?? Operator

Starting with how to assign null value type to non-nullable type.
Nullable types are declared in one of two ways:
System.Nullable<T> variable
T? variable
Where T can be any value type including struct; it cannot be a reference type.

Examples of Nullable Types
int? i = 10;
double? d1 = 3.14;
bool? flag = null;
char? letter = 'a';
int?[] arr = new int?[10];

Now coming to the main point, ?? operator

One of the best features of C# is the ?? "null coalescing" operator.  This provides a nice way to check whether a value is null, and if so return an alternate value.
The ?? operator defines the default value to be returned when a nullable type is assigned to a non-nullable type. If you try to assign a nullable type to a non-nullable type without using the ?? operator, you will generate a compile-time error.
The ?? operator returns the left-hand operand if it is not null, or else it returns the right operand.

class NullCoalesce
    static int? GetNullableInt()
        return null;

    static string GetStringValue()
        return null;

    static void Main()
        // ?? operator example.
        int? x = null;

        // y = x, unless x is null, in which case y = -1.
        int y = x ?? -1;

        // Assign i to return value of method, unless
        // return value is null, in which case assign
        // default value of int to i.
        int i = GetNullableInt() ?? default(int);

        string s = GetStringValue();
        // ?? also works with reference types. 
        // Display contents of s, unless s is null, 
        // in which case display "Unspecified".
        Console.WriteLine(s ?? "Unspecified");
The ?? operator works for both reference types and value types.
We can achieve the same by using ternary operator (that most are familiar with), then why to use null coalescing operator ?


Well, first of all, it's much easier to chain than the standard ternary:
string anybody = parm1 ?? localDefault ?? globalDefault;
string anyboby = (parm1 != null) ? parm1 
               : ((localDefault != null) ? localDefault 
               : globalDefault);
It also works well if null-possible object isn't a variable:
string anybody = Parameters["Name"] 
              ?? Settings["Name"] 
              ?? GlobalSetting["Name"];
string anybody = (Parameters["Name"] != null ? Parameters["Name"] 
                 : (Settings["Name"] != null) ? Settings["Name"]
                 :  GlobalSetting["Name"];
The chaining is a big plus for the operator, removes a bunch of redundant IFs

Secondly, The ternary operator requires a double evaluation or a temporary variable.
string result = MyMethod() ?? "default value";
while with the ternary operator you are left with either:
string result = (MyMethod () != null ? MyMethod () : "default value"); 
which calls MyMethod twice, or:
string methodResult = MyMethod ();
string result = (methodResult != null ? methodResult : "default value"); 
Either way, the null coalescing operator is cleaner and, I guess, more efficient.

Third it is readable


Only problem is the null-coalesce operator doesn't detect empty strings.
string result1 = string.empty ?? "dead code!";
string result2 = null ?? "coalesced!";

result1 = ""
result2 = coalesced!

Well it is the null -coalescing operator, not the nullOrEmpty -coalescing operator.
You can achieve the same with Extension methods


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