Check if every List element matches the predicate conditions in C#

To check if every List element matches the predicate conditions in C#, you can use the List<T>.TrueForAll() method. This method takes a predicate delegate and returns true if all elements in the list satisfy the condition, or false if any element fails the test.

Syntax

Following is the syntax for the TrueForAll() method −

public bool TrueForAll(Predicate<T> match)

Parameters

• match: A delegate that defines the conditions to check against the elements. It takes an element of type T and returns a boolean.

Return Value

Returns true if every element in the list matches the predicate conditions, otherwise false. Returns true for an empty list.

Using TrueForAll() with Custom Predicate Method

You can define a custom method as a predicate to check specific conditions −

Example - Checking Divisibility by 10

using System;
using System.Collections.Generic;

public class Demo {
   private static bool IsMultipleOfTen(int i) {
      return ((i % 10) == 0);
   }
   
   public static void Main(String[] args) {
      List<int> list = new List<int>();
      list.Add(200);
      list.Add(215);
      list.Add(310);
      list.Add(500);
      list.Add(600);
      
      Console.WriteLine("List elements...");
      foreach (int i in list) {
         Console.WriteLine(i);
      }
      
      bool result = list.TrueForAll(IsMultipleOfTen);
      Console.WriteLine("All elements divisible by 10: " + result);
   }
}

The output of the above code is −

List elements...
200
215
310
500
600
All elements divisible by 10: False

Example - All Elements Match Condition

using System;
using System.Collections.Generic;

public class Demo {
   private static bool IsMultipleOfTen(int i) {
      return ((i % 10) == 0);
   }
   
   public static void Main(String[] args) {
      List<int> list = new List<int>();
      list.Add(100);
      list.Add(200);
      list.Add(300);
      list.Add(400);
      list.Add(500);
      list.Add(600);
      list.Add(700);
      list.Add(800);
      list.Add(900);
      list.Add(1000);
      
      Console.WriteLine("List elements...");
      foreach (int i in list) {
         Console.WriteLine(i);
      }
      
      bool result = list.TrueForAll(IsMultipleOfTen);
      Console.WriteLine("All elements divisible by 10: " + result);
   }
}

The output of the above code is −

List elements...
100
200
300
400
500
600
700
800
900
1000
All elements divisible by 10: True

Using TrueForAll() with Lambda Expression

You can also use lambda expressions for inline predicate conditions −

Example

using System;
using System.Collections.Generic;

public class Demo {
   public static void Main(String[] args) {
      List<string> names = new List<string> { "Alice", "Bob", "Charlie", "David" };
      
      // Check if all names have length greater than 2
      bool allLongNames = names.TrueForAll(name => name.Length > 2);
      Console.WriteLine("All names longer than 2 characters: " + allLongNames);
      
      // Check if all names start with uppercase letter
      bool allCapitalized = names.TrueForAll(name => char.IsUpper(name[0]));
      Console.WriteLine("All names start with uppercase: " + allCapitalized);
      
      List<int> numbers = new List<int> { 2, 4, 6, 8, 10 };
      bool allEven = numbers.TrueForAll(num => num % 2 == 0);
      Console.WriteLine("All numbers are even: " + allEven);
   }
}

The output of the above code is −

All names longer than 2 characters: True
All names start with uppercase: True
All numbers are even: True

Conclusion

The List<T>.TrueForAll() method provides an efficient way to check if all elements in a list satisfy a given condition. You can use it with custom predicate methods or lambda expressions to validate data according to your specific requirements.