0

why c# code

            DateTime? temp = null;
            Console.WriteLine(string.IsNullOrWhiteSpace(temp.ToString()));

just print out "True" and do not throw null object Exception?

have run this code in Debug and Release build both do not throw exception using .NET Framework 4.7.2

Howard Yang
  • 36
  • 3

2 Answers2

3

Nullable is a struct. You can call methods on empty structs just fine, and ToString for an empty nullable returns string.Empty. No magic involved :)

The only real magic with nullables has to do with boxing. When you box, say, an integer, you get a boxed integer ((object)10 returns an object which can be cast to int - and actually only int). But when you box a nullable integer without a value, you actually get null, not a boxed nullable integer (while a nullable integer with value will turn into a perfectly normal int when boxed, not int?). So:

DateTime? temp = null;
var boxedTemp = (object)temp;
Console.WriteLine(string.IsNullOrWhiteSpace(boxedTemp.ToString()));

will actually throw, because boxedTemp is a null reference (while temp is not a reference in the first place, so it can't be a null reference specifically).

Luaan
  • 62,244
  • 7
  • 97
  • 116
1

Because the Nullable<DateTime> returns an empty string if its HasValue property is "false".

Here's the documentation: https://learn.microsoft.com/en-us/dotnet/api/system.nullable-1.tostring?view=netframework-4.7.2

Shazi
  • 1,490
  • 1
  • 10
  • 22