Due to a bug that was fixed in C# 4, the following program prints true
. (Try it in LINQPad)
void Main() { new Derived(); }
class Base {
public Base(Func<string> valueMaker) { Console.WriteLine(valueMaker()); }
}
class Derived : Base {
string CheckNull() { return "Am I null? " + (this == null); }
public Derived() : base(() => CheckNull()) { }
}
In VS2008 in Release mode, it throws an InvalidProgramException. (In Debug mode, it works fine)
In VS2010 Beta 2, it doesn't compile (I didn't try Beta 1); I learned that the hard way
Is there any other way to make this == null
in pure C#?
This observation has been posted on StackOverflow in another question earlier today.
Marc's great answer to that question indicates that according to the spec (section 7.5.7), you shouldn't be able to access this
in that context and the ability to do so in C# 3.0 compiler is a bug. C# 4.0 compiler is behaving correctly according to the spec (even in Beta 1, this is a compile time error):
§ 7.5.7 This access
A this-access consists of the reserved word
this
.this-access:
this
A this-access is permitted only in the block of an instance constructor, an instance method, or an instance accessor.