LINQ to DB is the fastest LINQ database access library offering a simple, light, fast, and type-safe layer between your POCO objects and your database.

Architecturally it is one step above micro-ORMs like Dapper, Massive, or PetaPoco, in that you work with LINQ expressions, not with magic strings, while maintaining a thin abstraction layer between your code and the database. Your queries are checked by the C# compiler and allow for easy refactoring.

However, it's not as heavy as LINQ to SQL or Entity Framework. There is no change-tracking, so you have to manage that yourself, but on the plus side you get more control and faster access to your data.

Code examples and demos can be found [here] (https://github.com/linq2db/examples).

From NuGet: Install-Package linq2db

In your web.config or app.config make sure you have a connection string:

<connectionStrings>
  <add name="Northwind" 
    connectionString = "Server=.\;Database=Northwind;Trusted_Connection=True;Enlist=False;" 
    providerName     = "SqlServer" />
</connectionStrings>

Now let's create a POCO class:

[Table(Name = "Products")]
public class Product
{
  [PrimaryKey, Identity]
  public int ProductID { get; set; }

  [Column(Name = "ProductName"), NotNull]
  public string Name { get; set; }

  // ... other columns ...
}

At this point LINQ to DB doesn't know how to connect to our database or which POCOs go with what database. All this mapping is done through a DataConnection class:

public class DbNorthwind : LinqToDB.Data.DataConnection
{
  public DbNorthwind() : base("Northwind") { }

  public Table<Product> Product { get { return GetTable<Product>(); } }
  // ... other tables ...
}

We call the base constructor with the "Northwind" parameter. This parameter has to match the name="Northwind" we defined above in our connection string. We also have to register our Product class we defined above to allows us to write LINQ queries.

And now let's get some data:

public static List<Product> All()
{
  using (var db = new DbNorthwind())
  {
    var query = from p in db.Product
                where p.ProductID > 25
                orderby p.Name descending
                select p;
    return query.ToList();
  }
}

Make sure you always wrap your DataConnection class (in our case DbNorthwind) in a using statement. This is required for proper resource management, like releasing the database connections back into the pool.

Most times we get the entire row from the database:

from p in db.Product
where p.ProductID == 5
select p;

However, sometimes getting all the fields is too wasteful so we want only certain fields, but still use our POCOs; something that is challenging for libraries that rely on object tracking, like LINQ to SQL.

from p in db.Product
orderby p.Name descending
select new Product
{
  Name = p.Name
};

Rather than concatenating strings we can 'compose' LINQ expressions. In the example below the final SQL will be different if onlyActive is true or false, or if searchFor is not null.

using (var db = new DbNorthwind())
{
  var products = from p in db.Product select p;

  if (onlyActive)
  {
    products = from p in products where !p.Discontinued select p;
  }

  if (searchFor != null)
  {
    products = from p in products where p.Name.Contains(searchFor) select p;
  }

  return products.ToList();
}

This assumes we added a Category class, just like we did with the Product class, defined all the fields, and registered it in our DbNorthwind data access class. We can now write an INNER JOIN query like this:

from p in db.Product
join c in db.Category on p.CategoryID equals c.CategoryID
select new Product
{
  Name = p.Name,
  Category = c
};

and a LEFT JOIN query like this:

from p in db.Product
from c in db.Category.Where(q => q.CategoryID == p.CategoryID).DefaultIfEmpty()
select new Product
{
  Name = p.Name,
  Category = c
};

In the previous example we assign an entire Category object to our product, but what if we want all the fields in our Product class, but we don't want to specify every field by hand? Unfortunately, we cannot write this:

from p in db.Product
from c in db.Category.Where(q => q.CategoryID == p.CategoryID).DefaultIfEmpty()
select new Product(c);

The query above assumes the Product class has a constructor that takes in a Category object. The query above won't work, but we can work around that with the following query:

from p in db.Product
from c in db.Category.Where(q => q.CategoryID == p.CategoryID).DefaultIfEmpty()
select Product.Build(p, c);

For this to work, we need a function in the Product class that looks like this:

public static Product Build(Product product, Category category)
{
  if (product != null)
  {
    product.Category = category;
  }
  return product;
}

One caveat with this approach is that if you're using it with composed queries (see example above) the select Build part has to come only in the final select.

At some point we will need to add a new Product to the database. One way would be to call the Insert extension method found in the LinqToDB namespace; so make sure you import that.

using (var db = new DbNorthwind())
{
  db.Insert(product);
}

This inserts all the columns from our Product class, but without retrieving the generated identity value. To do that we can use InsertWithIndentity, like this:

using (var db = new DbNorthwind())
{
  product.ProductID = Convert.ToInt32(db.InsertWithIdentity(product));
}

We need to convert the returned value to an integer since an identity field could be something other than an integer, like a GUID for example. There is also InsertOrReplace that updates a database record if found or adds it otherwise.

If you need to insert only certain fields, or use values generated by the database, you could write:

using (var db = new DbNorthwind())
{
  db.Product
    .Value(p => p.Name, product.Name)
    .Value(p => p.UnitPrice, 10.2m)
    .Value(p => p.Added, () => Sql.CurrentTimestamp)
    .Insert();
}

Using this method also allows us to build insert statements like this:

using (var db = new DbNorthwind())
{
  var statement = db.Product
                    .Value(p => p.Name, product.Name)
                    .Value(p => p.UnitPrice, 10.2m);

  if (storeAdded) statement.Value(p => p.Added, () => Sql.CurrentTimestamp);

  statement.Insert();
}

Updating records follows a similar pattern to Insert. We have an extension method that updates all the columns in the database:

using (var db = new DbNorthwind())
{
  db.Update(product);
}

And we also have a lower level update mechanism:

using (var db = new DbNorthwind())
{
  db.Product
    .Where(p => p.ProductID == product.ProductID)
    .Set(p => p.Name, product.Name)
    .Set(p => p.UnitPrice, product.UnitPrice)
    .Update();
}

Similarly, we can break an update query into multiple pieces if needed:

using (var db = new DbNorthwind())
{
  var statement = db.Product
                    .Where(p => p.ProductID == product.ProductID)
                    .Set(p => p.Name, product.Name);

  if (updatePrice) statement = statement.Set(p => p.UnitPrice, product.UnitPrice);

  statement.Update();
}

You're not limited to updating a single field. For example, we could discontinue all the products that are no longer in stock:

using (var db = new DbNorthwind())
{
  db.Product
    .Where(p => p.UnitsInStock == 0)
    .Set(p => p.Discontinued, true)
    .Update();
}

Similar to how you update records, you can also delete records:

using (var db = new DbNorthwind())
{
  db.Product
    .Where(p => p.Discontinued)
    .Delete();
}

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