handling dbnull data in vb.net

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?

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最佳答案

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

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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.

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You can use the IsDbNull function:

  If  IsDbNull(myItem("sID")) = False AndAlso myItem("sID")==sID Then
// Do something
End If
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You can also use the Convert.ToString() and Convert.ToInteger() methods to convert items with DB null effectivly.

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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.
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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.

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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)

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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
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Hello Friends

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

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

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Simple, but not obvious.

DbNull.Value.Equals(myValue)

I hate VB.NET

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For your problem, you can use following special workaround coding that only exists in VB.Net.

Dim nId As Integer = dr("id") + "0"

This code will replace DBNull value contained in id column by integer 0.

The only acceptable default value is "0" because this expression must also be used when dr("id") is not NULL !

So, using this technic, your code would be

   Dim myDataTable as DataTable
Dim s 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
s = nameItem("lastname") + " " + nameItem("initials")
If myItem("sID") + "0" = sID Then
' Do something
End If
Next

I have tested this solution and it works on my PC on Visual Studio 2022.

PS: if sID can be equal to 0 and you want to do something distinct when dr("sID") value is NULL, you must also adept you program and perhaps use Extension as proposed at end of this answer.

I have tested following statements

Dim iNo1 As Integer = dr("numero") + "0"
Dim iNo2 As Integer = dr("numero") & "0" '-> iNo = 10 when dr() = 1
Dim iNo3 As Integer = dr("numero") + "4" '-> iNo = 5  when dr() = 1
Dim iNo4 As Integer = dr("numero") & "4" '-> iNo = 14 when dr() = 1
Dim iNo5 As Integer = dr("numero") + "" -> System.InvalidCastException : 'La conversion de la chaîne "" en type 'Integer' n'est pas valide.'
Dim iNo6 As Integer = dr("numero") & "" -> System.InvalidCastException : 'La conversion de la chaîne "" en type 'Integer' n'est pas valide.'
Dim iNo7 As Integer = "" + dr("numero") -> System.InvalidCastException : 'La conversion de la chaîne "" en type 'Integer' n'est pas valide.'
Dim iNo8 As Integer = "" & dr("numero") -> System.InvalidCastException : 'La conversion de la chaîne "" en type 'Integer' n'est pas valide.'
Dim iNo9 As Integer = "0" + dr("numero")
Dim iNo0 As Integer = "0" & dr("numero")

Following statements works also correctly

Dim iNo9 As Integer = "0" + dr("numero")
Dim iNo0 As Integer = "0" & dr("numero")

I recognize that is a little tricky.

If trick are not your tips, you can also define an Extension so that following code works.

Dim iNo = dr.GetInteger("numero",0)

where GetInteger() code can be following

Module Extension
'***********************************************************************
'* GetString()
'***********************************************************************


<Extension()>
Public Function GetString(ByRef rd As SqlDataReader, ByRef sName As String, Optional ByVal sDefault As String = "") As String
Return GetString(rd, rd.GetOrdinal(sName), sDefault)
End Function


<Extension()>
Public Function GetString(ByRef rd As SqlDataReader, ByVal iCol As Integer, Optional ByVal sDefault As String = "") As String
If rd.IsDBNull(iCol) Then
Return sDefault
Else
Return rd.Item(iCol).ToString()
End If
End Function


'***********************************************************************
'* GetInteger()
'***********************************************************************


<Extension()>
Public Function GetInteger(ByRef rd As SqlDataReader, ByRef sName As String, Optional ByVal iDefault As Integer = -1) As Integer
Return GetInteger(rd, rd.GetOrdinal(sName), iDefault)
End Function


<Extension()>
Public Function GetInteger(ByRef rd As SqlDataReader, ByVal iCol As Integer, Optional ByVal iDefault As Integer = -1) As Integer
If rd.IsDBNull(iCol) Then
Return iDefault
Else
Return rd.Item(iCol)
End If
End Function


End Module

These methods are more explicitely and less tricky.

In addition, it is possible to define default values other than ZERO and also specific version as GetBoolean() or GetDate(), etc ...

Another possibility is to report SQL default conversion in SQL command using COALESCE SQL command !


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