DBNull.Value 字段

定义

命名空间:

System

程序集:

System.Runtime.dll

Source:

DBNull.cs

表示 DBNull 类的唯一实例。

public: static initonly DBNull ^ Value;

public static readonly DBNull Value;

 staticval mutable Value : DBNull

Public Shared ReadOnly Value As DBNull 

字段值

DBNull

示例

以下示例调用 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值。 如果存在，则字段值将追加到标签中的字符串输出中。

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;
}

member this.OutputLabels(dt: DataTable) =
    let mutable label = ""

    // Iterate rows of table
    for row in dt.Rows do
        let mutable label = String.Empty
        label <- label + this.AddFieldValue(label, row, "Title")
        label <- label + this.AddFieldValue(label, row, "FirstName")
        label <- label + this.AddFieldValue(label, row, "MiddleInitial")
        label <- label + this.AddFieldValue(label, row, "LastName")
        label <- label + this.AddFieldValue(label, row, "Suffix")
        label <- label + "\n"
        label <- label + this.AddFieldValue(label, row, "Address1")
        label <- label + this.AddFieldValue(label, row, "AptNo")
        label <- label + "\n"
        let labelLen = label.Length
        label <- label + this.AddFieldValue(label, row, "Address2")
        let labelLen =
            if label.Length <> labelLen then
                label + "\n"
            else label
        label <- label + this.AddFieldValue(label, row, "City")
        label <- label + this.AddFieldValue(label, row, "State")
        label <- label + this.AddFieldValue(label, row, "Zip")
        printfn $"{label}"
        printfn ""

member _.AddFieldValue(label: string, row: DataRow, fieldName: string) =
    if DBNull.Value.Equals row[fieldName] |> not then
        (string row[fieldName]) + " "
    else
        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 是单一实例类，这意味着只能存在此类的此实例。

如果数据库字段缺少数据，则可以使用 DBNull.Value 属性将对象值显式分配给 DBNull 该字段。 但是，大多数数据提供程序会自动执行此操作。

若要评估数据库字段以确定其值是否为 DBNull，可以将字段值传递给 DBNull.Value.Equals 方法。 但是，此方法很少使用，因为有多种其他方法可以评估数据库字段是否存在缺失数据。 其中包括 Visual Basic IsDBNull 函数、 Convert.IsDBNull 方法、 DataTableReader.IsDBNull 方法、 IDataRecord.IsDBNull 方法和几个其他方法。