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DataColumn.MaxLength property always returns -1 ?

Posted on 2009-04-08
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Last Modified: 2012-05-06
The client application receives the DataSet via a web service call.

Here is the code inside the web service that populates the dataset which is sent back to the client.
Dim DA As SqlDataAdapter
                Dim DS As New DataSet
                Cmd.CommandText = strSelect
                DA = New SqlDataAdapter(Cmd)
                DA.Fill(DS, "RunSQL")
                DA.Dispose()
                Con.Close()
                Return DS

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Question by:tfsln
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Expert Comment

by:HarryNS
ID: 24104005
Are you sure that dataset is getting filled up in your web service?

If so, then in your client application you can see the count of columns, DS.Tables[0].Columns.Count
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Expert Comment

by:Sreedhar Vengala
ID: 24104007
What is your strSelect value ?
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Expert Comment

by:Chandan_Gowda
ID: 24104088
check the dataset and make sure, the dataset is properly filled.
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Expert Comment

by:oobayly
ID: 24115263
What's the datatype of the column? According the MSDN, "The MaxLength property is ignored for non-text columns."
http://msdn.microsoft.com/en-us/library/system.data.datacolumn.maxlength(VS.80).aspx
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Author Comment

by:tfsln
ID: 24128751
HarryNS: I dont want a column count, i want the max lengths for the columns in the dataset (as specified in the database table design)

The select query is, lets say, "Select * from tablename"

I am not trying to get the MaxLength of any other columns other than varchar (the db is sql server 2005).
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Accepted Solution

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oobayly earned 500 total points
ID: 24128999
Add the following line after you create the DataAdapter, it will tell the DataAdapter to fetch the table schema as well as the data

More information at:
http://msdn.microsoft.com/en-us/library/system.data.missingschemaaction.aspx
DA.MissingSchemAction = MissingSchemaAction.AddWithKey

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Author Comment

by:tfsln
ID: 24133165
This doesnt seem to work when i use a complex query... Its sensitive information so i cant paste it, but are you aware of any limitations in using this property?

I often batch queries together to return multiple datatable's inside the dataset - is this a problem? Or using joins/unions?
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Expert Comment

by:oobayly
ID: 24133312
To be quite honest, this isn't something I've ever used, so I can't say how multiple joins or returning multiple datasets will effect using the property.

However, just doing some simple testing, selecting multiple simple tables doesn't cause any issues and neither does a query that has a single inner join.
My only thought is that you may be returning composite fields, so the schema can't be returned.
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Author Comment

by:tfsln
ID: 24133334
I do include composite fields... but i would have expected those fields to return the same value for MaxLength as any other field which doesnt have a max length (i.e. datetime)

If they do error, then thats a problem... Ill have to abandon this path
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Expert Comment

by:oobayly
ID: 24133405
Interesting, I ran the following code and in all 5 tables returned the columns had the expected MaxLength properties set.
For the composite fields, the MaxLength property set was the Length of the longest string returned, whilst for the standard fields, MaxLength = 64 for a VARCHAR(64) field etc.
      SqlDataAdapter adapter= new SqlDataAdapter("SELECT * FROM tblVehicles;SELECT * FROM tblBodyTypes;"+
        "SELECT * FROM tblVehicles INNER JOIN tblBodyTypes ON tblVehicles.fldBodyID = tblBodyTypes.fldBodyID;" +
        "SELECT 'Foo' + ' ' + 'bar' AS [Foo];" +
        "SELECT fldModelDescription + 'Foo' AS [Foo] FROM tblVehicles", conn);
      adapter.MissingSchemaAction = MissingSchemaAction.AddWithKey;
      DataSet ds = new DataSet();
      adapter.Fill(ds);

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Author Comment

by:tfsln
ID: 24133414
Interesting... Im going to need to run more tests. It logged in to our application but then failed on one of the most complex queries we have.

Its difficult, because when i test my web service it only allows me to enter text on one line - but i need to paste it a huge query over many lines just so that i can get the error message. At the moment we use the ASPAccelerator to compress web service output and a side effect is that all errors are returned as "Xml was not well formed"
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