Question

We have a program that uses Linq-To-Sql and does a lot of similar queries on our tables. In particular, we have a timestamp column, and we pull the most recent timestamp to see if the table has changed since our last query.

System.Data.Linq.Binary ts;
ts = context.Documents
    .Select(r => r.m_Timestamp)
    .OrderByDescending(r => r)
    .FirstOrDefault();

We repeat this query often on different tables that’s relevant for the current form/query whatever. What I would like to do is create a "function" or something that can repeat the last 3 lines of this query (and ideally would work on every table). Here’s what I would like to write:

System.Data.Linq.Binary ts;
ts = context.Documents
    .GetMostRecentTimestamp();

But I have no idea how to create this "GetMostRecentTimestamp". Also, these queries are never this simple. They usually filter by the Customer, or by the current order, so a more valid query might be

ts = context.Documents
    .Where(r => r.CustomerID == ThisCustomerID)
    .GetMostRecentTiemstamp(); 

Any help? Thanks!

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Update [Solution]

I selected Bala R’s answer, here’s the code updated so it compiles:

public static System.Data.Linq.Binary GetMostRecentTimestamp(this IQueryable<Data.Document> docs)
{
    return docs
        .Select(r => r.m_Timestamp)
        .OrderByDescending(r => r)
        .FirstOrDefault();
    }

The only drawback to this solution is that I will have to write this function for each table. I would have loved Tejs’s answer, if it actually worked, but I’m not re-designing my database for it. Plus DateTime is a not a good way to do timestamps.

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Update #2 (Not so fast)

While I can do a query such as Documents.Where( … ).GetMostRecentTimestamp(), this solution fails if I try to do an association based query such as MyCustomer.Orders.GetMostRecentTimestamp(), or MyCustomer.Orders.AsQueryable().GetMostRecentTimestamp();

Answer 1

How about an extension like this

public static DateTime GetMostRecentTimestamp (this IQueryable<Document> docs)
{
    return docs.Select(r => r.m_Timestamp)
               .OrderByDescending(r => r)
               .FirstOrDefault();
}