What does double? mean in C#? [duplicate]

Question

Possible Duplicate:
C# newbie: what’s the difference between “bool” and “bool?” ?

Hi, While reading the code of the NUnit project's assert class, I came across this particular construct -

public static void AreEqual(double expected, double? actual, double delta)
{
     AssertDoublesAreEqual(expected, (double)actual, delta ,null, null);
}

In this function the second input parameter is entered as double?. The interesting thing is that this code compiles without issue in VS2010 (C# 4.0). Anyone know why this is NOT throwing an error ? Why is double? considered a valid keyword and is there any special significance to the ?.

Answer 1

double? is just shorthand for Nullable<double>; basically, a double that can be null. But the code is not very safe. If actual is null, (double)actual will throw an exception.

Answer 2

It's a nullable type. So it's a double that can also be null.

See here for more info.