DataAdapter.Update() problem

I fill the data set from an Odbc data source (An EXCEL sheet).

Then I want to update the Excel sheet (I made a few changes in my data set).

But, if in the original Excell sheet I have an empty string cell
( the table.row[col]="" ), the odbcDataAdapter.Update() method makes an incorect format Excel sheet. (un unreadable sheet).

I tried to "PrepareForUpdate()" before the odbcDataAdapter.Update() statement but it doesn't work.

private void PrepareForUpdate() {
foreach (DataRow row in table.Rows) {
foreach (DataColumn col in table.Columns ) {
if (row[col].ToString()=="")
row [col]=" "; //System.DBNull.Value;
}
}
}

It doesn't work either if I try to set the value in that cell to a DBNull.Value.

I mention that I use C# in Visual Studio .NET and the Odbc .NET Data Provider from http://msdn.microsoft.com/downloads.

Please, if anyoane has encountered this problem or want to give me a clue don't hesitate 'cause i'm in a desperate situation !

I don't much about Excel in PHP.
I would look at "Advanced PHP for the World Wide Web" by Larry Ullman.
It has some good stuff about Excel and PHP in it.
Hope this helps some....