Expressions

An expression is a sequence of one or more operands and zero or more operators that can be evaluated to a single value, object, method, or namespace. -- Microsoft's C# Programming Guide

LINQ Query Expressions

Language-Integrated Query (LINQ) integrates query capabilities directly into the C# language. Although it is commonly used by Entity Framework, its usage applies to a variety of cases.

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 + " ");
    }
}

Query Expression Overview

• Can be used to query and to transform data from any LINQ-enabled data source, such as SQL databases.
• Easy to master.
• The variables in a query expression are strongly typed, although they can inferred by the compiler.
• The query is not executed until iterated through a foreach loop
• Queries are translated to Standard Query Operator method calls.
• Query expressions are preferred over method calls, due to its readability.
• Some query operations, such as Count or Max, have no equivalent query expression claus and must be expressed as a method call.
• Query expressions can be compiled to expression trees or to delegates.

Lambda Expressions

A lambda expression is an anonymous function used to create delegates or expression tree types. They are often used in Entity Framework.

Delegate.

delegate int del(int i);
static void Main(string[] args)
{
    del myDelegate = x => x * x;
    int j = myDelegate(5); //j = 25
}

Expression Tree.

static void Main(string[] args)
{
    Expression<del> myET = x => x * x;
}

Entity Framework.

var adultUserList = db.Users.Where(u => u.Age >= 18).ToList();

Expression Lambdas

Lambda expressions with an expression on the right side of the => operator.

(input parameters) => expression

(int x, string s) => s.Length > x

Statement Lambdas

Statement(s) is enclosed in braces

(input parameters) => {statement;}

delegate void TestDelegate(string s);

TestDelegate myDel = n => { string s = n + " " + "World"; Console.WriteLine(s); };
myDel("Hello");

Async Lambdas

Lambda expressions and statements that incorporate asynchronous processing.

Parallel.For(0, 10, async i => {
  await Task.Delay(1000);
});

Lambdas with the Standard Query Operators

Often used when working with Entity Framework.

Func<int, bool> isEqualToFive = x => x == 5;

bool result = isEqualToFive(4); // false

var activeUsers = db.Users.Where(u => u.Active == true).ToList();

Type Inference in Lambdas

The compiler infers the type of the result.

var customersFromLondon = customers.Where(c => c.City == "London");

Expression Trees

Read about expression trees here.