DBNull Class
Represents a nonexistent value. This class cannot be inherited.
Assembly: mscorlib (in mscorlib.dll)
The DBNull type exposes the following members.
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 it is reclaimed by garbage collection. (Inherited from Object.) | |
GetHashCode | Serves as a hash function for a particular type. (Inherited from Object.) | |
GetObjectData | Implements the ISerializable interface and returns the data needed to serialize the DBNull 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. |
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. |
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.
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 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; }
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.