如何找出 ExecuteScalar() 是 dbnull

Question

如何查看ExecuteScalar()的结果是否没有数据？检查是否为空无效。

我的代码：

Dim cmdCode As New OracleCommand
cmdCode.Connection = sgcnn
cmdCode.CommandText = "select max(SUBSTR(client_code,4,7)) from CLIENT_INFO where (client_code Like '%' || :SysUser || '%') "
cmdCode.Parameters.Add("@SysUser", OracleDbType.Varchar2).Value = strMessage

'Dim sqlResult As Object = cmdCode.ExecuteScalar()
'If sqlResult Is Nothing Then
'    ' No result found
'    strMessage.Append("0001")
'    txtClientCode.Text = strMessage.ToString
'Else
'    'result found
'    Dim newProdID As Int32 = 0
'    newProdID = Convert.ToInt32(cmdCode.ExecuteScalar())
'    newProdID = newProdID + 1
'    strMessage.Append(newProdID.ToString("0000"))
'    txtClientCode.Text = strMessage.ToString
'End If

Answer 1

If sqlResult Is DBNull.Value Then

一旦确定它不是 NULL，就不要再调用 ExecuteScalar。你已经得到了结果，所以使用它。此外，它已经是 Integer，因此无需转换。刚投：

Dim newProdID = CInt(sqlResult) + 1

Answer 2

        If IsDBNull(cmdCode.ExecuteScalar()) Then
        '    ' No result found
        '    strMessage.Append("0001")
        '    txtClientCode.Text = ""
    Else
        '    'result found
        '    Dim newProdID As Int32 = 0
        '    newProdID = Convert.ToInt32(cmdCode.ExecuteScalar())
        '    newProdID = newProdID + 1
        '    strMessage.Append(newProdID.ToString("0000"))
        '    txtClientCode.Text = strMessage.ToString
    End If

如何找出 ExecuteScalar() 是 dbnull

how to find out ExecuteScalar() is dbnull

vb.net

if-statement

dbnull

executescalar