How can you handle concurrency at field level in LINQ to SQL?

LINQ Interview Questions

Handling Concurrency at Field Level in LINQ to SQL

Handling concurrency at the field level in LINQ to SQL allows you to detect and manage conflicts when multiple users update specific fields of the same record simultaneously. This ensures that data remains consistent and avoids overwriting changes made by other users. Let's illustrate this with a complete source code example.

Example:

Suppose you have a database table named Products with columns ProductID, ProductName, and UnitsInStock. You want to allow multiple users to update the UnitsInStock field of a product while handling concurrency conflicts.

1. Database Table

Assume you have a database table named Products with the following structure:


CREATE TABLE Products
(
	ProductID INT PRIMARY KEY,
	ProductName NVARCHAR(100),
	UnitsInStock INT
)

2. LINQ to SQL DataContext

Create a LINQ to SQL DataContext to interact with the database. You should add this DataContext to your project using the LINQ to SQL designer in Visual Studio.

3. Code for Updating Stock with Field-Level Concurrency

Now, let's write C# code to update the UnitsInStock field of a product while handling concurrency conflicts using LINQ to SQL:


using System;
using System.Linq;

class Program
{
    static void Main()
    {
        // Create a DataContext instance
        MyDataContext dataContext = new MyDataContext();

        // Retrieve a product by ProductID
        int productId = 1;
        var product = dataContext.Products.Single(p => p.ProductID == productId);

        // Simulate another user updating the same product's UnitsInStock in the database
        Console.WriteLine($"Another user updated the UnitsInStock of Product {productId} in the database.");
        Console.WriteLine("Press Enter to continue...");
        Console.ReadLine();

        try
        {
            // Modify the UnitsInStock field
            product.UnitsInStock = 20;

            // Submit changes to the database with field-level concurrency
            dataContext.SubmitChanges(ConflictMode.ContinueOnConflict);
            Console.WriteLine($"UnitsInStock of Product {productId} updated successfully.");
        }
        catch (System.Data.Linq.ChangeConflictException)
        {
            // Handle concurrency conflict
            Console.WriteLine($"Concurrency conflict detected. Another user modified Product {productId}.");
            Console.WriteLine("Press Enter to reload the product and try again...");
            Console.ReadLine();

            // Refresh the product from the database
            dataContext.Refresh(System.Data.Linq.RefreshMode.OverwriteCurrentValues, product);

            // Update the UnitsInStock field and try again
            product.UnitsInStock = 20;
            dataContext.SubmitChanges();
            Console.WriteLine($"UnitsInStock of Product {productId} updated successfully after resolving the conflict.");
        }
    }
}

In this code:

We retrieve a product by its ProductID.
We simulate another user updating the same product's UnitsInStock in the database.
We modify the UnitsInStock field of the product and submit changes to the database using ConflictMode.ContinueOnConflict.
If a concurrency conflict is detected, we handle it by refreshing the product from the database and updating the UnitsInStock field again.

Output:


Another user updated the UnitsInStock of Product 1 in the database.
Press Enter to continue...

Concurrency conflict detected. Another user modified Product 1.
Press Enter to reload the product and try again...

UnitsInStock of Product 1 updated successfully after resolving the conflict.