handling dbnull data in vb.net

问题

I want to generate some formatted output of data retrieved from an MS-Access database and stored in a DataTable object/variable, myDataTable. However, some of the fields in myDataTable cotain dbNull data. So, the following VB.net code snippet will give errors if the value of any of the fields lastname, intials, or sID is dbNull.

   dim myDataTable as DataTable
   dim tmpStr as String
   dim sID as Integer = 1

   ...
   myDataTable = myTableAdapter.GetData() ' Reads the data from MS-Access table
   ...

   For Each myItem As DataRow In myDataTable.Rows

    tmpStr = nameItem("lastname") + " " + nameItem("initials")

    If myItem("sID")=sID Then
        ' Do something
    End If

    ' print tmpStr

   Next

So, how do i get the above code to work when the fields may contain dbNull without having to check each time if the data is dbNull as in this question?

回答1:

The only way that i know of is to test for it, you can do a combined if though to make it easy.

If NOT IsDbNull(myItem("sID")) AndAlso myItem("sID") = sId Then
   'Do success
ELSE
   'Failure
End If

I wrote in VB as that is what it looks like you need, even though you mixed languages.

Edit

Cleaned up to use IsDbNull to make it more readable

回答2:

I got tired of dealing with this problem so I wrote a NotNull() function to help me out.

Public Shared Function NotNull(Of T)(ByVal Value As T, ByVal DefaultValue As T) As T
        If Value Is Nothing OrElse IsDBNull(Value) Then
                Return DefaultValue
        Else
                Return Value
        End If
End Function

Usage:

If NotNull(myItem("sID"), "") = sID Then
  ' Do something
End If

My NotNull() function has gone through a couple of overhauls over the years. Prior to Generics, I simply specified everything as an Object. But I much prefer the Generic version.

回答3:

You can also use the Convert.ToString() and Convert.ToInteger() methods to convert items with DB null effectivly.

回答4:

A variation on Steve Wortham's code, to be used nominally with nullable types:

Private Shared Function GetNullable(Of T)(dataobj As Object) As T
    If Convert.IsDBNull(dataobj) Then
        Return Nothing
    Else
        Return CType(dataobj, T)
    End If
End Function

e.g.

mynullable = GetNullable(Of Integer?)(myobj)

You can then query mynullable (e.g., mynullable.HasValue)

回答5:

Microsoft came up with DBNull in .NET 1.0 to represent database NULL. However, it's a pain in the behind to use because you can't create a strongly-typed variable to store a genuine value or null. Microsoft sort of solved that problem in .NET 2.0 with nullable types. However, you are still stuck with large chunks of API that use DBNull, and they can't be changed.

Just a suggestion, but what I normally do is this:

All variables containing data read from or written to a database should be able to handle null values. For value types, this means making them Nullable(Of T). For reference types (String and Byte()), this means allowing the value to be Nothing.
Write a set of functions to convert back and forth between "object that may contain DBNull" and "nullable .NET variable". Wrap all calls to DBNull-style APIs in these functions, then pretend that DBNull doesn't exist.

回答6:

If you are using a BLL/DAL setup try the iif when reading into the object in the DAL

While reader.Read()
 colDropdownListNames.Add(New DDLItem( _
 CType(reader("rid"), Integer), _
 CType(reader("Item_Status"), String), _
 CType(reader("Text_Show"), String), _
 CType( IIf(IsDBNull(reader("Text_Use")), "", reader("Text_Use")) , String), _
 CType(reader("Text_SystemOnly"), String), _
 CType(reader("Parent_rid"), Integer)))
End While

回答7:

You can use the IsDbNull function:

  If  IsDbNull(myItem("sID")) = False AndAlso myItem("sID")==sID Then
    // Do something
End If

回答8:

For the rows containing strings, I can convert them to strings as in changing

tmpStr = nameItem("lastname") + " " + nameItem("initials")

tmpStr = myItem("lastname").toString + " " + myItem("intials").toString

For the comparison in the if statement myItem("sID")=sID, it needs to be change to

myItem("sID").Equals(sID)

Then the code will run without any runtime errors due to vbNull data.

回答9:

   VB.Net
   ========
    Dim da As New SqlDataAdapter
    Dim dt As New DataTable
    Call conecDB()        'Connection to Database
    da.SelectCommand = New SqlCommand("select max(RefNo) from BaseData", connDB)

    da.Fill(dt)

    If dt.Rows.Count > 0 And Convert.ToString(dt.Rows(0).Item(0)) = "" Then
        MsgBox("datbase is null")

    ElseIf dt.Rows.Count > 0 And Convert.ToString(dt.Rows(0).Item(0)) <> "" Then
        MsgBox("datbase have value")

    End If

回答10:

Hello Friends

This is the shortest method to check db Null in DataGrid and convert to string

create the cell validating event and write this code
If Convert.ToString(dgv.CurrentCell.Value) = "" Then
CurrentCell.Value = ""
End If

回答11:

This is BY FAR the easiest way to convert DBNull to a string. The trick is that you CANNOT use the TRIM function (which was my initial problem) when referring to the fields from the database:

BEFORE (produced error msg):

Me.txtProvNum.Text = IIf(Convert.IsDBNull(TRIM(myReader("Prov_Num"))), "", TRIM(myReader("Prov_Num")))

AFTER (no more error msg :-) ):

Me.txtProvNum.Text = IIf(Convert.IsDBNull(myReader("Prov_Num")), "", myReader("Prov_Num"))

回答12:

I think this should be much easier to use:

select ISNULL(sum(field),0) from tablename

Copied from: http://www.codeproject.com/Questions/736515/How-do-I-avoide-Conversion-from-type-DBNull-to-typ

来源：https://stackoverflow.com/questions/222834/handling-dbnull-data-in-vb-net

标签

vb.net

dbnull