Language Integrated Query (LINQ)

Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language. Traditionally, queries against data are expressed as simple strings without type checking at compile time or IntelliSense support. Furthermore, you have to learn a different query language for each type of data source: SQL databases, XML documents, various Web services, and so on. With LINQ, a query is a first-class language construct, just like classes, methods, events.

For a developer who writes queries, the most visible “language-integrated” part of LINQ is the query expression. Query expressions are written in a declarative query syntax. By using query syntax, you can perform filtering, ordering, and grouping operations on data sources with a minimum of code. You use the same basic query expression patterns to query and transform data in SQL databases, ADO .NET Datasets, XML documents and streams, and .NET collections.

class LINQQueryExpressions

{

    static void Main()

    {

    // Specify the data source.
    int[] scores = new int[] { 97, 92, 81, 60 };

    // Define the query expression.
    IEnumerable<int> scoreQuery =
        from score in scores
        where score > 80
        select score;

    // Execute the query.
    foreach (int i in scoreQuery)
    {
        Console.Write(i + " ");
    }            
}

}

// Output: 97 92 81

Sources:

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/linq/