How to export a DataTable to Xml with ALL columns as attributes?

Question

Question: I'm exporting a System.Data.DataTable to XML. So far it works fine. But I want to have all the data in attributes, which works fine as well. But my problem now, if in one column, all rows are NULL, no empty attributes are written. So if I read the XML back to a DataTable, it lacks this column...

How can I force write all columns even when they are empty ?
(DataType not necessarely string)

public void ExportTable(string strDirectory, DataTable dtt)
{
    using (System.Data.DataSet ds = new System.Data.DataSet()) {
        string strTable = dtt.TableName;

        ds.Tables.Add(dtt);
        ds.DataSetName = strTable;

        // Move data to attributes 
        foreach (DataTable dt in ds.Tables) {

            foreach (DataColumn dc in dt.Columns) {
                dc.ColumnMapping = MappingType.Attribute;
            }

        }

        System.Xml.XmlWriterSettings settings = new System.Xml.XmlWriterSettings();
        settings.Indent = true;
        //settings.Encoding = System.Text.Encoding.GetEncoding("ISO-8859-1") 
        settings.Encoding = System.Text.Encoding.UTF8;
        settings.CloseOutput = true;
        settings.CheckCharacters = true;
        settings.NewLineChars = "\r\n";
        // vbCr & vbLf 

        // Write as UTF-8 with indentation 
        using (System.Xml.XmlWriter w = System.Xml.XmlWriter.Create(System.IO.Path.Combine(strDirectory, strTable + ".xml"), settings)) {

            // Strip out timezone 
            foreach (DataTable dt in ds.Tables) {

                foreach (DataColumn dc in dt.Columns) {

                    if (object.ReferenceEquals(dc.DataType, typeof(DateTime))) {
                        dc.DateTimeMode = DataSetDateTime.Unspecified;
                    }

                }

            }

            ds.Tables[0].WriteXml(w, XmlWriteMode.IgnoreSchema);
            w.Flush();
            w.Close();
        }
        // w 

    }
    // ds 

}
// ExportTable 

VB.NET original:

 Public Sub ExportTable(strDirectory As String, dtt As DataTable)
        Using ds As New System.Data.DataSet()
            Dim strTable As String = dtt.TableName

            ds.Tables.Add(dtt)
            ds.DataSetName = strTable

            ' Move data to attributes
            For Each dt As DataTable In ds.Tables

                For Each dc As DataColumn In dt.Columns
                    dc.ColumnMapping = MappingType.Attribute
                Next dc

            Next dt

            Dim settings As New System.Xml.XmlWriterSettings()
            settings.Indent = True
            'settings.Encoding = System.Text.Encoding.GetEncoding("ISO-8859-1")
            settings.Encoding = System.Text.Encoding.UTF8
            settings.CloseOutput = True
            settings.CheckCharacters = True
            settings.NewLineChars = vbCrLf ' vbCr & vbLf

            ' Write as UTF-8 with indentation
            Using w As System.Xml.XmlWriter = System.Xml.XmlWriter.Create(System.IO.Path.Combine(strDirectory, strTable & ".xml"), settings)

                ' Strip out timezone
                For Each dt As DataTable In ds.Tables

                    For Each dc As DataColumn In dt.Columns

                        If dc.DataType Is GetType(DateTime) Then
                            dc.DateTimeMode = DataSetDateTime.Unspecified
                        End If

                    Next dc

                Next dt

                ds.Tables(0).WriteXml(w, XmlWriteMode.IgnoreSchema)
                w.Flush()
                w.Close()
            End Using ' w

        End Using ' ds

    End Sub ' ExportTable

Answer 1

Every XML attribute must be assigned a value that is enclosed in a pair of single or double quotation marks. There is no equivalent in plain text to denote a NULL value. A pair of quotation marks with no value to represent an empty string is not the same as a NULL value. Therefore, the only way to represent a NULL attribute is to omit the attribute.

This means that you will need to either set AllowDBNull to false and assign a suitable DefaultValue on the DataColumn, or include the schema.

Also, see Handling Null Values (ADO.NET)., particularly this section which explains the behavior:

In addition, the following rules apply for an instance of DataRow.["columnName"] null assignments:

1.The default default value is DbNull.Value for all except the strongly typed null columns where it is the appropriate strongly typed null value.

2.Null values are never written out during serialization to XML files (as in "xsi:nil").

3.All non-null values, including defaults, are always written out while serializing to XML. This is unlike XSD/XML semantics where a null value (xsi:nil) is explicit and the default value is implicit (if not present in XML, a validating parser can get it from an associated XSD schema). The opposite is true for a DataTable: a null value is implicit and the default value is explicit.

4.All missing column values for rows read from XML input are assigned NULL. Rows created using NewRow or similar methods are assigned the DataColumn's default value.

5.The IsNull method returns true for both DbNull.Value and INullable.Null.