ZetCode

C# lambda expression

last modified November 3, 2020

C# lambda expression tutorial shows how to use lambda expressions in C#. C# tutorial is a comprehensive tutorial on C# language.

A lambda expression is an anonymous function not bound to an identifier. With lambda expressions, we can create more concise code.

A lambda expression has two forms:

(input-parameters) => expression

This form has an expression as its body.

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

This form has a statement block as its body.

C# lambda expression simple example

The following is a simple example with lambda expressions.

Program.cs
using System;

Func<int, int> square = x => x * x;
Func<int, int> cube = x => x * x * x;
Func<int, int> inc = x => x++;
Func<int, int, int> add = (x, y) => x + y;

Console.WriteLine(square(5));
Console.WriteLine(cube(5));
Console.WriteLine(inc(5));
Console.WriteLine(add(5, 7));

In the example, we define four functions.

Func<int, int> square = x => x * x;

A Func delegate refers to a lambda expression, which squares the input value. This is the closest equivalent of plain functions in other languages.

$ dotnet run
25
125
5
12

This is the output.

C# lambda statement

The lambda statement may have multiple statements, which are enclosed in {} braces.

Program.cs
using System;

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

greet("Pau");
greet("Lucia");

In the example, we use an Action delegate. On the right side of the equation, we have a lambda statement, which consists of two statements. The Action delegate takes one input parameter and does not return anything.

$ dotnet run
Hello Pau!
Hello Lucia!

This is the output.

C# lambda expression with arrays

In the following examples, we work with arrays.

Program.cs
using System;

int[] vals = { 1, -2, 3, 4, 0, -3, 2, 1, 3 };

var v1 = Array.FindIndex(vals, x => x == 3);
Console.WriteLine(v1);

var v2 = Array.FindLastIndex(vals, x => x == 3);
Console.WriteLine(v2);

var positive = Array.FindAll(vals, x => x > 0);
Console.WriteLine(string.Join(",", positive));

The array functions expect a predicate function, which is applied for all elements of the array.

var positive = Array.FindAll(vals, x => x > 0);

We pass the FindAll method a lambda expressions as its second parameter; the expression is a predicate which returns true for values greater than zero.

$ dotnet run
2
8
1,3,4,2,1,3

This is the output.

C# lambda expression with LINQ

We can use lambda expressions in many LINQ methods.

Program.cs
using System;
using System.Linq;
using System.Collections.Generic;

var vals = new List<int> { -1, 2, -2, 0, 3, 4, -5 };

var squared = vals.Select(x => x * x);
Console.WriteLine(string.Join(", ", squared));

var filtered = vals.Where(x => x > 0);
Console.WriteLine(string.Join(", ", filtered));

In the example, we use lambda expression in LINQ's Select and Where methods.

$ dotnet run
1, 4, 4, 0, 9, 16, 25
2, 3, 4

This is the output.

C# array of lambda expressions

In the following example, we create an array of lambda expressions.

Program.cs
using System;

var funs = new Func<int, int>[]
{
    x => x * x,
    x => ++x,
    x => --x
};

for (int i=0; i<6; i++) 
{
    Console.WriteLine(funs[0](i));
    Console.WriteLine(funs[1](i));
    Console.WriteLine(funs[2](i));
    Console.WriteLine();
}

In the example, we put three lambda expression in an array. Then in the for loop, we pass integers 0..5 to the three lambda expressions.

C# lambda expression discards

Since C# 9.0, we can use discards (_) as parameters of lambdas and anonymous methods.

Program.cs
using System;
using System.Windows.Forms;
using System.Drawing;

namespace QuitButton
{
    class MyForm : Form
    {
        private FlowLayoutPanel flowPanel;

        public MyForm()
        {
            InitComponents();
        }

        private void InitComponents()
        {
            Text = "Quit button";
            ClientSize = new Size(800, 450);

            flowPanel = new FlowLayoutPanel();

            flowPanel.Dock = DockStyle.Fill;
            flowPanel.BorderStyle = BorderStyle.FixedSingle;

            var button = new Button();
            button.Margin = new Padding(10, 10, 0, 0);

            button.Text = "Quit";
            button.AutoSize = true;
            button.Click += (_, _) => Close();

            flowPanel.Controls.Add(button);
            Controls.Add(flowPanel);

            CenterToScreen();
        }

        [STAThread]
        static void Main()
        {
            Application.SetHighDpiMode(HighDpiMode.SystemAware);
            Application.EnableVisualStyles();
            Application.Run(new MyForm());
        }
    }
}

The example is a simple GUI application created with Winforms; it works only on Windows.

button.Click += (_, _) => Close();

Since we do not work with the sender object and the event arguments, we can use discards.

In this tutorial, we have worked with lambda expressions in C#.

Read C# tutorial or list all C# tutorials.