Question

Whilst playing around in an open source project, my attempt to ToString a DateTime object was thwarted by the compiler. When I jumped to the definition, I saw this:

public DateTime? timestamp;

Might someone please enlighten me on what this is called and why it might be useful?

Answer 1

This is a nullable type. Nullable types allow value types (e.g. ints and structures like DateTime) to contain null.

The ? is syntactic sugar for Nullable<DateTime> since it’s used so often.

To call ToString():

if (timstamp.HasValue) {        // i.e. is not null
    return timestamp.Value.ToString();
}
else {
    return "<unknown>";   // Or do whatever else that makes sense in your context
}