如何在C＃中检测空引用？

Question

How do I determine if an object reference is null in C# w/o throwing an exception if it is null?

ie If I have a class reference being passed in and I don't know if it is null or not.

Answer 1

testing against null will never* throw an exception

void DoSomething( MyClass value )
{
    if( value != null )
    {
        value.Method();
    }
}

---

* never as in should never . As @Ilya Ryzhenkov points out, an incorrect implementation of the != operator for MyClass could throw an exception. Fortunately Greg Beech has a good blog post on implementing object equality in .NET .

Answer 2

What Robert said, but for that particular case I like to express it with a guard clause like this, rather than nest the whole method body in an if block:

void DoSomething( MyClass value )
{
    if ( value == null ) return;
    // I might throw an ArgumentNullException here, instead

    value.Method();
}

Answer 3

Note, that having operator != defined on MyClass would probably lead do different result of a check and NullReferenceException later on. To be absolutely sure, use object.ReferenceEquals(value, null)

Answer 4

if(p != null)
{
   DoWork(p);
}

Also, the 'as' keyword is helpful if you want to detect if a class is of the right type and use it all at once.

IExample e = p as IExample;
if(e != null)
    DoWork(e);

In the above example if you were to cast e like (IExample)e it will throw an exception if e does not implement IExapmle. If you use 'as' and e doesn't implement IExample e will simply be null.

Answer 5

If you look in the majority of the .NET framework source code you will see they put checks like this at the top of their functions.

public void DoSomething(Object myParam)
{
  if (myParam == null) throw new ArgumentNullException("myParam");

  // Carry on
}

Answer 6

With C# 6.0 this is much more elegant; you can do it in one line :-)

value?.Method();

If "value" is null, nothing will happen - and no exception.

Answer 7

It's nit picky, but I always code these like ...

if (null == obj) {
   obj = new Obj();
}

instead of

if (obj == null) {
   obj = new Obj();
}

to avoid accidently writing

if (obj = null) {
   obj = new Obj();
}

because

if (null = obj) {
   obj = new Obj();
}

will give you a compiler error

Answer 8

(YourObject != Null)

you can compare to null?

If it's null instead of throwing an exception you can initialize your object. You can use the Null Pattern.

Answer 9

或者，如果您使用值类型，则可以阅读有关可空类型的内容： http ： //www.c-sharpcorner.com/UploadFile/mosessaur/nullabletypes08222006164135PM/nullabletypes.aspx

Answer 10

I have in the application's xaml.cs application derivative definition:

private SortedList myList;

And I want to be able to re-use my constructors. So I needed:

if ( myList == null)
   myList = new SortedList();

Thanks Robert!