Null handling

When the underlying primitive is a reference type, such as string, null is never a valid value. How it is treated depends on the method you use to create or parse an instance.

Instantiation

Passing null to From(...) will lead to ArgumentNullException:

var username = Username.From(null!); // throws ArgumentNullException

At the same time, TryFrom(...) will not throw on null and instead will return false:

Username.TryFrom(null!, out var username); // returns false

Parsing

Parsing follows the same rules. Parse(...) parses the input into the primitive and then validates it, so a null input leads to ArgumentNullException:

var username = Username.Parse(null!); // throws ArgumentNullException

TryParse(...) will not throw on null and instead will return false:

Username.TryParse(null, out var username); // returns false

Null propagation

Sometimes you might want to treat null as "no value" instead of invalid input and propagate it through. In such case you need to use FromNullable(...) instead of From(...):

Username? username = Username.FromNullable(null); // returns null

TryFromNullable(...) does the same for the Try pattern:

Username.TryFromNullable(null, out var username); // returns true, username is null