# Lambda Expressions

# Using lambda syntax to create a closure

See remarks for discussion of closures. Suppose we have an interface:

public interface IMachine<TState, TInput>
    TState State { get; }
    public void Input(TInput input);

and then the following is executed:

IMachine<int, int> machine = ...;
Func<int, int> machineClosure = i => {
    return machine.State;

Now machineClosure refers to a function from int to int, which behind the scenes uses the IMachine instance which machine refers to in order to carry out the computation. Even if the reference machine goes out of scope, as long as the machineClosure object is maintained, the original IMachine instance will be retained as part of a 'closure', automatically defined by the compiler.

Warning: this can mean that the same function call returns different values at different times (e.g. In this example if the machine keeps a sum of its inputs). In lots of cases, this may be unexpected and is to be avoided for any code in a functional style - accidental and unexpected closures can be a source of bugs.

# Basic lambda expressions

Func<int, int> add1 = i => i + 1;

Func<int, int, int> add = (i, j) => i + j;

// Behaviourally equivalent to:

int Add1(int i)
    return i + 1;

int Add(int i, int j)
    return i + j;


Console.WriteLine(add1(42)); //43
Console.WriteLine(Add1(42)); //43
Console.WriteLine(add(100, 250)); //350
Console.WriteLine(Add(100, 250)); //350

# Basic lambda expressions with LINQ

// assume source is {0, 1, 2, ..., 10}

var evens = source.Where(n => n%2 == 0);
// evens = {0, 2, 4, ... 10}

var strings = source.Select(n => n.ToString());
// strings = {"0", "1", ..., "10"}

# Lambda syntax with statement block body

Func<int, string> doubleThenAddElevenThenQuote = i => {
    var doubled = 2 * i;
    var addedEleven = 11 + doubled;
    return $"'{addedEleven}'";

# Lambda expressions with System.Linq.Expressions

Expression<Func<int, bool>> checkEvenExpression = i => i%2 == 0;
// lambda expression is automatically converted to an Expression<Func<int, bool>>

# Remarks

# Closures

Lambda expressions will implicitly capture variables used and create a closure. A closure is a function along with some state context. The compiler will generate a closure whenever a lambda expression 'encloses' a value from its surrounding context.

E.g. when the following is executed

Func<object, bool> safeApplyFiltererPredicate = o => (o != null) && filterer.Predicate(i);

safeApplyFilterPredicate refers to a newly created object which has a private reference to the current value of filterer, and whose Invoke method behaves like

o => (o != null) && filterer.Predicate(i);

This can be important, because as long as the reference to the value now in safeApplyFilterPredicate is maintained, there will be a reference to the object which filterer currently refers to. This has an effect on garbage collection, and may cause unexpected behaviour if the object which filterer currently refers to is mutated.

On the other hand, closures can be used to deliberate effect to encapsulate a behaviour which involves references to other objects.


var logger = new Logger();
Func<int, int> Add1AndLog = i => {
    logger.Log("adding 1 to " + i);
    return (i + 1);

Closures can also be used to model state machines:

Func<int, int> MyAddingMachine() {
    var i = 0;
    return x => i += x;