为什么0，而不是VB System.Data.DataTable中的Nothing

Question

代码：

        Private m_log_dataTable As System.Data.DataTable = Nothing
        Private m_freq As String = Nothing
        Private m_r As Single = Nothing
        Private m_l As Single = Nothing
        Private m_c As Single = Nothing
        Private m_rp As Single = Nothing
        Private m_rs As Single = Nothing
        Private m_z As Single = Nothing
        Private m_esr As Single = Nothing
        Private m_dcr As Single = Nothing
        Private m_q As Single = Nothing
        Private m_d As Single = Nothing

...

    Private Sub LOG()
        Try
            m_freq = Nothing
            m_r = Nothing
            m_l = Nothing
            m_c = Nothing
            m_rp = Nothing
            m_rs = Nothing
            m_z = Nothing
            m_esr = Nothing
            m_dcr = Nothing
            m_q = Nothing
            m_d = Nothing
            m_value = Nothing
            m_unit = Nothing
            m_log_dataTable.Rows.Add(DateTime.Now, getDUT(), getMode(), m_freq, m_r, m_l, m_c, m_r, m_rs, m_z, m_esr, m_dcr, m_q, m_d)'Line1
            m_log_dataTable.Rows.Add(DateTime.Now, getDUT(), getMode(), Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing, Nothing)'Line2
        Catch ex As Exception
            MsgBox("Exception when logging:" + ex.Message)
        End Try
    End Sub

输出：

为什么line1（在上面的代码中）将0写入数据表而不是Nothing？ 我该怎么办？

由于

Answer 1

VB.NET中的

Nothing等于C＃中的default(T)（而不是null）。值类型不能是null因此在db中它们以not null约束的列表示。

您确实应该为您的字段使用Nullable(Of Single)（与Single?相同）。

但当然这还不够。您应该修改数据库中的列，以便它们具有null约束而不是not null，System.Data.DataTable应该以相同的方式配置。

Answer 2

您需要使用Nullable type。将您的所有single类型更改为single?，然后重试。

Private m_r As Single? = Nothing
Private m_l As Single? = Nothing

// etc

正如Gabor所评论的那样，您需要访问Value属性或Nullable(Of T)可用的其他方法之一。

m_r.Value                 ' Access the underlying value

m_r.GetValueOrDefault()   ' Underlying value or, if none, default for the underlying type

m_r.GetValueOrDefault(3)  ' Underlying value or, if none, some default value you decide

Answer 3

Coz Single的默认值为0.您可以使用字符串列，也可以定义目标datagridview的列。

DataGridViewCellStyle1.Format = "N"
Column1.DefaultCellStyle = DataGridViewCellStyle1