DBNull Class
Represents a nonexistent value. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
The DBNull class represents a nonexistent value. In a database, for example, a column in a row of a table might not contain any data whatsoever. That is, the column is considered to not exist at all instead of merely not having a value. A DBNull object represents the nonexistent column. Additionally, COM interop uses the DBNull class to distinguish between a VT_NULL variant, which indicates a nonexistent value, and a VT_EMPTY variant, which indicates an unspecified value.
The DBNull type is a singleton class, which means only one DBNull object exists. The DBNull.Value member represents the sole DBNull object. DBNull.Value can be used to explicitly assign a nonexistent value to a database field, although most ADO.NET data providers automatically assign values of DBNull when a field does not have a valid value. You can determine whether a value retrieved from a database field is a DBNull value by passing the value of that field to the DBNull.Value.Equals method. However, some languages and database objects supply methods that make it easier to determine whether the value of a database field is DBNull.Value. These include the Visual Basic IsDBNull function, the Convert.IsDBNull method, the DataTableReader.IsDBNull method, and the IDataRecord.IsDBNull method.
Do not confuse the notion of Nothing in an object-oriented programming language with a DBNull object. In an object-oriented programming language, Nothing means the absence of a reference to an object. DBNull represents an uninitialized variant or nonexistent database column.
The following example calls the DBNull.Value.Equals method to determine whether a database field in a contacts database has a valid value. If it does, the field value is appended to the string output in a label.
Private Sub OUtputLabels(dt As DataTable) Dim label As String ' Iterate rows of table For Each row As DataRow In dt.Rows Dim labelLen As Integer label = String.Empty label += AddFieldValue(label, row, "Title") label += AddFieldValue(label, row, "FirstName") label += AddFieldValue(label, row, "MiddleInitial") label += AddFieldValue(label, row, "LastName") label += AddFieldValue(label, row, "Suffix") label += vbCrLf label += AddFieldValue(label, row, "Address1") label += AddFieldValue(label, row, "AptNo") label += vbCrLf labelLen = Len(label) label += AddFieldValue(label, row, "Address2") If Len(label) <> labelLen Then label += vbCrLf label += AddFieldValue(label, row, "City") label += AddFieldValue(label, row, "State") label += AddFieldValue(label, row, "Zip") Console.WriteLine(label) Console.WriteLine() Next End Sub Private Function AddFieldValue(label As String, row As DataRow, _ fieldName As String) As String If Not DbNull.Value.Equals(row.Item(fieldName)) Then Return CStr(row.Item(fieldName)) & " " Else Return Nothing End If End Function
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.