[C#] Feature introduced in C# 2.0

• Feature introduced in C# 2.0

• Feature introduced
• Generics 泛型 (C# 程式設計手冊)
• 泛型型別參數 T
• Code

// Declare the generic class.
public class GenericList< t>
{
    void Add(T input) { }
}
class TestGenericList
{
    private class ExampleClass { }
    static void Main()
    {
        // Declare a list of type int.
        GenericList< int> list1 = new GenericList< int>();

        // Declare a list of type string.
        GenericList< string> list2 = new GenericList< string>();

        // Declare a list of type ExampleClass.
        GenericList< exampleclass> list3 = new GenericList< exampleclass>();
    }
}

• Anonymous
 Methods 匿名函式 (C# 程式設計手冊)
• 匿名函式是可用於任何需要委派類型之處的「內嵌」陳述式或運算式
• Code

class Test
{
    delegate void TestDelegate(string s);
    static void M(string s)
    {
        Console.WriteLine(s);
    }

    static void Main(string[] args)
    {
        // Original delegate syntax required 
        // initialization with a named method.
        TestDelegate testDelA = new TestDelegate(M);

        // C# 2.0: A delegate can be initialized with
        // inline code, called an "anonymous method." This
        // method takes a string as an input parameter.
        TestDelegate testDelB = delegate(string s) { Console.WriteLine(s); };

        // C# 3.0. A delegate can be initialized with
        // a lambda expression. The lambda also takes a string
        // as an input parameter (x). The type of x is inferred by the compiler.
        TestDelegate testDelC = (x) => { Console.WriteLine(x); };

        // Invoke the delegates.
        testDelA("Hello. My name is M and I write lines.");
        testDelB("That's nothing. I'm anonymous and ");
        testDelC("I'm a famous author.");

        // Keep console window open in debug mode.
        Console.WriteLine("Press any key to exit.");
        Console.ReadKey();
    }
}
/* Output:
    Hello. My name is M and I write lines.
    That's nothing. I'm anonymous and
    I'm a famous author.
    Press any key to exit.
 */

• Nullable Type 可為 Null 的類型 (C# 程式設計手冊)
• 可為 Null 的型別是 System.Nullable<T> 結構的執行個體
• 可為 Null 的型別可以代表其基礎值型別的正確值範圍，再加上額外的 null 值
• Code

/// 

/// 使用 HasValue 成員先測試變數是否包含值，再嘗試顯示它
/// 
int? x = 10;
if (x.HasValue)
{
    System.Console.WriteLine(x.Value);
}
else
{
    System.Console.WriteLine("Undefined");
}

/// 

/// 使用 null 成員先測試變數是否包含值，再嘗試顯示它
/// 
int? y = 10;
if (y != null)
{
    System.Console.WriteLine(y.Value);
}
else
{
    System.Console.WriteLine("Undefined");
}

/// 

/// 明確轉換
/// 
int? n = null;

//int m1 = n;      // Will not compile.
int m2 = (int)n;   // Compiles, but will create an exception if n is null.
int m3 = n.Value;  // Compiles, but will create an exception if n is null.

• Code 2

        int? n = null;
        int m = 0;
        // 1. 一般判別
        if ( n == null )
            m = 0;
        else 
            m = n;
        // 2. (三元)運算子
        // https://msdn.microsoft.com/zh-tw/library/zakwfxx4(v=vs.100).aspx
        m = (n == null ? 0 : n);

        // 3. null 聯合運算子
        // https://docs.microsoft.com/zh-tw/dotnet/csharp/language-reference/operators/null-conditional-operator
        m = n ?? n;

• Partial Class 部分類別和方法 (C# 程式設計手冊)
• 處理大型專案時，將類別分散到不同的檔案可讓多位程式設計人員同時處理它
• 處理自動產生的來源時，程式碼可以加入類別，不必重新建立來源檔案
• 若要分割類別定義，請使用 partial 關鍵字修飾詞
• Code

public partial class Employee
{
    public void DoWork()
    {
    }
}

public partial class Employee
{
    public void GoToLunch()
    {
    }
}

• Covariance Contra variance 泛型中的共變數和反變數
• 因在 C# 4.0 版本有加強此方式的使用方法
• 此先不做討論
• Ref:
• https://www.slideshare.net/ssusercab70d/c35-55576724