20.C#编程指南-构造函数

使用构造函数

任何时候，只要创建类或结构，就会调用它的构造函数。如果您没有为对象提供构造函数，则默认情况下C#将创建一个构造函数，该构造函数实例化对象，并将成员变量设置为默认值表中列出的默认值。静态类和结构也可以有构造函数。

如果您为对象提供了构造函数，C#将不会创建默认构造函数（不带参数的构造函数）。

类和 structs 都可以定义具有参数的构造函数。 带参数的构造函数必须通过 new 语句或 base 语句来调用。 类和 structs 还可以定义多个构造函数，并且二者均不需要定义默认构造函数。 例如：

View Code

public class Bus
{
// Static variable used by all Bus instances.
// Represents the time the first bus of the day starts its route.
protected static readonly DateTime globalStartTime;

// Property for the number of each bus.
protected int RouteNumber { get; set; }

// Static constructor to initialize the static variable.
// It is invoked before the first instance constructor is run.
static Bus()
{
globalStartTime = DateTime.Now;

// The following statement produces the first line of output,
// and the line occurs only once.
Console.WriteLine("Static constructor sets global start time to {0}",
globalStartTime.ToLongTimeString());
}

// Instance constructor.
public Bus(int routeNum)
{
RouteNumber = routeNum;
Console.WriteLine("Bus #{0} is created.", RouteNumber);
}

// Instance method.
public void Drive()
{
TimeSpan elapsedTime = DateTime.Now - globalStartTime;

// For demonstration purposes we treat milliseconds as minutes to simulate
// actual bus times. Do not do this in your actual bus schedule program!
Console.WriteLine("{0} is starting its route {1:N2} minutes after global start time {2}.",
this.RouteNumber,
elapsedTime.TotalMilliseconds,
globalStartTime.ToShortTimeString());
}
}

class TestBus
{
static void Main()
{
// The creation of this instance activates the static constructor.
Bus bus1 = new Bus(71);

// Create a second bus.
Bus bus2 = new Bus(72);

// Send bus1 on its way.
bus1.Drive();

// Wait for bus2 to warm up.
System.Threading.Thread.Sleep(25);

// Send bus2 on its way.
bus2.Drive();

// Keep the console window open in debug mode.
System.Console.WriteLine("Press any key to exit.");
System.Console.ReadKey();
}
}
/* Sample output:
Static constructor sets global start time to 3:57:08 PM.
Bus #71 is created.
Bus #72 is created.
71 is starting its route 6.00 minutes after global start time 3:57 PM.
72 is starting its route 31.00 minutes after global start time 3:57 PM.
*/