In C #? What is the meaning

Specific examples

if (Datetime?) dbsources) {       var backurl = request["backurl"string. Empty;}

The example code above means:

1. The first is if the question mark function of the If judgment bracket, is to let itself cannot take the value of NULL value type can take null values, such as datetime and int type.
2. The double question mark in the IF statement is called the null merge double operator, and its usage is that if the left-hand value of this operator is not NULL, the left value is returned, and the value from the right is returned.

In C #? What is the meaning