DBNull DBNull DBNull DBNull Class

定义

public ref class DBNull sealed : IConvertible, System::Runtime::Serialization::ISerializable

[System.Runtime.InteropServices.ComVisible(true)]
[System.Serializable]
public sealed class DBNull : IConvertible, System.Runtime.Serialization.ISerializable

type DBNull = class
    interface ISerializable
    interface IConvertible

Public NotInheritable Class DBNull
Implements IConvertible, ISerializable

继承

ObjectObjectObjectObject

DBNullDBNullDBNullDBNull

属性

ComVisibleAttribute SerializableAttribute

实现

IConvertibleIConvertibleIConvertibleIConvertible ISerializableISerializableISerializableISerializable

示例

下面的示例调用DBNull.Value.Equals方法，以确定联系人数据库中的数据库字段是否具有有效的值。The following example calls the DBNull.Value.Equals method to determine whether a database field in a contacts database has a valid value. 如果是这样，字段值追加到字符串输出在标签中。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;
}

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

注解

DBNull类表示不存在的值。The DBNull class represents a nonexistent value. 例如，在数据库中，表的行中的列不可能包含任何数据。In a database, for example, a column in a row of a table might not contain any data whatsoever. 也就是说，列被视为根本不存在，而不是只是不具有值。That is, the column is considered to not exist at all instead of merely not having a value. 一个DBNull对象都表示不存在的列。A DBNull object represents the nonexistent column. 此外，COM 互操作使用DBNull类，以区分 VT_NULL 变体，用于指示不存在的值和 VT_EMPTY 变体，用于指示未指定的值。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.

DBNull类型是一个单一实例类，这意味着只有一个DBNull对象存在。The DBNull type is a singleton class, which means only one DBNull object exists. DBNull.Value成员表示单独DBNull对象。The DBNull.Value member represents the sole DBNull object. DBNull.Value 可用于显式将不存在的值分配到数据库字段中，尽管大多数的 ADO.NET 数据提供程序自动分配的值DBNull字段没有有效的值。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. 您可以确定从数据库字段中检索某个值是否DBNull通过将为该字段的值传递值DBNull.Value.Equals方法。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. 但是，某些语言和数据库对象提供一些方法，使其更轻松地确定数据库字段的值是否为DBNull.Value。However, some languages and database objects supply methods that make it easier to determine whether the value of a database field is DBNull.Value. 其中包括 Visual BasicIsDBNull函数，Convert.IsDBNull方法，DataTableReader.IsDBNull方法，和IDataRecord.IsDBNull方法。These include the Visual Basic IsDBNull function, the Convert.IsDBNull method, the DataTableReader.IsDBNull method, and the IDataRecord.IsDBNull method.

不要混淆这一概念null在面向对象的编程语言与DBNull对象。Do not confuse the notion of null in an object-oriented programming language with a DBNull object. 在面向对象的编程语言中，null表示不存在的对象的引用。In an object-oriented programming language, null means the absence of a reference to an object. DBNull 表示一个未初始化的变量或不存在的数据库列。DBNull represents an uninitialized variant or nonexistent database column.

字段

方法

显式界面实现

适用于

另请参阅

• DBNull
• IsDBNull(Object)
• VarEnum