DBNull Class (System)

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.NET Framework Class Library
DBNull Class

Represents a nonexistent value. This class cannot be inherited.

Inheritance Hierarchy
System.Object
  System.DBNull

Namespace:  System
Assembly:  mscorlib (in mscorlib.dll)
Syntax
Visual Basic
<SerializableAttribute> _
<ComVisibleAttribute(True)> _
Public NotInheritable Class DBNull _
	Implements ISerializable, IConvertible
C#
[SerializableAttribute]
[ComVisibleAttribute(true)]
public sealed class DBNull : ISerializable, 
	IConvertible
Visual C++
[SerializableAttribute]
[ComVisibleAttribute(true)]
public ref class DBNull sealed : ISerializable, 
	IConvertible
F#
[<Sealed>]
[<SerializableAttribute>]
[<ComVisibleAttribute(true)>]
type DBNull =  
    class
        interface ISerializable
        interface IConvertible
    end

The DBNull type exposes the following members.

Methods
  Name Description
Public method Supported by the XNA Framework Equals(Object) Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
Protected method Supported by the XNA Framework Finalize Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public method Supported by the XNA Framework GetHashCode Serves as a hash function for a particular type. (Inherited from Object.)
Public method GetObjectData Implements the ISerializable interface and returns the data needed to serialize the DBNull object.
Public method Supported by the XNA Framework GetType Gets the Type of the current instance. (Inherited from Object.)
Public method Supported by the XNA Framework GetTypeCode Gets the TypeCode value for DBNull.
Protected method Supported by the XNA Framework MemberwiseClone Creates a shallow copy of the current Object. (Inherited from Object.)
Public method Supported by the XNA Framework ToString() Returns an empty string (String.Empty). (Overrides Object.ToString().)
Public method Supported by the XNA Framework ToString(IFormatProvider) Returns an empty string using the specified System.IFormatProvider.
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Fields
  Name Description
Public field Static member Supported by the XNA Framework Value Represents the sole instance of the DBNull class.
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Explicit Interface Implementations
  Name Description
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToBoolean Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToByte Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToChar Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToDateTime Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToDecimal Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToDouble Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToInt16 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToInt32 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToInt64 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToSByte Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToSingle Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToType Infrastructure. Converts the current DBNull object to the specified type.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToUInt16 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToUInt32 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
Explicit interface implemetation Private method Supported by the XNA Framework IConvertible.ToUInt64 Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
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Remarks

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 null in an object-oriented programming language with a DBNull object. In an object-oriented programming language, null means the absence of a reference to an object. DBNull represents an uninitialized variant or nonexistent database column.

Examples

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.

Visual Basic

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
C#

private void OutputLabels(DataTable dt)
{
   string label; 

   // Iterate rows of table
   foreach (DataRow row in dt.Rows)
   {
      int labelLen;
      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 += "\n";
      label += AddFieldValue(label, row, "Address1");
      label += AddFieldValue(label, row, "AptNo");
      label += "\n";
      labelLen = label.Length;
      label += AddFieldValue(label, row, "Address2");
      if (label.Length != labelLen)
         label += "\n";
      label += AddFieldValue(label, row, "City");
      label += AddFieldValue(label, row, "State");
      label += AddFieldValue(label, row, "Zip");
      Console.WriteLine(label);
      Console.WriteLine();
   }
}

private string AddFieldValue(string label, DataRow row, 
                             string fieldName) 
{                                
   if (! DBNull.Value.Equals(row[fieldName])) 
      return (string) row[fieldName] + " ";
   else
      return String.Empty;
}
Version Information

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
Thread Safety
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.
See Also

Reference

System Namespace
Convert.DBNull
Convert.IsDBNull
System.Runtime.InteropServices.VarEnum