1.设计一个类，我们只能生成该类的一个实例（Singleton模式）
1.2.解法1：利用静态构造函数1
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11public sealed class Singleton4 {
private Singleton4() {
}
private static Singleton4 instance = new Singleton4();
public static Singleton4 Instance{
get{
return instance;
}
}
}
1.3 解法2：实现按需创建实例1
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17public sealed class Singleton5 {
Singleton5() {
}
public static Singleton5 Instance{
get{
return Nested.instance;
}
}
class Nested {
static Nested() {
}
internal satic readonly Singleton5 instance = new Singleto5();
}
}