Equivalent to Java's Optional.orElse in C#

You can use the ?? operator.

Your code will be updated to:

string x = null;
string y = x ?? "NeedToCheckforNull"; 

See: ?? Operator (C# Reference)

C# has the special Nullable<T> type which can be declared with int?, decimal?, etc. These can provide a default value by using .GetValueOrDefault(), T GetValueOrDefault(T defaultValue), and the ?? opterator.

string x = null;
Console.WriteLine(x ?? "NeedToCheckforNull");