vb.net DBNull 到字符串错误
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vb.net DBNull to string error
提问by user1532468
I have a sub which just adds the values from a access db to textboxes. However, some of the fields in the db contain null values or are empty and this is causing vb to throw an error, 'Cannot convert from DBNull to string'.
我有一个 sub 只是将访问数据库中的值添加到文本框。但是，db 中的某些字段包含空值或为空，这会导致 vb 抛出错误“无法从 DBNull 转换为字符串”。
How can I fix this based on my posted code or if someone could post a tutorial for this type of event, as I am a new user struggling to make sense of this. Many thanks
我如何根据我发布的代码解决这个问题，或者如果有人可以发布此类事件的教程，因为我是一个新用户，正在努力理解这一点。非常感谢
Sub add()
While dr.Read()
txtname.Text = dr(0).ToString()
txtfathername.Text = dr(1).ToString()
txtaddress.Text = dr(2).ToString()
txtemail.Text = dr(3).ToString()
End While
End Sub
UPDATE:
更新：
Sub filllistview()
Try
'creatconn()
cn.Open()
Dim cmd As OleDbCommand = New OleDbCommand("Select * from Postings", cn)
dr = cmd.ExecuteReader()
While dr.Read()
ListView1.Items.Add(dr(0).ToString())
ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(dr(1))
ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(dr(2))
ListView1.Items(ListView1.Items.Count - 1).SubItems.Add(dr(3))
End While
Catch ex As Exception
System.Windows.Forms.MessageBox.Show(ex.Message)
Finally
dr.Close()
cn.Close()
End Try
End Sub
Sub showcontectsinlistview()
Try
'creatconn()
cn.Open()
Dim cmd As OleDbCommand = New OleDbCommand("select * from Postings where [Code]='" & ListView1.Text & "'", cn)
dr = cmd.ExecuteReader()
add()
Catch ex As Exception
System.Windows.Forms.MessageBox.Show(ex.Message)
Finally
dr.Close()
cn.Close()
End Try
End Sub
采纳答案by Malcolm Salvador
Detect if the data coming from the row is empty first, then replace it with other values with will make it into a string, then you can pass it onto your textbox
首先检测来自行的数据是否为空，然后将其替换为其他值将其变成字符串，然后您可以将其传递到您的文本框
Use the nothing function:
使用无功能：
dim x as string
if dr(0).value = nothing then
x = "Null data"
else
x = dr(0).value
end if
It seems the problems stems on the part when you read the data from the database. can you try changing your query to not output null values?
当您从数据库中读取数据时，问题似乎出在部分上。您可以尝试将查询更改为不输出空值吗？
something like
就像是
SELECT Field
FROM Table
WHERE Not(Field) is null;
回答by Merijn Den Houting
I think you should just check if the values are not NULL
我认为您应该检查值是否不为 NULL
Sub add()
While dr.Read()
If dr(0) Then txtname.Text = dr(0).ToString()
If dr(1) Then txtfathername.Text = dr(1).ToString()
If dr(2) Then txtaddress.Text = dr(2).ToString()
If dr(3) Then txtemail.Text = dr(3).ToString()
End While
End Sub
There probably is a better way though...
虽然可能有更好的方法......
回答by Ric
You will need to check if the value IsDBNullfirst like so
您需要先检查该值是否IsDBNull像这样
Dim value As String = If(dr.IsDBNull(dr(0)), "", dr(0).ToString())
then use the value
然后使用 value
you can put this logic into a method and do the return from there.
您可以将此逻辑放入一个方法中并从那里返回。
or in your method:
或在您的方法中：
Sub add()
While dr.Read()
If dr(0) Then txtname.Text = If(dr.IsDBNull(dr(0)), "", dr(0).ToString())
If dr(1) Then txtfathername.Text = If(dr.IsDBNull(dr(1)), "", dr(1).ToString())
If dr(2) Then txtaddress.Text = If(dr.IsDBNull(dr(2)), "", dr(2).ToString())
If dr(3) Then txtemail.Text = If(dr.IsDBNull(dr(3)), "", dr(3).ToString())
End While
End Sub
also check other SO answers: SQL Data Reader - handling Null column values
还要检查其他 SO 答案：SQL Data Reader - processing Null column values
.Add(dr(0).ToString())
becomes:
变成：
.Add(If(dr.IsDBNull(dr(0)), "", dr(0).ToString()))
