Lambda expressions and anonymous functions

You use a lambda expression to create an anonymous function. Use the lambda declaration operator => to separate the lambda's parameter list from its body. A lambda expression can be of any of the following two forms:

• Expression lambda that has an expression as its body:

  (input-parameters) => expression
  

• Statement lambda that has a statement block as its body:

  (input-parameters) => { <sequence-of-statements> }
  

To create a lambda expression, you specify input parameters (if any) on the left side of the lambda operator and an expression or a statement block on the other side.

Any lambda expression can be converted to a delegate type. The types of its parameters and return value define the delegate type to which a lambda expression can be converted. If a lambda expression doesn't return a value, it can be converted to one of the Action delegate types; otherwise, it can be converted to one of the Func delegate types. For example, a lambda expression that has two parameters and returns no value can be converted to an Action<T1,T2> delegate. A lambda expression that has one parameter and returns a value can be converted to a Func<T,TResult> delegate. In the following example, the lambda expression x => x * x, which specifies a parameter named x and returns the value of x squared, is assigned to a variable of a delegate type:

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

Expression lambdas can also be converted to the expression tree types, as the following example shows:

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

You use lambda expressions in any code that requires instances of delegate types or expression trees. One example is the argument to the Task.Run(Action) method to pass the code that should be executed in the background. You can also use lambda expressions when you write LINQ in C#, as the following example shows:

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, for example in LINQ to Objects and LINQ to XML, the parameter is a delegate type System.Func<T,TResult>. When you call the Queryable.Select method in the System.Linq.Queryable class, for example in LINQ to SQL, the parameter type is an expression tree type Expression<Func<TSource,TResult>>. In both cases, you can use the same lambda expression to specify the parameter value. That makes the two Select calls to look similar although in fact the type of objects created from the lambdas is different.

Expression lambdas

A lambda expression with an expression on the right side of the => operator is called an expression lambda. An expression lambda returns the result of the expression and takes the following basic form:

(input-parameters) => expression

The body of an expression lambda can consist of a method call. However, if you're creating expression trees that are evaluated outside the context of the .NET Common Language Runtime (CLR), such as in SQL Server, you shouldn't use method calls in lambda expressions. The methods have no meaning outside the context of the .NET Common Language Runtime (CLR).

Statement lambdas

A statement lambda resembles an expression lambda except that its statements are enclosed in braces:

(input-parameters) => { <sequence-of-statements> }

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!

You can't use statement lambdas to create expression trees.

Input parameters of a lambda expression

You enclose input parameters of a lambda expression in parentheses. Specify zero input parameters with empty parentheses:

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

If a lambda expression has only one input parameter, parentheses are optional:

Func<double, double> cube = x => x * x * x;

Two or more input parameters are separated by commas:

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

Sometimes the compiler can't infer the types of input parameters. 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.

You can use discards to specify two or more input parameters of a lambda expression that aren't used in the expression:

Func<int, int, int> constant = (_, _) => 42;

Lambda discard parameters can be useful when you use a lambda expression to provide an event handler.

Beginning with C# 12, you can provide default values for parameters on lambda expressions. The syntax and the restrictions on default parameter values are the same as for methods and local functions. The following example declares a lambda expression with a default parameter, then calls it once using the default and once with two explicit parameters:

var IncrementBy = (int source, int increment = 1) => source + increment;

Console.WriteLine(IncrementBy(5)); // 6
Console.WriteLine(IncrementBy(5, 2)); // 7

You can also declare lambda expressions with params arrays or collections as parameters:

var sum = (params IEnumerable<int> values) =>
{
    int sum = 0;
    foreach (var value in values) 
        sum += value;
    
    return sum;
};

var empty = sum();
Console.WriteLine(empty); // 0

var sequence = new[] { 1, 2, 3, 4, 5 };
var total = sum(sequence);
Console.WriteLine(total); // 15

As part of these updates, when a method group that has a default parameter is assigned to a lambda expression, that lambda expression also has the same default parameter. A method group with a params collection parameter can also be assigned to a lambda expression.

Lambda expressions with default parameters or params collections as parameters don't have natural types that correspond to Func<> or Action<> types. However, you can define delegate types that include default parameter values:

delegate int IncrementByDelegate(int source, int increment = 1);
delegate int SumDelegate(params int[] values);
delegate int SumCollectionDelegate(params IEnumerable<int> values);

Or, you can use implicitly typed variables with var declarations to define the delegate type. The compiler synthesizes the correct delegate type.

For more information about default parameters on lambda expressions, see the feature spec for default parameters on lambda expressions.

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

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 Item1, Item2, and so on. 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 Tuple types.

Lambdas with the standard query operators

LINQ to Objects, among other implementations, has 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 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 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 finds 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

You don't use lambda expressions directly in query expressions, but you can use them in method calls within query expressions, as the following example shows:

var numberSets = new List<int[]>
{
    new[] { 1, 2, 3, 4, 5 },
    new[] { 0, 0, 0 },
    new[] { 9, 8 },
    new[] { 1, 0, 1, 0, 1, 0, 1, 0 }
};

var setsWithManyPositives = 
    from numberSet in numberSets
    where numberSet.Count(n => n > 0) > 3
    select numberSet;

foreach (var numberSet in setsWithManyPositives)
{
    Console.WriteLine(string.Join(" ", numberSet));
}
// Output:
// 1 2 3 4 5
// 1 0 1 0 1 0 1 0

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're 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.

Natural type of a lambda expression

A lambda expression in itself doesn'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. That informal "type" refers to the delegate type or Expression type to which the lambda expression is converted.

Beginning with C# 10, a lambda expression can have a natural type. Instead of forcing you to declare a delegate type, such as Func<...> or Action<...> for a lambda expression, the compiler can infer the delegate type from the lambda expression. For example, consider the following declaration:

var parse = (string s) => int.Parse(s);

The compiler can infer parse to be a Func<string, int>. The compiler chooses an available Func or Action delegate, if a suitable one exists. Otherwise, it synthesizes a delegate type. For example, the delegate type is synthesized if the lambda expression has ref parameters. When a lambda expression has a natural type, it can be assigned to a less explicit type, such as System.Object or System.Delegate:

object parse = (string s) => int.Parse(s);   // Func<string, int>
Delegate parse = (string s) => int.Parse(s); // Func<string, int>

Method groups (that is, method names without parameter lists) with exactly one overload have a natural type:

var read = Console.Read; // Just one overload; Func<int> inferred
var write = Console.Write; // ERROR: Multiple overloads, can't choose

If you assign a lambda expression to System.Linq.Expressions.LambdaExpression, or System.Linq.Expressions.Expression, and the lambda has a natural delegate type, the expression has a natural type of System.Linq.Expressions.Expression<TDelegate>, with the natural delegate type used as the argument for the type parameter:

LambdaExpression parseExpr = (string s) => int.Parse(s); // Expression<Func<string, int>>
Expression parseExpr = (string s) => int.Parse(s);       // Expression<Func<string, int>>

Not all lambda expressions have a natural type. Consider the following declaration:

var parse = s => int.Parse(s); // ERROR: Not enough type info in the lambda

The compiler can't infer a parameter type for s. When the compiler can't infer a natural type, you must declare the type:

Func<string, int> parse = s => int.Parse(s);

Explicit return type

Typically, the return type of a lambda expression is obvious and inferred. For some expressions that doesn't work:

var choose = (bool b) => b ? 1 : "two"; // ERROR: Can't infer return type

Beginning with C# 10, you can specify the return type of a lambda expression before the input parameters. When you specify an explicit return type, you must parenthesize the input parameters:

var choose = object (bool b) => b ? 1 : "two"; // Func<bool, object>

Attributes

Beginning with C# 10, you can add attributes to a lambda expression and its parameters. The following example shows how to add attributes to a lambda expression:

Func<string?, int?> parse = [ProvidesNullCheck] (s) => (s is not null) ? int.Parse(s) : null;

You can also add attributes to the input parameters or return value, as the following example shows:

var concat = ([DisallowNull] string a, [DisallowNull] string b) => a + b;
var inc = [return: NotNullIfNotNull(nameof(s))] (int? s) => s.HasValue ? s++ : null;

As the preceding examples show, you must parenthesize the input parameters when you add attributes to a lambda expression or its parameters.

Capture of outer variables and variable scope in lambda expressions

Lambdas can refer to outer variables. These outer variables are the variables 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 won't be garbage-collected until the delegate that references it becomes eligible for garbage collection.
• Variables introduced within a lambda expression aren't visible in the enclosing method.
• A lambda expression can't directly capture an in, ref, or out parameter from the enclosing method.
• A return statement in a lambda expression doesn't cause the enclosing method to return.
• A lambda expression can't contain a goto, break, 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.

You can apply the static modifier to a lambda expression to prevent unintentional capture of local variables or instance state by the lambda:

Func<double, double> square = static x => x * x;

A static lambda can't capture local variables or instance state from enclosing scopes, but can reference static members and constant definitions.

C# language specification

For more information, see the Anonymous function expressions section of the C# language specification.

For more information about these features, see the following feature proposal notes:

• Lambda discard parameters
• Static anonymous functions
• Lambda improvements (C# 10)

See also

• Use local function instead of lambda (style rule IDE0039)
• C# operators and expressions
• LINQ (Language-Integrated Query)
• Expression trees
• Local functions vs. lambda expressions
• LINQ sample queries
• XQuery sample
• 101 LINQ samples