In any programming language, be careful when using Nulls. The example above shows another issue. If you use a type of Nullable, that means that the variables instantiated from that type can hold the value System. DBNull. Value; not that it has changed the interpretation of setting the value to default using = Nothing or that the Object of the value can now support a null reference. Just a warning. happy coding! You could create a separate class containing a value type. An object created from such a class would be a reference type, which could be assigned Nothing. An example: Public Class Date Time Nullable Private _value As Date Time 'properties Public Property Value As Date Time Get Return _value End Get Set( By Val value As Date Time) _value = value End Set End Property 'constructors Public Sub New Value = Date Time. Min Value End Sub Public Sub New( By Val dt As Date Time) Value = dt End Sub 'overridables Public Overrides Function To String As String Return Value. To String End Function End Class 'in Main Dim dtn As Date Time Nullable = Nothing Dim str Test1 As String = Falied Dim str Test2 As String = Failed If dtn Is Nothing Then str Test1 = Succeeded dtn = New Date Time Nullable( Date Time. Now) If dtn Is Nothing Then str Test2 = Succeeded Console. Write Line( test1: Or the longer form: Nullable< Date Time> My Nullable Date; And, finally, there's a built in way to reference the default of any type. This returns null for reference types, but for our Date Time example it will return the same as Date Time. Min Value: default( Date Time).
Stack Overflow is a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Join the Stack Overflow community to: Ask programming questions Answer and help your peers Get recognized for your expertise up vote 45 down vote favorite Is there an easy way within C to check to see if a Date Time instance has been assigned a value or not? up vote 33 down vote accepted The only way of having a variable which hasn't been assigned a value in C is for it to be a local variable - in which case at compile-time you can tell that it isn't definitely assigned by trying to read from it I suspect you really want Nullable< Date Time> (or Date Time? with the C syntactic sugar) - make it null to start with and then assign a normal Date Time value (which will be converted appropriately). Then you can just compare with null (or use the Has Value property) to see whether a real value has been set. do you mean like so: Date Time datetime = new Date Time if (datetime = Date Time. Min Value) /unassigned or you could use Nullable Date Time? datetime = null; if (!datetime. Has Value) /unassigned Date Time is value type, so it can not never be null. If you think Date Time? ( Nullable ) you can use: Date Time? something = Get Date Time bool is Null = (something = null bool is Null2 =!something. Has Value; I just found out that Get Hash Code for an unassigned datetime is always zero. I am not sure if this is a good way to check for null datetime, because, I can't find any documentation on why this behavior is displayed. if(dt. Get Hash Code =0) Console. Write Line( Date Time is unassigned put this somewhere: public static class Date Time Util /or whatever name public static bool Is Empty(this Date Time date Time) return date Time = default( Date Time then: Date Time datetime; if (datetime. Is Empty /unassigned I'd say the default value is always new.
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