"?" - Question mark in c# code (Nullable value types)

In a piece of auto generated code, I found the following property


public DateTime? EffettiveTime
{
get { return m_effettiveTime; }
set { this.m_effettiveTime = value; }
}


This was the first time I seen the question mark in the c# code (at least in this context) The problem arose when I wanted to do the following:


someInstance.date = EffettiveTime.ToString("yyyy-mm-dd");


It returned an error saying that the ToString method does not have any overload methods, which takes a string. I looked up DateTime and found the overloaded method, so why couldn't I use it.

After some googling I found the wikipedia site for c-sharp (link) where I found the following:


Nullable value types (denoted by a question mark, e.g. int? i = null;) which add null to the set of allowed values for any value type. This provides improved interaction with SQL databases, which can have nullable columns of types corresponding to C# primitive types: an SQL INTEGER NULL column type directly translates to the C# int?.


But then there is just the problem that this new type (DateTime?) did not implement the method that I needed, so I had to cast it to DateTime before continuing, on. This seemed to work:

DateTime eTime = (DateTime) EffettiveTime;
someInstance.date = eTime.ToString("yyyy-mm-dd");


---

The "?" question mark is also used for short if statements. The following can be written much shorter:


SomeClasse someInstance = null;
if(<someBoolean>)
{
someInstance= <doSomething>;
}
else
{
someInstance = <doSomethingElse>;
}



This could be written like this instead

SomeClasse someInstance = <someBoolean> ? <doSomething> : <doSomethingElse>;