handling dbnull data in vb.net

Azim_4612 0
Viewed 264680

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?

dbnull vb.net
13 Answers

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

Mitchel_Sellers_132790

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

Mitchel_Sellers_132790

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)

user_13046230

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:

  1. 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.
  2. 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.
user_1154130

You can use the IsDbNull function:

  If  IsDbNull(myItem("sID")) = False AndAlso myItem("sID")==sID Then
    // Do something
End If
user_2250

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

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

to

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.

Azim_46120 
   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
user_30424560

Simple, but not obvious.

DbNull.Value.Equals(myValue)

I hate VB.NET

user_15390010
user_44094620
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