从“DBNull”类型到“String”类型的转换是无效的 vb.net
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Conversion from type 'DBNull' to type 'String' is not valid vb.net
提问by Reshma
While using the given below code showing one error. The error is: "Conversion from type 'DBNull' to type 'String' is not valid." Help me to find a proper solution. Thank you.
使用下面给出的代码显示一个错误。错误是：“ Conversion from type 'DBNull' to type 'String' is not valid.”帮助我找到合适的解决方案。谢谢你。
Code:
代码：
cmd2.CommandText = "SELECT [first_name]+' ' +[middle_name]+' ' + [last_name] AS NAME, [staff_id] FROM [staff_profile]"
sdr2 = cmd2.ExecuteReader
While sdr2.Read
drop1l.Items.Add(New ListItem(sdr2("name"), sdr2("staff_id"))) // error popup here
End While
sdr2.Close()
回答by Rahul Tripathi
You should try like this:
你应该这样尝试：
If Not IsDBNull(dt.Rows(0)("name")) Then
sdr2.Value = dt.Rows(0)("name")
End If
If Not IsDBNull(dt.Rows(1)("staff_id")) Then
sdr2.Value = dt.Rows(1)("staff_id")
End If
or a dirty fix like this:
或者像这样的肮脏修复：
drop1l.Items.Add(New ListItem(sdr2("name").ToString(), sdr2("staff_id").ToString()))
回答by Reshma
You are getting this error because either sdr2("name")or sdr2("staff_id")is null. you can avoid it in two ways:
您收到此错误是因为sdr2("name")或sdr2("staff_id")是null。您可以通过两种方式避免它：
1.
1.
drop1l.Items.Add(New ListItem(sdr2("name").Tostring(), sdr2("staff_id").Tostring()))
2. or check for null in the query
2. 或者在查询中检查 null
回答by sabikp
try like this also
也这样试试
dt.Rows(0)("name").ToString()
回答by Icepickle
It means that one of the values you have received, is null, and it cannot be casted to string. You could implement a function that does the casting for you (and checks if a value is dbnull or nothing), something in the line of:
这意味着您收到的值之一为空，并且无法转换为字符串。您可以实现一个为您进行转换的函数（并检查值是否为 dbnull 或什么都没有），如下所示：
Function GetStringValue(value as Object) as String
if value is Nothing or IsDBNull(value)then
Return String.Empty
End If
Return DirectCast(value, GetType(String))
End Function
and then you could do
然后你可以做
drop1l.Items.Add(new ListItem(GetStringValue(sdr2("name")), GetStringValue(sdr2("staff_id")))
