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Using Entity Framework Core for Data Access & Relational Mapping

LINQ Query Operations

Explore how to build and execute LINQ queries in EF Core by defining data sources, creating query expressions, and understanding deferred and immediate execution. Learn the differences between query and method syntax to query and transform data effectively.

We'll cover the following...

Overview

Every LINQ query operation consists of three distinct actions:

  • Defining a data source
  • Defining the query
  • Executing the query

We’ll review these actions using the code sample below:

{
    "version": "0.2.0",
    "configurations": [
        {
            // Use IntelliSense to find out which attributes exist for C# debugging
            // Use hover for the description of the existing attributes
            // For further information visit https://github.com/OmniSharp/omnisharp-vscode/blob/master/debugger-launchjson.md
            "name": ".NET Core Launch (console)",
            "type": "coreclr",
            "request": "launch",
            "preLaunchTask": "build",
            // If you have changed target frameworks, make sure to update the program path.
            "program": "${workspaceFolder}/bin/Debug/net6.0/QueryData.dll",
            "args": [],
            "cwd": "${workspaceFolder}",
            // For more information about the 'console' field, see https://aka.ms/VSCode-CS-LaunchJson-Console
            "console": "internalConsole",
            "stopAtEntry": false
        },
        {
            "name": ".NET Core Attach",
            "type": "coreclr",
            "request": "attach"
        }
    ]
}
Using a LINQ query to output odd numbers

Click the “Run” button of the code widget above, then execute the command below in the terminal:

C#
dotnet run

Defining a data source

In a LINQ query operation, the first step is to define the data source. A LINQ data source is any object that supports the generic IEnumerable<T> interface or an interface that inherits from it, such as IQueryable<T>.

  • Line 3 of Program.cs in the SPA widget above uses an integer array named numbers as a data source. This data source implicitly supports the generic IEnumerable<T> interface.

Defining the query

The query defines what information to retrieve from a data source. In addition, it specifies how that information should be filtered, grouped, or sorted before returning it. The query is initialized with a query expression and stored in a query ...