How do I check whether a DATETIME field in SQL Server is NULL or not using VB.NET?

I have attempted this:

Public Function GetDate(ByVal FullDate As DateTime) As String

Dim MyNull As System.DBNull

If FullDate.ToString Is MyNull Then
GetDate = ""
Else
GetDate = FullDate.Date
End If

End Function

but it gives an error

TIA.