Lambda expressions (C# Programming Guide)

Source: Microsoft – https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/statements-expressions-operators/lambda-expressions

A lambda expression is a block of code (an expression or a statement block) that is treated as an object. It can be passed as an argument to methods, and it can also be returned by method calls. Lambda expressions are used extensively for:

Lambda expressions are code that can be represented either as a delegate, or as an expression tree that compiles to a delegate. The specific delegate type of a lambda expression depends on its parameters and return value. Lambda expressions that don’t return a value correspond to a specific Action delegate, depending on its number of parameters. Lambda expressions that return a value correspond to a specific Func delegate, depending on its number of parameters. For example, a lambda expression that has two parameters but returns no value corresponds to an Action<T1,T2> delegate. A lambda expression that has one parameter and returns a value corresponds to Func<T,TResult> delegate.

A lambda expression uses =>, the lambda declaration operator, to separate the lambda’s parameter list from its executable code. To create a lambda expression, you specify input parameters (if any) on the left side of the lambda operator, and you put the expression or statement block on the other side. For example, the single-line lambda expression x => x * x specifies a parameter that’s named x and returns the value of x squared. You can assign this expression to a delegate type, as the following example shows:

Func<int, int> square = x => x * x;
Console.WriteLine(square(5));
// Output:
// 25

You also can assign a lambda expression to an expression tree type:

System.Linq.Expressions.Expression<Func<int, int>> e = x => x * x;
Console.WriteLine(e);
// Output:
// x => (x * x)

Or you can pass it directly as a method argument:

int[] numbers = { 2, 3, 4, 5 };
var squaredNumbers = numbers.Select(x => x * x);
Console.WriteLine(string.Join(" ", squaredNumbers));
// Output:
// 4 9 16 25

When you use method-based syntax to call the Enumerable.Select method in the System.Linq.Enumerable class (as you do in LINQ to Objects and LINQ to XML) the parameter is a delegate type System.Func<T,TResult>. A lambda expression is the most convenient way to create that delegate. When you call the Queryable.Select method in the System.Linq.Queryable class (as you do in LINQ to SQL) the parameter type is an expression tree type Expression<Func<TSource,TResult>>. Again, a lambda expression is just a very concise way to construct that expression tree. The lambdas allow the Select calls to look similar although in fact the type of object created from the lambda is different.

All restrictions that apply to anonymous methods also apply to lambda expressions.

Expression lambdas

A lambda expression with an expression on the right side of the => operator is called an expression lambda. Expression lambdas are used extensively in the construction of expression trees. An expression lambda returns the result of the expression and takes the following basic form:

(input-parameters) => expression

The parentheses are optional only if the lambda has one input parameter; otherwise they are required.

Specify zero input parameters with empty parentheses:

Action line = () => Console.WriteLine();

Two or more input parameters are separated by commas enclosed in parentheses:

Func<int, int, bool> testForEquality = (x, y) => x == y;

Sometimes it’s impossible for the compiler to infer the input types. You can specify the types explicitly as shown in the following example:

Func<int, string, bool> isTooLong = (int x, string s) => s.Length > x;

Input parameter types must be all explicit or all implicit; otherwise, a CS0748 compiler error occurs.

The body of an expression lambda can consist of a method call. However, if you are creating expression trees that are evaluated outside the context of the .NET common language runtime, such as in SQL Server, you should not use method calls in lambda expressions. The methods will have no meaning outside the context of the .NET common language runtime.

Statement lambdas

A statement lambda resembles an expression lambda except that the statement(s) is enclosed in braces:

(input-parameters) => { statement; }

The body of a statement lambda can consist of any number of statements; however, in practice there are typically no more than two or three.

Action<string> greet = name => 
{ 
    string greeting = $"Hello {name}!";
    Console.WriteLine(greeting);
};
greet("World");
// Output:
// Hello World!

Statement lambdas, like anonymous methods, cannot be used to create expression trees.

Async lambdas

You can easily create lambda expressions and statements that incorporate asynchronous processing by using the async and await keywords. For example, the following Windows Forms example contains an event handler that calls and awaits an async method, ExampleMethodAsync.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        button1.Click += button1_Click;
    }

    private async void button1_Click(object sender, EventArgs e)
    {
        await ExampleMethodAsync();
        textBox1.Text += "\r\nControl returned to Click event handler.\n";
    }

    private async Task ExampleMethodAsync()
    {
        // The following line simulates a task-returning asynchronous process.
        await Task.Delay(1000);
    }
}

You can add the same event handler by using an async lambda. To add this handler, add an async modifier before the lambda parameter list, as the following example shows:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        button1.Click += async (sender, e) =>
        {
            await ExampleMethodAsync();
            textBox1.Text += "\r\nControl returned to Click event handler.\n";
        };
    }

    private async Task ExampleMethodAsync()
    {
        // The following line simulates a task-returning asynchronous process.
        await Task.Delay(1000);
    }
}

For more information about how to create and use async methods, see Asynchronous Programming with async and await.

Lambda expressions and tuples

Starting with C# 7.0, the C# language provides built-in support for tuples. You can provide a tuple as an argument to a lambda expression, and your lambda expression can also return a tuple. In some cases, the C# compiler uses type inference to determine the types of tuple components.

You define a tuple by enclosing a comma-delimited list of its components in parentheses. The following example uses tuple with three components to pass a sequence of numbers to a lambda expression, which doubles each value and returns a tuple with three components that contains the result of the multiplications.

Func<(int, int, int), (int, int, int)> doubleThem = ns => (2 * ns.Item1, 2 * ns.Item2, 2 * ns.Item3);
var numbers = (2, 3, 4);
var doubledNumbers = doubleThem(numbers);
Console.WriteLine($"The set {numbers} doubled: {doubledNumbers}");
// Output:
// The set (2, 3, 4) doubled: (4, 6, 8)

Ordinarily, the fields of a tuple are named Item1Item2, etc. You can, however, define a tuple with named components, as the following example does.

Func<(int n1, int n2, int n3), (int, int, int)> doubleThem = ns => (2 * ns.n1, 2 * ns.n2, 2 * ns.n3);
var numbers = (2, 3, 4);
var doubledNumbers = doubleThem(numbers);
Console.WriteLine($"The set {numbers} doubled: {doubledNumbers}");

For more information about C# tuples, see C# tuple types.

Lambdas with the standard query operators

LINQ to Objects, among other implementations, have an input parameter whose type is one of the Func<TResult> family of generic delegates. These delegates use type parameters to define the number and type of input parameters, and the return type of the delegate. Func delegates are very useful for encapsulating user-defined expressions that are applied to each element in a set of source data. For example, consider the Func<T,TResult> delegate type:

public delegate TResult Func<in T, out TResult>(T arg)

The delegate can be instantiated as a Func<int, bool> instance where int is an input parameter and bool is the return value. The return value is always specified in the last type parameter. For example, Func<int, string, bool> defines a delegate with two input parameters, int and string, and a return type of bool. The following Func delegate, when it’s invoked, returns Boolean value that indicates whether the input parameter is equal to five:

Func<int, bool> equalsFive = x => x == 5;
bool result = equalsFive(4);
Console.WriteLine(result);   // False

You can also supply a lambda expression when the argument type is an Expression<TDelegate>, for example in the standard query operators that are defined in the Queryable type. When you specify an Expression<TDelegate> argument, the lambda is compiled to an expression tree.

The following example uses the Count standard query operator:

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
int oddNumbers = numbers.Count(n => n % 2 == 1);
Console.WriteLine($"There are {oddNumbers} odd numbers in {string.Join(" ", numbers)}");

The compiler can infer the type of the input parameter, or you can also specify it explicitly. This particular lambda expression counts those integers (n) which when divided by two have a remainder of 1.

The following example produces a sequence that contains all elements in the numbers array that precede the 9, because that’s the first number in the sequence that doesn’t meet the condition:

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var firstNumbersLessThanSix = numbers.TakeWhile(n => n < 6);
Console.WriteLine(string.Join(" ", firstNumbersLessThanSix));
// Output:
// 5 4 1 3

The following example specifies multiple input parameters by enclosing them in parentheses. The method returns all the elements in the numbers array until it encounters a number whose value is less than its ordinal position in the array:

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 };
var firstSmallNumbers = numbers.TakeWhile((n, index) => n >= index);
Console.WriteLine(string.Join(" ", firstSmallNumbers));
// Output:
// 5 4

Type inference in lambda expressions

When writing lambdas, you often don’t have to specify a type for the input parameters because the compiler can infer the type based on the lambda body, the parameter types, and other factors as described in the C# language specification. For most of the standard query operators, the first input is the type of the elements in the source sequence. If you are querying an IEnumerable<Customer>, then the input variable is inferred to be a Customer object, which means you have access to its methods and properties:

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

The general rules for type inference for lambdas are as follows:

  • The lambda must contain the same number of parameters as the delegate type.
  • Each input parameter in the lambda must be implicitly convertible to its corresponding delegate parameter.
  • The return value of the lambda (if any) must be implicitly convertible to the delegate’s return type.

Note that lambda expressions in themselves don’t have a type because the common type system has no intrinsic concept of “lambda expression.” However, it’s sometimes convenient to speak informally of the “type” of a lambda expression. In these cases the type refers to the delegate type or Expression type to which the lambda expression is converted.

Variable scope in lambda expressions

Lambdas can refer to outer variables (see Anonymous methods) that are in scope in the method that defines the lambda expression, or in scope in the type that contains the lambda expression. Variables that are captured in this manner are stored for use in the lambda expression even if the variables would otherwise go out of scope and be garbage collected. An outer variable must be definitely assigned before it can be consumed in a lambda expression. The following example demonstrates these rules:

public static class VariableScopeWithLambdas
{
    public class VariableCaptureGame
    {
        internal Action<int> updateCapturedLocalVariable;
        internal Func<int, bool> isEqualToCapturedLocalVariable;

        public void Run(int input)
        {
            int j = 0;

            updateCapturedLocalVariable = x =>
            {
                j = x;
                bool result = j > input;
                Console.WriteLine($"{j} is greater than {input}: {result}");
            };

            isEqualToCapturedLocalVariable = x => x == j;

            Console.WriteLine($"Local variable before lambda invocation: {j}");
            updateCapturedLocalVariable(10);
            Console.WriteLine($"Local variable after lambda invocation: {j}");
        }
    }

    public static void Main()
    {  
        var game = new VariableCaptureGame();
        
        int gameInput = 5;
        game.Run(gameInput);

        int jTry = 10;
        bool result = game.isEqualToCapturedLocalVariable(jTry);
        Console.WriteLine($"Captured local variable is equal to {jTry}: {result}");

        int anotherJ = 3;
        game.updateCapturedLocalVariable(anotherJ);

        bool equalToAnother = game.isEqualToCapturedLocalVariable(anotherJ);
        Console.WriteLine($"Another lambda observes a new value of captured variable: {equalToAnother}");
    }
    // Output:
    // Local variable before lambda invocation: 0
    // 10 is greater than 5: True
    // Local variable after lambda invocation: 10
    // Captured local variable is equal to 10: True
    // 3 is greater than 5: False
    // Another lambda observes a new value of captured variable: True
}

The following rules apply to variable scope in lambda expressions:

  • A variable that is captured will not be garbage-collected until the delegate that references it becomes eligible for garbage collection.
  • Variables introduced within a lambda expression are not visible in the enclosing method.
  • A lambda expression cannot directly capture an inref, or out parameter from the enclosing method.
  • return statement in a lambda expression doesn’t cause the enclosing method to return.
  • A lambda expression cannot contain a gotobreak, or continue statement if the target of that jump statement is outside the lambda expression block. It’s also an error to have a jump statement outside the lambda expression block if the target is inside the block.