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How to: Connect to a Database (LINQ to SQL)

The DataContext is the main conduit by which you connect to a database, retrieve objects from it, and submit changes back to it. You use the DataContext just as you would use an ADO.NET SqlConnection. In fact, the DataContext is initialized with a connection or connection string that you supply. For more information, see DataContext Methods (O/R Designer).

The purpose of the DataContext is to translate your requests for objects into SQL queries to be made against the database, and then to assemble objects out of the results. The DataContext enables Language-Integrated Query (LINQ) by implementing the same operator pattern as the Standard Query Operators, such as Where and Select.

Security noteSecurity Note

Maintaining a secure connection is of the highest importance. For more information, see Security in LINQ to SQL.

In the following example, the DataContext is used to connect to the Northwind sample database and to retrieve rows of customers whose city is London.


' DataContext takes a connection string.
Dim db As New DataContext("…\Northwnd.mdf")

' Get a typed table to run queries.
Dim Customers As Table(Of Customer) = db.GetTable(Of Customer)()

' Query for customer from London.
Dim Query = _
    From cust In Customers _
    Where cust.City = "London" _
    Select cust

For Each cust In Query
    Console.WriteLine("id=" & cust.CustomerID & _
        ", City=" & cust.City)
Next


Each database table is represented as a Table collection available by way of the GetTable method, by using the entity class to identify it.

Best practice is to declare a strongly typed DataContext instead of relying on the basic DataContext class and the GetTable method. A strongly typed DataContext declares all Table collections as members of the context, as in the following example.


Partial Public Class Northwind
    Inherits DataContext
    Public Customers As Table(Of Customer)
    Public Orders As Table(Of Order)
    Public Sub New(ByVal connection As String)
        MyBase.New(connection)
    End Sub
End Class


You can then express the query for customers from London more simply as:


Dim db As New Northwind("...\Northwnd.mdf")

Dim query = _
    From cust In db.Customers _
    Where cust.City = "London" _
    Select cust

For Each cust In query
    Console.WriteLine("id=" & cust.CustomerID & _
        ", City=" & cust.City)
Next


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