如何检查是否已创建类实例或为空C＃

Question

How do you check if classinstance has been created/initialised or is null ?

private MyClass myclass;

if(!check class has been initialised)      //<- What would that check be?
    myclass = new MyClass();

Thanks in advance

Answer 1

Just check if it is null

if (myclass == null)

As mentioned in your comment,

if (myclass.Equals(null)) 

will not work because if myclass is null, that's translated to

if (null.Equals(null))

which leads to a NullReferenceException .

By the way, objects declared in class scope are automatically initialized to null by the compiler, so your line:

private MyClass myclass;

is the same as

private MyClass myclass = null;

Objects declared in a method are forced to be assigned some initial value (even if that initial value is null).

Answer 2

if (myclass == null)
    myclass = new MyClass();

Answer 3

如果您希望将类限制为单个实例，请查看Singleton设计模式： http ： //msdn.microsoft.com/en-us/library/ff650316.aspx

Answer 4

you can use this constructions for avoid checking

    public class SomeClass
    {
        private MyClass _myClass;
        public MyClass MyClass
        {
            get { return _myClass ?? (_myClass = new MyClass()); }
            set { _myClass = value; }
        }
    }

    public class SomeClass
    {
        private readonly MyClass _myClass = new MyClass();
        public MyClass MyClass
        {
            get { return _myClass; }
        }
    }