DBNull Class

 
System_CAPS_noteNote

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Represents a nonexistent value. This class cannot be inherited.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)


<SerializableAttribute>
<ComVisibleAttribute(True)>
Public NotInheritable Class DBNull
	Implements ISerializable, IConvertible

NameDescription
System_CAPS_pubmethodEquals(Object)

Determines whether the specified object is equal to the current object.(Inherited from Object.)

System_CAPS_pubmethodGetHashCode()

Serves as the default hash function. (Inherited from Object.)

System_CAPS_pubmethodGetObjectData(SerializationInfo, StreamingContext)

Implements the ISerializable interface and returns the data needed to serialize the DBNull object.

System_CAPS_pubmethodGetType()

Gets the Type of the current instance.(Inherited from Object.)

System_CAPS_pubmethodGetTypeCode()

Gets the TypeCode value for DBNull.

System_CAPS_pubmethodToString()

Returns an empty string (String.Empty).(Overrides Object.ToString().)

System_CAPS_pubmethodToString(IFormatProvider)

Returns an empty string using the specified System.IFormatProvider.

NameDescription
System_CAPS_pubfieldSystem_CAPS_staticValue

Represents the sole instance of the DBNull class.

NameDescription
System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToBoolean(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToByte(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToChar(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToDateTime(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToDecimal(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToDouble(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToInt16(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToInt32(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToInt64(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToSByte(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToSingle(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToType(Type, IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. Converts the current DBNull object to the specified type.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToUInt16(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToUInt32(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

System_CAPS_pubinterfaceSystem_CAPS_privmethodIConvertible.ToUInt64(IFormatProvider)

This API supports the product infrastructure and is not intended to be used directly from your code. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.

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.Valuecan 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.

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

Universal Windows Platform
Available since 10
.NET Framework
Available since 1.1
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0

Any public static ( Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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