一、多線程不安全方式實現
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
|
public sealed class SingleInstance
{
private static SingleInstance instance;
private SingleInstance() { }
public static SingleInstance Instance
{
get
{
if (null == instance)
{
instance = new SingleInstance();
}
return instance;
}
}
}
|
sealed表示SingleInstance不能被繼承。其實構造函數私有化已經達到了這個效果，私有的構造函數不能被繼承。爲了可讀性，可以加個sealed。
不安全的單例指的是在多線程環境下可能有多個線程同時進入if語句，創建了多次單例對象。
二、安全的單例模式
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
|
public sealed class SingleInstance
{
private static volatile SingleInstance instance;
private static readonly object obj = new object();
private SingleInstance() { }
public static SingleInstance Instance
{
get
{
if (null == instance)
{
lock (obj)
{
if (null == instance)
{
instance = new SingleInstance();
}
}
}
return instance;
}
}
}
|
加鎖保護，在多線程下可以確保實例值被創建一次。缺點是每次獲取單例，都要進行判斷，涉及到的鎖和解鎖比較耗資源。
三、只讀屬性式
|
1
2
3
4
5
6
7
8
9
10
11
12
|
public sealed class SingleInstance
{
private static readonly SingleInstance instance = new SingleInstance();
private SingleInstance() { }
public static SingleInstance Instance
{
get
{
return instance;
}
}
}
|
藉助readonly屬性，instance只被初始化一次，同樣達到了單例的效果。在Main函數執行第一句話之前，instance其實已經被賦值了，並不是預期的 只有到訪問Instance變量時才創建對象。
如下代碼：
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
|
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Begin");
var temp = SingleInstance.instance; ;
}
}
public sealed class SingleInstance
{
public static readonly SingleInstance instance = new SingleInstance();
private SingleInstance()
{
Console.WriteLine("初始化初始化！");
}
public static SingleInstance Instance
{
get { return instance; }
}
}
|
輸出：![]()
在執行第一句代碼之前，實例已經被初始化。
解決方法是在SingleInstance中加上靜態構造函數。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
|
public sealed class SingleInstance
{
public static readonly SingleInstance instance = new SingleInstance();
static SingleInstance() { }
private SingleInstance()
{
Console.WriteLine("初始化初始化！");
}
public static SingleInstance Instance
{
get { return instance; }
}
}
|
在運行輸出：
![]()
四、使用Lazy
|
1
2
3
4
5
6
7
8
9
10
11
12
|
public sealed class SingleInstance
{
private static readonly Lazy<SingleInstance> instance = new Lazy<SingleInstance>(() => new SingleInstance());
private SingleInstance(){}
public static SingleInstance Instance
{
get
{
return instance.Value;
}
}
}
|
Lazy默認是線程安全的。MSDN描述如下：
Will the lazily initialized object be accessed from more than one thread? If so, the Lazy<T> object might create it on any thread. You can use one of the simple constructors whose default behavior is to create a thread-safe Lazy<T> object, so that only one instance of the lazily instantiated object is created no matter how many threads try to access it. To create a Lazy<T> object that is not thread safe, you must use a constructor that enables you to specify no thread safety.
五、泛型單例
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
|
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Begin");
mySingle.Instance.age = 500;
Console.WriteLine(mySingle.Instance.age);
}
}
public abstract class SingleInstance<T>
{
private static readonly Lazy<T> _instance = new Lazy<T>(() =>
{
var ctors = typeof(T).GetConstructors(BindingFlags.Instance| BindingFlags.NonPublic| BindingFlags.Public);
if (ctors.Count() != 1)
throw new InvalidOperationException(String.Format("Type {0} must have exactly one constructor.", typeof(T)));
var ctor = ctors.SingleOrDefault(c => c.GetParameters().Count() == 0 && c.IsPrivate);
if (ctor == null)
throw new InvalidOperationException(String.Format("The constructor for {0} must be private and take no parameters.", typeof(T)));
return (T)ctor.Invoke(null);
});
public static T Instance
{
get{ return _instance.Value;}
}
}
public class mySingle : SingleInstance<mySingle>
{
private mySingle() { }
public int age;
}
|
