Can you call a method on an object that is null?

Lost User

Apparently no, isnt't it? But run this code:

class Program {

static void Main(string\[\] args) {
    object o = null;

    Console.WriteLine(o.IsNull());
}

}

public static class ExtensionMethods {

public static bool IsNull(this object obj) {
    return (obj == null);
}

}

If you expected it to throw a NullReferenceException (as I did), then you're in for a surprise. It actually prints true. Makes me wonder how extension methods are implemented internally. Something like a wrapper object over the null object? Any .NET experts here that can explain?

OriginalGriff

Extension methods are implemented as you would expect: as a call to a static method, with a parameter. So effectively, what you have written is:

public static bool IsNull(object o)
{
return (o == null);
}
...
if (IsNull(null))
{
...

Which nobody would expect to throw a null reference. The extension method bit is just syntactic sugar. Looks odd though!

Ideological Purity is no substitute for being able to stick your thumb down a pipe to stop the water

Lost User

Yes, I understand. But what intrigues me is the

o.IsNull()

syntax, the mental picture I get is that since o is null, any attempt to call a method/field would throw a NullReferenceException, replace IsNull() with ToString() and you'll know what I'm talking about.

object.IsNull(o)

would have made much sense.

Adriaan Davel

I often use this as an 'exploit', can help make your code more readable in some cases. Sometimes I rather create an extension method than an instance method and do the null checking in the method instead of the calling procedure. (Not recommended as default pattern though) :)

____________________________________________________________ Be brave little warrior, be VERY brave

BobJanova

The whole point of extension methods is to avoid having to make explicit static calls. I don't really like them because they're hiding what's actually going on, but on the other hand those Linq method trains on IEnumerable<T> (all extension methods obviously) are really convenient. I don't get the point of IsNull, though. o.IsNull() is more characters and less clear than o == null! (And although you can override == on a custom type so that does something different, anyone who overrides it in a way that makes o == null not do what you expect should be taken out and shot repeatedly.)

J4amieC

Ive taken huge advantage of this recently when using PerformanceCounters. You see I create performance counters if possible, but it's entirely expected that some developer machines might not yet have my PerformanceCounters and I didnt want my code to not work on their machines. Hence this Extension class

public static class PerformanceCounterExtensions
{
public static void SafeIncrement(this PerformanceCounter counter)
{
if (counter != null)
counter.Increment();
}

    public static void SafeDecrement(this PerformanceCounter counter)
    {
        if (counter != null)
            counter.Decrement();
    }

    public static void SafeIncrementBy(this PerformanceCounter counter, long value)
    {
        if (counter != null)
            counter.IncrementBy(value);
    }

    public static void SafeReset(this PerformanceCounter counter)
    {
        if (counter != null)
            counter.RawValue = 0;
    }
}

Florin Jurcovici

You can, in C++. And you don't get an exception/runtime error until your program tries to touch data members - much deeper in the call stack than the actual call point. Which makes debugging a very funny thing ...

onelopez

The compiler does not read an extension method as a method belonging to the instance object, instead it expands your code to something like:

Console.WriteLine(ExtensionMethods.IsNull(o));

Therefore, you do not get an exception.

Brisingr Aerowing

Or be burned alive. In hot lava. Along with the code he wrote.

Bill Gates is a very rich man today... and do you want to know why? The answer is one word: versions. Dave Barry Read more at [BrainyQuote](http://www.brainyquote.com/quotes/topics topic_technology.html#yAfSEbrfumitrteO.99)[^]

ekolis

If only you could have extension... parentheses... or whatever you'd call it... seriously, Microsoft, WHY did you have to choose null to represent an event that has no handlers?! Now instead of

MySpecialPropertyChanged();

I have to do

if (MySpecialPropertyChanged != null)
MySpecialPropertyChanged();

or have the program crash if nothing's listening to the event!:mad:

H Brydon

I am not a C# person, but in C++, you can call a static method through a pointer that is NULL:

{
CBlah* pBlah = NULL;
pBlah->SomeStaticMethod(); // legal if SomeStaticMethod() is static
CBlah::SomeStaticMethod(); // [same thing]
}

Calling the method requires the right access level (ie. private/protected/public) and a value may or may not be returned by the API (ie. 'void', 'int' etc.). This is not valid in java, and I don't know if it is valid in C#.

-- Harvey