LINQ Query Expressions in C#

LINQ Query Expressions in C#

Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language (also in Visual Basic and potentially any other .NET language). With LINQ, a query is now a first-class language construct, just like classes, methods, events and so on.

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 syntaxintroduced in C# 3.0. By using query syntax, you can perform even complex 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 MYLINQQueryExpressions
{
    static void Main()
    {

        // Specify the data source. 
        int[] marks= new int[] { 77, 110, 92, 125};

        // Define the query expression.
        IEnumerable<int> scoreQuery =
            from mark in marks
            where mark> 95
            select mark;

        // Execute the query. 
        foreach (int i in scoreQuery)
        {
            Console.Write(i + " ");
        }            
    }
}
// Output: 110 125

How to Write LINQ Query Expressions

Guidelines to write LINQ Query Expressions

• Query expressions can be used to query and to transform data from any LINQ-enabled data source. For example, a single query can retrieve data from a SQL database, and produce an XML stream as output.
• Query expressions are easy to master because they use many familiar C# language constructs.
• The variables in a query expression are all strongly typed, although in many cases you do not have to provide the type explicitly because the compiler can infer it.
• A query is not executed until you iterate over the query variable in a foreach statement.
• At compile time, query expressions are converted to Standard Query Operator method calls according to the rules set forth in the C# specification. Any query that can be expressed by using query syntax can also be expressed by using method syntax. However, in most cases query syntax is more readable and concise. 
• As a rule when you write LINQ queries, we recommend that you use query syntax whenever possible and method syntax whenever necessary. There is no semantic or performance difference between the two different forms. Query expressions are often more readable than equivalent expressions written in method syntax.
• Some query operations, such as Count or Max, have no equivalent query expression clause and must therefore be expressed as a method call. Method syntax can be combined with query syntax in various ways.
• Query expressions can be compiled to expression trees or to delegates, depending on the type that the query is applied to. IEnumerable<T> queries are compiled to delegates. IQueryable and IQueryable<T> queries are compiled to expression trees.