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c#non-nullable

Why is null not allowed for DateTime in C#?

Why it is not allowed to assign null to a DateTime in C#? How has this been implemented? And can this feature be used to make your own classes non-nullable?

Example:

string stringTest = null; // Okay
DateTime dateTimeTest = null; // Compile error

I know that I can use DateTime? in C# 2.0 to allow null to be assigned to dateTimeTest and that I could use Jon Skeet's NonNullable class on my string to get a run time error on the assignment of stringTest. I'm just wondering why the two types behave differently.

Solution

  • DateTime is a value-type (struct), where-as string is a reference-type (class etc). That is the key difference. A reference can always be null; a value can't (unless it uses Nullable<T> - i.e. DateTime?), although it can be zero'd (DateTime.MinValue), which is often interpreted as the same thing as null (esp. in 1.1).