Question

我有一个包含复杂查询的页面。查询正常工作但只要有空字符串或空字符串就会抛出此错误。

从“DBNull”类型转换为“Double”类型无效。

这是我的代码

If Not Me.IsPostBack Then
            Dim dt As DataTable = Me.GetData("SELECT (`behaviour`+`treatment`+`charges`+`information`+`hygine`+`admission`)/6 AverageRating, COUNT(ID) RatingCount from ratings WHERE hospitalID Like '%" + var2 + "%';")
            avg.Text = FormatNumber(CDbl(dt.Rows(0)("AverageRating")), 1)
            totalVotes.Text = String.Format(dt.Rows(0)("RatingCount"))
        End If

avg.text从页面加载时的db获取值

Answer 1

在SQL中使用isnull函数。

示例：选择名称，isnull（评级，0）

如果rating = null，则sql将返回0

Answer 2

Double.TryParse(text, value)最擅长测试

    If Not Me.IsPostBack Then
    Dim testDouble as Double
    Dim dt As DataTable = Me.GetData("SELECT (`behaviour`+`treatment`+`charges`+`information`+`hygine`+`admission`)/6 AverageRating, COUNT(ID) RatingCount from ratings WHERE hospitalID Like '%" + var2 + "%';")

    If Double.TryParse((dt.Rows(0)("AverageRating")), testDouble) Then
        ' text is convertible to Double, and testDouble contains the Double value now'
        avg.Text = FormatNumber(testDouble, 1)
    Else
        ' Cannot convert text to Double'
    End If
        totalVotes.Text = String.Format(dt.Rows(0)("RatingCount"))
    End If

Answer 3

使用扩展方法DataRowExtensions.Field Method (DataRow, String)

来自文档：

如果指定的DataColumn的值为null且T是引用 type或nullable类型，返回类型将为null。

If Not Me.IsPostBack Then
    Dim dt As DataTable = Me.GetData("SELECT ....")
    Dim avgValue As Double? = dt.Rows(0).Field(Of Double?)("AverageRating")
    If avgValue.HasValue = True Then
        avg.Text = avgValue.Value.ToString()
    Else
        avg.Text = "no value"
    End If

End If

Answer 4

将您的查询更改为

SELECT (isnull(`behaviour`,0)+isnull(`treatment`,0)+isnull(`charges`,0)+isnull(`information`,0)+isnull(`hygine`,0)+isnull(`admission`,0))/6 AverageRating, COUNT(ID) RatingCount from ratings

返回空数据时，从“DBNull”类型到“Double”类型的转换无效

4 个答案: