Artikel
11/18/2011

DBNull Class

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Represents a nonexistent value. This class cannot be inherited.

Inheritance Hierarchy

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

Syntax

'Declaration
<ComVisibleAttribute(True)> _
Public NotInheritable Class DBNull _
    Implements IConvertible

[ComVisibleAttribute(true)]
public sealed class DBNull : IConvertible

The DBNull type exposes the following members.

Methods

	Name	Description
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	GetTypeCode	Gets the TypeCode value for DBNull.
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	ToString()	Returns an empty string (String.Empty). (Overrides Object.ToString().)
	ToString(IFormatProvider)	Returns an empty string using the specified System.IFormatProvider.

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Fields

	Name	Description
	Value	Represents the sole instance of the DBNull class.

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Explicit Interface Implementations

	Name	Description
	IConvertible.ToBoolean	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToByte	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToChar	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToDateTime	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToDecimal	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToDouble	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToInt16	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToInt32	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToInt64	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToSByte	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToSingle	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToType	Infrastructure. Converts the current DBNull object to the specified type.
	IConvertible.ToUInt16	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	IConvertible.ToUInt32	Infrastructure. This conversion is not supported. Attempting to make this conversion throws an InvalidCastException.
	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 and the Convert.IsDBNull method.

Do not confuse the notion of nulla null reference (Nothing in Visual Basic) in an object-oriented programming language with a DBNull object. In an object-oriented programming language, nulla null reference (Nothing in Visual Basic) means the absence of a reference to an object. DBNull represents an uninitialized variant or nonexistent database column.

Version Information

Silverlight

Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

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.

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