DBNull.Value 欄位
定義
重要
部分資訊涉及發行前產品,在發行之前可能會有大幅修改。 Microsoft 對此處提供的資訊,不做任何明確或隱含的瑕疵擔保。
表示 DBNull 類別的唯一執行個體。
public: static initonly DBNull ^ Value;
public static readonly DBNull Value;
staticval mutable Value : DBNull
Public Shared ReadOnly Value As DBNull
欄位值
範例
下列範例會 DBNull.Value.Equals
呼叫 方法來判斷連絡人資料庫中的資料庫欄位是否具有有效的值。 如果這樣做,則會將域值附加至標籤中的字串輸出。
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
備註
DBNull 是單一類別,這表示只有這個類別的實例可以存在。
如果資料庫欄位有遺漏的資料,您可以使用 DBNull.Value 屬性明確地將物件值指派 DBNull 給欄位。 不過,大部分的資料提供者都會自動執行這項操作。
若要評估資料庫欄位,以判斷其值是否為 DBNull ,您可以將域值傳遞至 DBNull.Value.Equals
方法。 不過,此方法很少使用,因為有一些其他方法來評估資料庫欄位是否有遺漏資料。 其中包括 Visual Basic IsDBNull
函式、 Convert.IsDBNull 方法、 DataTableReader.IsDBNull 方法、 IDataRecord.IsDBNull 方法,以及數個其他方法。