Basic usage C# with Example

Using the null-coalescing operator (??) allows you to specify a default value for a nullable type if the left-hand 
operand is null. 
string testString = null; 
Console.WriteLine("The specified string is - " + (testString ?? "not provided")); 
Live Demo on .NET Fiddle 
This is logically equivalent to: 
string testString = null; 
if (testString == null) 
{ 
Console.WriteLine("The specified string is - not provided"); 
} 
else 
{ 
Console.WriteLine("The specified string is - " + testString); 
} 
or using the ternary operator (?:) operator: 
string testString = null; 
Console.WriteLine("The specified string is - " + (testString == null ? "not provided" : 
testString));