Why is c# ReadXml returning wrong data?

Question

I am caching data to decrease SQL Server activity.

When I fetch data from SQL Server the first time it is requested, I then use WriteXml to store it on disk.

Then second time it is requested (ie a file called cacheName exists), I fetch the data with ReadXml .

The data in SQL Server has only one row with utcDT value of '2012-03-25 02:01' and when I write the DataTable at Code point 1 Test1() shows there is only one row. When I manually inspect Xml file on disk I also see only 1 row.

However, after reading with ReadXml at code point 2 Test1() shows 2 such rows!

How is this happening?

public static DataTable FetchCache()
{
    var cacheName = @"C:\MyCache.xml";

    if (File.Exists(cacheName))
    {
        var ds = new DataSet();
        var fs = new FileStream(cacheName, FileMode.Open, FileAccess.Read, FileShare.Read);

        using (var sr = new StreamReader(fs))
        {
            ds.ReadXml(sr, XmlReadMode.ReadSchema);
        }

        table = ds.Tables[0];
        Test1(table);  //Code point 2
    }
    else
    {
        table = FetchDataTable(connectionString, sqlCommand, nullOnError: nullOnError);
        Test1(table); //Code point 1
        table.WriteXml(cacheName, XmlWriteMode.WriteSchema);
    }

    return table;
}

public static void Test1(DataTable table)
{
    var rows = table.AsEnumerable()
                    .Where(x => x.Field<DateTime>("utcDT").Equals(new DateTime(2012, 03, 25, 02, 01, 00)))
                    .ToArray();
}

public static DataTable FetchDataTable(string connectionString, string sqlCommand, bool nullOnError= false)
{
    DataSet ds = new DataSet();
    DataTable dt = new DataTable();

    try
    {
        using (SqlConnection conn = new SqlConnection(connectionString))
        {
            conn.Open();

            SqlDataAdapter da = new SqlDataAdapter(sqlCommand, conn);
            da.SelectCommand.CommandTimeout = 600;
            ds.Reset();

            da.Fill(ds);
            dt = ds.Tables[0];
        }
    }
    catch (Exception err)
    {
        if (nullOnError)
            return dt;

        throw new Exception("[Utils.MsSQLS.FetchDataTable] Could not get data: " + err.Message);
    }

    return dt;
}

Answer 1

Just in case anyone else has same issue. I found the answer.

My SQL table DateTime columns were originally specified as UnspecifiedLocal, but the data they contain is actually UTC.

In order to save to cache as Xml, I have to create a new DataTable clone from table I fetch from SQL, and change DateTimeMode to UTC , then I copy from the old table to the new (row by row using ItemArray copy).

This is cumbersome but gets rid of the issue.