs, is generic:
// Demonstrate a generic struct using System; // This structure is generic struct KeyValue<TKey, TValue> { public TKey key; public TValue val; public KeyValue(TKey a, TValue b) { key = a; val = b; } } class GenStructDemo { static void Main() { KeyValue<string, int> kv = new KeyValue<string, int>("Tom", 20); KeyValue<string, bool> kv2 = new KeyValue<string, bool>("Fan On", false); ConsoleWriteLine(kvkey + " is " + kvval + " years old"); ConsoleWriteLine(kv2key + " is " + kv2val ); } }
A generic structure
C# 30: A Beginner s Guide
The output is shown here:
Tom is 20 years old Fan On is False
Like generic classes, generic structures can have constraints For example, this version of KeyValue restricts TValue to value types:
struct KeyValue<TKey, TValue> where TValue : struct { //
Generic Methods
As the preceding examples have shown, methods inside a generic class can make use of a class type parameter and are, therefore, automatically generic relative to the type parameter However, it is possible to declare a generic method that uses one or more type parameters of its own Furthermore, it is possible to create a generic method that is enclosed within a nongeneric class Let s begin with an example The following program declares a non-generic class called ArrayUtils and a static generic method within that class called CopyInsert( ) The CopyInsert( ) method copies the contents of one array to another, inserting a new element at a specified location in the process It can be used with any type of array
// Demonstrate a generic method using System; // A class of array utilities // a generic class class ArrayUtils { Notice that this is not
// Copy an array, inserting a new element in the process // This is a generic method A generic method public static bool CopyInsert<T>(T e, int idx, T[] src, T[] target) { // See if target array is big enough if(targetLength < srcLength+1) return false; // Copy src to target, inserting e at idx in the process for(int i=0, j=0; i < srcLength; i++, j++) { if(i == idx) { target[j] = e; j++; } target[j] = src[i]; }
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