Question

在运行程序期间没有数据/值被提取时，我遇到了问题，我收到以下错误：

无法将对象从DBNull强制转换为其他类型。

我尝试使用dr.hasrow()验证它，但我仍然遇到错误。

drqry = "SELECT max(num) FROM tbCVinfo WHERE cvno LIKE '%" & cvstr & "%'"

cmd2.CommandText = drqry
cmd2.Connection = con

Dim result As String = ""

cv = Convert.ToInt32(cmd2.ExecuteScalar())
'''''''''in this section I'm getting the error.

cv = cv + 1
If cv >= 0 And cv <= 9 Then
  ...............
Else
    cn = "0001"
End If
cn = result

cvno = cn

Answer 1

解决此问题的一种简单方法是检查返回的值是否为DBNull：

Dim cv As Integer
Dim queryResult = cmd2.ExecuteScalar()
If IsDBNull(queryResult) Then
    ' No matching records. Do something about it.
Else
    cv = DirectCast(queryResult, Integer)
End If

Answer 2

可以为c＃executescalar编写代码吗？

SqlCommand smd = new SqlCommand("Select Min(GidenEvrakSira) FROM GidenEvrak Where UserID is null", con);

minum = Convert.ToInt32(smd.ExecuteScalar());

executioncalar：无法将对象从DBNull转换为其他类型

2 个答案: