meaning of ??= in csharp

Code Example - meaning of ??= in csharp

                
                        ??=
//assigns the right hand value to the left if left one is null
                    
                
 

what is the meaning of ?? in csharp

                        
                                //the null-coalescing operator ?? returns the value of its left-hand operand if 
//it isn't null; otherwise, it evaluates the right-hand operand and returns 
//its result.