How to Use Null-Coalescing Operator (??)

Introduction

• The ?? operator is called the null-coalescing operator
• It's used to define a default value for nullable value types or reference types
• It returns the left-hand operand if the operand is not null
• Otherwise it returns the right operand

Compile-Time Errors and Exceptions

• A nullable type can contain a value, or it can be undefined
• 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 value type to a non-nullable value type without using the ?? operator, you will generate a compile-time error

It's like this

int? x = null;//x is nullable value type
int z = 0;//z is non-nullable value type

z = x; 

But if you use ?? operator. You won't have any problem.

z = x ?? 1;//with ?? operator No issues 

• If you use a cast, and the nullable value type is currently undefined, an InvalidOperationException exception will be thrown

It's like this

int? x = null;//x is nullable value type
int z = 0;//z is non-nullable value type

z = (int)x;//when you run the app,
           //It'll give ==>

But If you use ?? operator, you won't have any problem.

z = x ?? 1;//with ?? operator No issues

• The result of a ?? operator is not considered to be a constant even if both its arguments are constants

Example

class Program
    {
        static void Main(string[] args)
        {
            //EXAMPLE 01 :    

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

            int y = x ?? -1;

 
            //EXAMPLE 02 : ?? Operator With Value Type

            // 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);

 
            //EXAMPLE 03 : ?? Operator With Reference Type

            string s = GetStringValue();

            // Assign content of s to z, unless s is null, 
            // in which case assign "Unspecified"

            var z = s ?? "Unspecified";
        }

        static int? GetNullableInt()
        {
            return null;
        }

        static string GetStringValue()
        {
            return null;
        }
    }

Output

Example 01

Example 02

Example 03

Usage of ?? Operator

• The chaining is a big plus for the ?? operator. It removes a bunch of redundant IFs

  e.g.

  C#

  string finalVal = parm1 ?? localDefault ?? globalDefault; 

• You can easily convert nullable value types into non-nullable types

  e.g.

  C#

  int? test = null;
  var result = test ?? 0; //now the result is int, not nullable int?

• You can use null-coalescing operator in lazy instantiated private variables

  e.g.

  C#

  private IList<Car> _car;
  
  public ILis<Car> CarList
   {
     get { return _car ?? (_car = new List<Car>()); }
   }

References

?? Operator

Conclusion

• It can greatly enhance your code, making it concise and pleasant to read.
• Enjoy this ?? operator and let me know if you have any issues.

I hope this helps you. Comments and feedback are greatly appreciated.

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