SQL vb.net - 从数据库读取空值
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vb.net - read null value from database
提问by Milkncookiez
I have a database where in column there is no value (so it's null), but I can't handle this in vb.net. I tried with this code:
我有一个数据库，其中列中没有值（所以是null），但我无法在 vb.net 中处理此问题。我尝试使用此代码：
reader.Read()
If String.IsNullOrEmpty(reader.GetString(0)) Then
Return
Else
tilbulfolderTextBox.Text = reader.GetString(0)
End If
and also with:
还有：
If reader.Read() = False Then
and with:
与：
If IsDBNull(reader.Read()) Then
But apparently it doesn't work because I get exception on the statement after the Elsethat I can't get Nullvalues with this method.
I guess you will figure out what I require from the program by reading the code itself.
但显然它不起作用，因为我在Else无法Null使用此方法获取值之后的语句中出现异常。我想你会通过阅读代码本身来弄清楚我对程序的要求。
回答by Steve
The IsDBNullmethod of the DbDataReader base object is defined to handle this situation.
Of course you can't try to read something if the reader.Read() returns with false (meaning no more rows are available)
DbDataReader 基对象的IsDBNull方法被定义为处理这种情况。
当然，如果 reader.Read() 返回 false（意味着没有更多行可用），则您不能尝试读取某些内容
If reader.Read() Then
If reader.IsDBNull(0) Then
Return
Else
tilbulfolderTextBox.Text = reader.GetString(0)
End If
End If
Also, I don't see more of your code, but keep in mind that returning in this way could be very wrong if you don't close the connection and dispose the objects involved in this operation
另外，我没有看到您的更多代码，但请记住，如果您不关闭连接并处理此操作中涉及的对象，以这种方式返回可能是非常错误的
And, yes, as others have pointed out, there is also a function called IsDBNull from the Microsoft.VisualBasic assembly, but, I prefer to use the methods provided by the classes defined in the .NET framework and not the ones provided for compatibility with previous versions of VB
而且，是的，正如其他人指出的那样，Microsoft.VisualBasic 程序集中还有一个名为 IsDBNull 的函数，但是，我更喜欢使用 .NET 框架中定义的类提供的方法，而不是为与以前版本的 VB
回答by Egor4eg
You should use IsDbNullfunction to comapre it with null:
您应该使用IsDbNull函数将其与null：
reader.Read()
If IsDbNull(reader(0)) OrElse String.IsNullOrEmpty(reader.GetString(0)) Then
Return
Else
tilbulfolderTextBox.Text = reader.GetString(0)
End If
回答by Mandeep Singh
try this
尝试这个
reader.Read()
If IsDbNull(reader.GetString(0)) Then
Return
Else
tilbulfolderTextBox.Text = reader.GetString(0)
End If
回答by SysDragon
.Net has a different type for handling SQL NULLs: DbNull.
.Net 有一种不同的类型来处理 SQL NULL：DbNull.
Check this post about how to handle it: handling dbnull data in vb.net
查看这篇关于如何处理它的帖子：处理 vb.net 中的 dbnull 数据
In your case, you need the IsDbNullfunction, check this MSDN Documentationabout it.
在您的情况下，您需要该IsDbNull功能，请查看有关它的MSDN 文档。
If IsDbNull(data) Then
return
Else
tilbulfolderTextBox.Text = data
End If
回答by Rajaprabhu Aravindasamy
回答by Cubsoft
Sounds like your IF may not be working properly
听起来您的 IF 可能无法正常工作
Try using IsDBNull instead.
尝试改用 IsDBNull。
IsDBNull is a boolean variable and can be used like the following;
IsDBNull 是一个布尔变量，可以像下面这样使用；
reader.Read()
If Not IsDbNull(reader.GetString(0)) Then
tilbulfolderTextBox.Text = reader.GetString(0)
Else
return
End If
