Predicate delegate type in C Sharp (C#) - Mini Project: Build & Apply

Using Predicate Delegate Type in C#

📖 Scenario: You are working on a simple program to filter a list of numbers. You want to find which numbers are even using a special kind of function called a Predicate. This helps you check each number and decide if it meets your rule.

🎯 Goal: Build a C# program that uses a Predicate<int> delegate to find all even numbers from a list of integers.

📋 What You'll Learn

Create a list of integers with exact values: 1, 2, 3, 4, 5, 6

Create a Predicate<int> delegate called isEven that checks if a number is even

Use the FindAll method with the isEven predicate to get all even numbers

Print the resulting list of even numbers separated by spaces

💡 Why This Matters

🌍 Real World

Filtering lists based on conditions is common in apps like shopping filters, contact lists, or game scores.

💼 Career

Understanding Predicate delegates helps you write clean, reusable code for filtering data in C# applications.

1

Create the list of numbers

Create a List<int> called numbers with these exact values: 1, 2, 3, 4, 5, 6.

C Sharp (C#)

using System.Collections.Generic;

// Create the list of numbers
// Your code here

Hint

Use new List<int> { 1, 2, 3, 4, 5, 6 } to create the list.

2

Create the Predicate delegate

Create a Predicate<int> delegate called isEven that returns true if a number is even (divisible by 2), otherwise false.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };

        // Create the Predicate delegate called isEven
        // Your code here
    }
}

Hint

Use a lambda expression like n => n % 2 == 0 to define the predicate.

3

Use FindAll with the Predicate

Use the FindAll method on numbers with the isEven predicate to create a new list called evenNumbers containing only even numbers.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };

        Predicate isEven = n => n % 2 == 0;

        // Use FindAll with isEven to get evenNumbers
        // Your code here
    }
}

Hint

Call numbers.FindAll(isEven) to get the filtered list.

4

Print the even numbers

Print the numbers in evenNumbers separated by spaces using Console.WriteLine and string.Join.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List numbers = new List { 1, 2, 3, 4, 5, 6 };

        Predicate isEven = n => n % 2 == 0;

        List evenNumbers = numbers.FindAll(isEven);

        // Print the evenNumbers separated by spaces
        // Your code here
    }
}

Hint

Use Console.WriteLine(string.Join(" ", evenNumbers)) to print the list.

Practice

1.

What does a Predicate<T> delegate represent in C#?

easy

A. A method that takes a parameter of type T and returns a boolean indicating a condition.

B. A method that takes no parameters and returns void.

C. A method that takes two parameters and returns an integer.

D. A method that returns a string representation of an object.

Solution

1. Step 1: Understand the Predicate delegate signature

   A Predicate delegate always takes one parameter of type T and returns a boolean value.

2. Step 2: Match the description to the options

   A method that takes a parameter of type T and returns a boolean indicating a condition correctly describes this signature as a method taking T and returning bool.

3. Final Answer:

   A method that takes a parameter of type T and returns a boolean indicating a condition. -> Option A

4. Quick Check:

   Predicate<T> = method(T) returns bool [OK]

Hint: Predicate always returns true or false for a given input [OK]

Common Mistakes:

• Confusing Predicate with Action or Func delegates
• Thinking Predicate returns void or other types
• Assuming Predicate takes multiple parameters

2.

Which of the following is the correct syntax to declare a Predicate delegate for integers?

?

easy

A. Predicate<int> isPositive = x => x > 0;

B. Predicate<int> isPositive = (int x) => { return x; };

C. Predicate<int> isPositive = x => x + 1;

D. Predicate<int> isPositive = () => true;

Solution

1. Step 1: Check the lambda expression return type

   Predicate<int> expects a method returning bool. Predicate<int> isPositive = x => x > 0; returns x > 0, which is bool.

2. Step 2: Verify parameter and return correctness

   Predicate<int> isPositive = (int x) => { return x; }; returns x (int), not bool. Predicate<int> isPositive = x => x + 1; returns x + 1 (int), not bool. Predicate<int> isPositive = () => true; has no parameter but Predicate requires one.

3. Final Answer:

   Predicate<int> isPositive = x => x > 0; -> Option A

4. Quick Check:

   Predicate<int> = int input => bool output [OK]

Hint: Predicate must return bool, not int or void [OK]

Common Mistakes:

• Returning int instead of bool
• Missing parameter in lambda
• Using incorrect lambda syntax

3.

What will be the output of the following C# code?

using System;
class Program {
    static void Main() {
        Predicate<string> isLong = s => s.Length > 5;
        string[] words = {"apple", "banana", "pear"};
        int count = 0;
        foreach (var word in words) {
            if (isLong(word)) count++;
        }
        Console.WriteLine(count);
    }
}

medium

A. 3

B. 2

C. 1

D. 0

Solution

1. Step 1: Understand the Predicate condition

   The predicate checks if the string length is greater than 5.

2. Step 2: Count words with length > 5

   "apple" length 5 (false), "banana" length 6 (true), "pear" length 4 (false). Only "banana" passes.

3. Final Answer:

   2 -> Option B

4. Quick Check:

   Words with length > 5 are "banana" only, count = 1 [Incorrect]

Hint: Count items where predicate returns true [OK]

Common Mistakes:

• Counting words with length equal to 5
• Misreading predicate condition
• Assuming all words pass the test

4.

Identify the error in this code using a Predicate delegate:

Predicate<int> isEven = n => n % 2 == 0;
int[] numbers = {1, 2, 3, 4};
int evenCount = 0;
foreach (int num in numbers) {
    if (isEven) evenCount++;
}
Console.WriteLine(evenCount);

medium

A. The Predicate delegate is not assigned correctly.

B. The foreach loop variable should be declared as var.

C. The if condition should call the delegate with an argument.

D. The Console.WriteLine statement is missing parentheses.

Solution

1. Step 1: Check how Predicate is used in if condition

   The code uses if (isEven) which checks the delegate itself, not its result.

2. Step 2: Correct usage requires calling the delegate with an argument

   It should be if (isEven(num)) to evaluate the condition for each number.

3. Final Answer:

   The if condition should call the delegate with an argument. -> Option C

4. Quick Check:

   Call Predicate with input: isEven(num) [OK]

Hint: Always call Predicate with an argument inside if [OK]

Common Mistakes:

• Using delegate name without parentheses
• Confusing delegate variable with method call
• Ignoring parameter requirement

5.

You want to filter a list of integers to keep only positive even numbers using a Predicate delegate. Which of the following implementations correctly combines these conditions?

hard

A. Predicate<int> isPositiveEven = x => x % 2 == 0;

B. Predicate<int> isPositiveEven = x => x > 0 || x % 2 == 0;

C. Predicate<int> isPositiveEven = x => x < 0 && x % 2 == 0;

D. Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0;

Solution

1. Step 1: Understand the filtering requirement

   We want numbers that are both positive and even.

2. Step 2: Analyze each option's condition

   Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0; uses AND (&&) to require both conditions true. Predicate<int> isPositiveEven = x => x > 0 || x % 2 == 0; uses OR (||), allowing positives or evens, which is incorrect. Predicate<int> isPositiveEven = x => x < 0 && x % 2 == 0; checks for negative and even, wrong sign. Predicate<int> isPositiveEven = x => x % 2 == 0; checks only even, missing positivity.

3. Final Answer:

   Predicate<int> isPositiveEven = x => x > 0 && x % 2 == 0; -> Option D

4. Quick Check:

   Use AND to combine conditions for filtering [OK]

Hint: Use && to combine multiple conditions in Predicate [OK]

Common Mistakes:

• Using || instead of && for combined conditions
• Checking wrong sign for positivity
• Ignoring one of the required conditions