Throwing null literal

19 Sep 2019 1 mins C#

“Every” week I discover something new (and I’m not afraid to share it). Today isn’t any different. Apparently, C# compiler is fine with code throwing null literal. Yes, it looks weird, but if you think about it, it makes sense. Let me show you.

The code below compiles fine and results in NullReferenceException being thrown.

static void Test1()
{
	throw null;
}

I don’t know about you, but this, if not weird, looks funny. The NullReferenceException thrown isn’t anything special and you can catch it is as usual.

static void Test2()
{
	try
	{
		throw null;
	}
	catch (NullReferenceException ex)
	{
		Console.WriteLine(ex.ToString());
	}
}

So why is throw null even allowed? It looks like a special case, to be disallowed, but it’s not. Following simple code is absolutely acceptable.

static void Test3()
{
	Exception GetMeException() => DateTime.Now.DayOfWeek == DayOfWeek.Thursday ? null : new InvalidOperationException();

	throw GetMeException();
}

Basically “sometimes” my code returns null and that is then thrown. Discussion whether that is a bug or not is another discussion.

When I first saw (and it was in reference assemblies) the throw null I was perplexed. But trying it and then thinking about it in broader context, I realized it’s just a valid (but pretty stupid, don’t do it) code stripped to absolute minimum.

Update (based on comment below)

This, exactly, is part of the specification.

Jiří Činčura is .NET, C# and Firebird expert. He focuses on data and business layers, language constructs, parallelism, databases and performance. For almost two decades he contributes to open-source, i.e. FirebirdClient. He works as a senior software engineer for Microsoft. Frequent speaker and blogger at www.tabsoverspaces.com.