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Datasource column size (max length)

Posted on 2009-04-09
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Last Modified: 2013-11-26
Hi,
Is it possible to retrieve the datasource columns max length, as specified in the db, through VB.Net code ? If so what would it be ? I don't need a loop, I need to be able to only work with the columns I want either with some sort of an index or with the column name. Thanks
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Question by:GPSAddict
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11 Comments
 
LVL 53

Expert Comment

by:Dhaest
ID: 24106762
Did you take a look at
size = dataTables(0).Columns(0).MaxLength
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Expert Comment

by:sybe
ID: 24106788
Starting from a SqlDataReader, you need to get the Schema


oDR = oCmd.ExecuteReader()
oSchema = oDR.GetSchemaTable()
 
' get the columnsize for field by (numeric) index
iColumnSize = oSchema.Rows.Item(index)("ColumnSize")

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Author Comment

by:GPSAddict
ID: 24106832
Dhaest: datables(0), 0 beeing ??? I have many sql datasources, how to reference the good one ?
sybe: anymore documentation on that ? I'm not using tableadapters or anything.
Thanks
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LVL 28

Expert Comment

by:sybe
ID: 24106914
I could point you to some Googles links on "GetSchemaTable()", but I guess those are easy to find by yourself.

The few lines of code I posted are useful to me, in the sense that sometimes I need to auto-truncate submitted texts. For that I need to compare the length of the text with the column size of string-fields in the database (char, varchar etc). After some browsing around, I puzzled the code together bit by bit.

I rewrote that code as below, to make clear how to get the max length.



Dim oSchema As DataTable
Dim oCmd As New SqlCommand(sSQL, oSQLConnection)
Dim oDR As SqlDataReader
oDR = oCmd.ExecuteReader()
oSchema = oDR.GetSchemaTable()
 
 
' loop through the fields (not rows) in the SQLDataReader
Dim i As Integer
Dim sFieldName As String
Dim iColumnSize As Integer
 
For i = 0 To oDR.FieldCount - 1
    sFieldName = oDR.GetName(i)
    iColumnSize = oSchema.Rows.Item(i)("ColumnSize")
    If oSchema.Rows.Item(i)("DataType").ToString = "System.String" Then
        Response.write "field of name = " & sFieldName & " is of type String, and has a maximal length of " & iColumnSize & "<br>"
    End If
Next

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Author Comment

by:GPSAddict
ID: 24107020
Thanks, my sqldatasource is named SubCatDataSource. Now I didn't write a single line of code and whent he page loads, the select query is returned. Since everything is embedded through the objects, I tried, in the form load even, the code below. It doesn't work...Any ideas why ? Thanks

    Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim myReader As SqlClient.SqlDataReader = SubCategoryDataSource.SelectCommand.ExecuteReader()
 
    End Sub

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Expert Comment

by:sybe
ID: 24107105
I don't know about that. It seems a totally different question.
But in my code, the connection if created by a line like this, not from a datasource in the page. I don't know if it is possible to create a connection object from that.

New SqlConnection(sConnectionString)
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Expert Comment

by:Dhaest
ID: 24115378
Can you try the following:

How To Retrieve Column Schema Using the DataReader GetSchemaTable Method and Visual Basic .NET
http://support.microsoft.com/kb/310108

http://vb.net-informations.com/ado.net-dataproviders/ado.net-schema-information-sqldatareader.htm
Private Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        Dim myReader As SqlClient.SqlDataReader = SubCategoryDataSource.SelectCommand.ExecuteReader()
 'Retrieve column schema into a DataTable. 
schemaTable = myReader.GetSchemaTable() 
 
'For each field in the table... 
For Each myField As DataRow In schemaTable.Rows 
    'For each property of the field... 
    For Each myProperty As DataColumn In schemaTable.Columns 
        'Display the field name and value. 
        Console.WriteLine((myProperty.ColumnName & " = ") + myField(myProperty).ToString()) 
    Next 
    Console.WriteLine() 
    
    'Pause. 
    Console.ReadLine() 
Next 
 
    End Sub

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Author Comment

by:GPSAddict
ID: 24151162
Thanks daest, I've read those links but I am unable to work it through the datasource object. Using your code, it fails on the below line. VS.NET won't let me write .ExecuteReader at all. All the rest seems ok. Anymore ideas ?
Dim myReader As SqlClient.SqlDataReader = SubCategoryDataSource.SelectCommand.ExecuteReader()

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Author Comment

by:GPSAddict
ID: 24294331
Hello ?
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Accepted Solution

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Dhaest earned 2000 total points
ID: 24295237
Dim cmd As New SqlCommand("select * from yourtable", cn)
Dim rdr As SqlDataReader = cmd.ExecuteReader(CommandBehavior.KeyInfo)
...
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