All

This is a C# extension method. It tells us if all the elements in a collection match a certain condition. It returns true or false.

System.Linq

All() is part of the System.Linq namespace in .NET. We often want to call All() on the result of a query expression using other LINQ features.

An example

To begin, include the System.Linq namespace. Next, pass in a lambda expression to the All method. We pass 1 argument to the All() method, usually a lambda expression.

using System;
using System.Linq;

int[] array = { 10, 20, 30 };

// Are all elements >= 10? YES
bool a = array.All(element => element >= 10);

// Are all elements >= 20? NO
bool b = array.All(element => element >= 20);

// Are all elements < 40? YES
bool c = array.All(element => element < 40);

Console.WriteLine(a);
Console.WriteLine(b);
Console.WriteLine(c);
True
False
True

List, query example

We can use All() on a query expression to create more complex meanings. Here we use All() on a List generic type.

using System;
using System.Linq;
using System.Collections.Generic;

var colors = new List<string>() { "BLUE", "GREY", "white" };

// True if all strings of length 4 are in uppercase.
var result = (from color in colors
              where color.Length == 4
              select color).All(element => element == element.ToUpper());

Console.WriteLine("RESULT: {0}", result);
RESULT: True

Benefits

Why would we use All() instead of looping over elements and using an if-statement? The All method is more compact. And it can be part of a more complex LINQ statement.

As part of the LINQ extensions, All() serves a specific purpose. For arrays, we can also use the Array.TrueForAll method, which uses the same syntax but is a static method.