SQL数据读取器 - 如何优雅地处理空列值

Question

我正在使用SQLDataReader从数据库中检索可能为null的值.我已经解决了如何处理Null字符串值,但无法使用相同的技巧来处理整数或布尔值:

Using cmd As DbCommand = store.GetStoredProcCommand("RetrievePOCO")
    store.AddInParameter(cmd, "ID", DbType.Int32, ID)
    Using reader As IDataReader = store.ExecuteReader(cmd)
        If reader.Read() = True Then
            Dim newPOCO As New POCO()
            With newPOCO
                'If the source column is null TryCast will return nothing without throwing an error
                .StatusXML = TryCast(reader.GetString(reader.GetOrdinal("StatusXML")), String)
                'How can a null integer or boolean be set elegantly?
                .AppType = TryCast(reader.GetInt32(reader.GetOrdinal("AppType")), System.Nullable(Of Integer))
                .Archived = TryCast(reader.GetBoolean(reader.GetOrdinal("Archived")), Boolean)

那么如何优雅地设置null整数或布尔值呢？我已经在C#中看到了建议,但它们没有正确转换为VB,给出'TryCast操作数必须是引用类型,但是整数？是值类型'编译器错误.

Answer 1

我在这种情况下使用以下函数:

Public Shared Function NoNull(ByVal checkValue As Object, ByVal returnIfNull As Object) As Object
    If checkValue Is DBNull.Value Then
        Return returnIfNull
    Else
        Return checkValue
    End If
End Function

您的代码看起来像这样:

With newPOCO
    .StatusXML = NoNull(reader("StatusXML"), "")
    .AppType = NoNull(reader("AppType"), -1)
    .Archived = NoNull(reader("Archived"), False)
End With

请注意,如果值为DbNUll作为第二个参数,则此函数要求您传递应使用的值.