如何解决“无法将对象从 DBNull 转换为其他类型”。

Question

如何解决“无法将对象从 DBNull 转换为其他类型。”？ 我正在使用组合框来检索服务的价格和折扣价，如果我在combobox3中选择一个项目一次，将不会发生错误，但如果我在combobox3中再次选择一个项目，“对象不能从DBNull投射到其他类型。”会发生..

这是我的代码

Private Sub ComboBox3_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox3.SelectedIndexChanged

    Dim ds As New DataSet

    ds = getPrice(arr(ComboBox2.SelectedIndex), ComboBox3.Text)


    If ds.Tables("getPrice").Rows.Count > 0 Then
        For i As Integer = 0 To ds.Tables("getPrice").Rows.Count - 1

            TextBox1.Text = Convert.ToDecimal(ds.Tables("getPrice").Rows(i).Item(0))
            TextBox2.Text = Convert.ToDecimal(ds.Tables("getPrice").Rows(i).Item(1))

        Next
    End If

End Sub

这是我的查询

 Public Function getPrice(ByVal serviceId As String, ByVal breedSize As String)

    If breedSize = "Small Breed" Then
        sqlStr = "Select price_small_breed, discount_small_breed From tblServicePrice Where service_id = " + serviceId + ""
    ElseIf breedSize = "Medium Breed" Then
        sqlStr = "Select price_medium_breed, discount_medium_breed From tblServicePrice Where service_id = " + serviceId + ""
    ElseIf breedSize = "Big Breed" Then
        sqlStr = "Select price_big_breed, discount_big_breed From tblServicePrice Where service_id = " + serviceId + ""
    End If

    ds.Clear()
    da = New OleDbDataAdapter(sqlStr, con.ConnectionString)
    da.Fill(ds, "getPrice")


    Return ds

End Function

这是图片

出现此问题的原因可能是什么？

Answer 1

TextBox1.Text = If(ds.Tables("getPrice").Rows(i).Item(0) Is DBNull.Value, "", Convert.ToDecimal(ds.Tables("getPrice").Rows(i).Item(0)).ToString()) 
TextBox2.Text = If(ds.Tables("getPrice").Rows(i).Item(0) Is DBNull.Value, "", Convert.ToDecimal(ds.Tables("getPrice").Rows(i).Item(1)).ToString()) 

Answer 2

如果您针对数据库运行在 GetPrice 中执行的特定查询语句，您将看到至少有一行在您正在读取的序号位置具有空值。

很可能 serviceID（注意大写）不是您假设的值。

Answer 3

如果数据库中有 NULL 值会发生什么？如果可以接受使用某个值，例如零，你可以像这样修改查询

sqlStr = "Select COALESCE(price_small_breed, 0), COALESCE(discount_small_breed, 0) From tblServicePrice Where service_id = " & serviceId & ""

Answer 4

我的答案是这样。我声明了新数据集，我没有使用代码顶部的数据集

Public Function getPrice(ByVal serviceId As String, ByVal breedSize As String) As DataSet

        Dim dsa As New DataSet


        If breedSize = "Small Breed" Then
            sqlStr = "Select price_small_breed, discount_small_breed From tblServicePrice Where service_id = " + serviceId + ""
        ElseIf breedSize = "Medium Breed" Then
            sqlStr = "Select price_medium_breed, discount_medium_breed From tblServicePrice Where service_id = " + serviceId + ""
        ElseIf breedSize = "Big Breed" Then
            sqlStr = "Select price_big_breed, discount_big_breed From tblServicePrice Where service_id = " + serviceId + ""
        End If

        dsa.Clear()
        da = New OleDbDataAdapter(sqlStr, con.ConnectionString)
        da.Fill(dsa, "getPrice")

        Return dsa

    End Function

这是我之前的代码..

Public Function getPrice(ByVal serviceId As String, ByVal breedSize As String)

    If breedSize = "Small Breed" Then
        sqlStr = "Select price_small_breed, discount_small_breed From tblServicePrice Where service_id = " + serviceId + ""
    ElseIf breedSize = "Medium Breed" Then
        sqlStr = "Select price_medium_breed, discount_medium_breed From tblServicePrice Where service_id = " + serviceId + ""
    ElseIf breedSize = "Big Breed" Then
        sqlStr = "Select price_big_breed, discount_big_breed From tblServicePrice Where service_id = " + serviceId + ""
    End If

    ds.Clear()
    da = New OleDbDataAdapter(sqlStr, con.ConnectionString)
    da.Fill(ds, "getPrice")


    Return ds

End Function