接线员'？'不能应用于'方法组'类型的操作数

Question

This is a question about C#'s newly introduced null-checking operator.

Assuming I have an interface like:

interface ILogger
{
    void Log(string message);
}

and a function that expects a logging action:

void DoWork(Action<string> logAction)
{
    // Do work and use @logAction
}

Why do I get the following compiler error if I try to write:

void Main(string[] args)
{
    ILogger logger = GetLogger(); // Assume GetLogger() may return null

    //
    // Compiler error: 
    // Operator '?' cannot be applied to operand of type 'method group'
    //
    DoWork(logger?.Log);   
}

Answer 1

There's nothing special going on with ?. here, it works just like it does with ?: : logger?.Log would give the same result as logger == null ? null : logger.Log logger == null ? null : logger.Log , except for logger only being evaluated once.

The problem is that logger == null ? null : logger.Log logger == null ? null : logger.Log is just as invalid in earlier versions of C#. ?: requires one operand to be convertible to the type of the other, but neither null nor logger.Log have a type. You'd have to write that as eg logger == null ? null : (Action<string>) logger.Log logger == null ? null : (Action<string>) logger.Log .

Unfortunately, the introduction of that cast means there's no simple pretty C# 6 shortened version you can use, as the same applies to ?. : logger?.Log is invalid because logger.Log does not have a type, so logger?.Log doesn't have a type either, but if it's an expression without a type, and it's not a method group, then there's nothing C# would let you do with it.

Answer 2

If you are able to change your interface to

interface ILogger
{
    //void Log(string message);
    Action<string> Log {get; }
}

Then you could use the logger?.Log notation.

The ILogger implementation would then look something like

class Logger : ILogger
{
    Action<string> Log => str => { /* Do stuff with str */ };
}

Answer 3

Adding to @hvd statement

We can also use Null Conditional Operator ( ?. ) with Null-Coalescing Operator ( ?? ) If you write following code

string userName = null;
int len = userName.Length;

then

userName.Length

will throw the exception: "An unhandled exception of type 'System.NullReferenceException' occurred in ConsoleApplication... Additional information: Object reference not set to an instance of an object."

if you use the following code

string userName = null;
int? len = userName.Length;

still you will get the same exception

But

int? len = userName?.Length;

It will return null and will not throw the exception

But we are still having an issue. The issue is I am using int? (Nullable int) which is not good because people will prefer that if string is null then it should return 0.

So we can do this by combination of null Conditional Operator(?.) and null-coalescing Operator(??)

int len = userName?.Length ?? 0;

Added

we have another usage of null conditional operator too.

like when you are working with event handling in that case we can use

myCustomEventHandler?.Invoke(this, EventArgs.Empty);

where "myCustomEventHandler" is public event before C#6.0 we have to create local copy of "myCustomEventHandler" otherwise it may throw exception in case of muti-threaded application. whereas in C# 6 we can directly call the "myCustomEventHandler" without creating local copy and it is thread safe.

eg in C# 5

var handler= this.myCustomEventHandler;
if (handler!= null)
    handler(…)

in C# 6

myCustomEventHandler?.Invoke(e)