将空字符串转换为日期

Question

我到处搜索都无济于事，这是完成我的项目之前的最后一步，所以请帮助！提前致谢！

用户将在网格视图中选择一个条目，然后将其重定向到填充有所选行中的数据的表单（从而使网格视图可以以更用户友好的方式进行编辑）。数据库接受空值，我想在相应的文本框中将空日期值显示为空白（或“”）。相反，我收到错误：

从“DBNull”类型到“Date”类型的转换无效。

这是我的代码：

'preceded by connection code     
Dim sqlcmd As String = "SELECT * from Master WHERE RecNum = @recnum"
        'Dim sqlCmd As New OleDb.OleDbCommand("SELECT * from Master WHERE RecNum = @recnum", connection)
        Dim FileCommand3 As New OleDb.OleDbCommand(sqlcmd, connection)
        FileCommand3.Parameters.AddWithValue("@recnum", user)
        Dim Reader3 As OleDb.OleDbDataReader = FileCommand3.ExecuteReader()
        If Reader3.Read Then

            stock = myCStr(Reader3("StockNum"))
            make = myCStr(Reader3("Make"))
            color = myCStr(Reader3("Color"))
            stockin = myCStr(Reader3("Stockin"))
            ucistart = myCStr(Reader3("UCIStartDate"))
            repairs = Reader3("Repairs")
            tires = Reader3("tiresneeded")
            onlot = Reader3("onlot")
            sold = Reader3("sold")
            year = myCStr(Reader3("year"))
            model = myCStr(Reader3("model"))
            location = Reader3("location")
            srvcRO = myCStr(Reader3("svcROnum"))
            ucicompldate = myCStr(Reader3("uciestcompletedate"))
            collRO = myCStr(Reader3("collisionROnum"))
            other = myCStr(Reader3("other"))
            offprop = Reader3("offProperty")
            detail = (Reader3("detail")
        End If
        connection.Close()

        SoldCheckBX.Checked = sold
        DetailTXTbox.Text = detail
        'etc, etc
    End Sub

我使用函数 mycstr 来修复 dbnull to string 错误，但适应“date”数据类型似乎并不那么简单

Function myCStr(ByVal test As Object) As String
    If isdbnull(test) Then
        Return ("")
    Else
        Return CStr(test)
    End If
End Function

Answer 1

当您从读取器读取所有日期的值时，请尝试此操作，这将首先测试日期是否为 dbnull，如果是，则它将分配一个空值，您应该得到所需的空单元格，否则它将显示日期：

ucistart = IIf(reader3("UCIStartDate") Is DBNull.Value, Nothing, reader3("UCIStartDate"))

Answer 2

您是否尝试过使用Convert.IsDBNull函数？

这里是官方文档。