Params keyword for variable arguments in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of method using params keyword

What is the output of this C# program?

C Sharp (C#)

using System;
class Program {
    static void PrintNumbers(params int[] numbers) {
        foreach (var num in numbers) {
            Console.Write(num + " ");
        }
    }
    static void Main() {
        PrintNumbers(1, 2, 3);
    }
}

A System.Int32[]

B 1, 2, 3

C 1 2 3

D Error: params keyword not allowed

💡 Hint

Remember, params allows passing multiple arguments as an array.

The method PrintNumbers accepts any number of integers using params. It prints each number followed by a space, so the output is 1 2 3 .

❓ Predict Output

intermediate

Output when passing an array to a params parameter

What will this program print?

C Sharp (C#)

using System;
class Program {
    static void ShowItems(params string[] items) {
        foreach (var item in items) {
            Console.Write(item + ",");
        }
    }
    static void Main() {
        string[] fruits = {"apple", "banana"};
        ShowItems(fruits);
    }
}

A System.String[],

B apple,banana,

C apple, banana,

D Error: Cannot convert string[] to params string[]

💡 Hint

Passing an array to a params parameter passes all elements as arguments.

The array fruits is passed to ShowItems which treats it as multiple arguments. It prints each fruit followed by a comma, resulting in apple,banana,.

❓ Predict Output

advanced

Output when mixing normal and params parameters

What is the output of this program?

C Sharp (C#)

using System;
class Program {
    static void Display(string title, params int[] numbers) {
        Console.Write(title + ": ");
        foreach (var n in numbers) {
            Console.Write(n + " ");
        }
    }
    static void Main() {
        Display("Numbers", 5, 10, 15);
    }
}

A Numbers: 5 10 15

B Numbers: 5,10,15

C Numbers: System.Int32[]

D Error: params must be last parameter

💡 Hint

The params parameter must be last and collects all extra arguments.

The method prints the title then all numbers separated by spaces. The output is Numbers: 5 10 15 .

❓ Predict Output

advanced

What error occurs with params not last?

What error will this code produce?

C Sharp (C#)

class Program {
    static void Test(params int[] numbers, string label) {
    }
}

A Compile-time error: missing return type

B Runtime error: invalid params usage

C No error, compiles fine

D Compile-time error: params must be last parameter

💡 Hint

Check the position of the params parameter.

The params parameter must be the last in the method signature. Here it is not last, so the compiler gives an error.

🧠 Conceptual

expert

How many items in params array when called with no arguments?

Consider this method:

void PrintAll(params string[] items) { Console.WriteLine(items.Length); }

What will be printed when calling PrintAll() with no arguments?

A 0

B 1

C null

D Compile-time error

💡 Hint

Think about what params does when no arguments are passed.

When no arguments are passed, the params array is an empty array, so items.Length is 0.

Practice

1. What does the params keyword do in a C# method?

easy

A. Makes the method run faster by optimizing loops.

B. Allows the method to accept a variable number of arguments as an array.

C. Declares a method as asynchronous.

D. Specifies that the method returns multiple values.

Solution

1. Step 1: Understand the purpose of params

   The params keyword allows a method to accept zero or more arguments as a single array parameter.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated features: performance, async methods, and multiple return values, which params does not do.

3. Final Answer:

   Allows the method to accept a variable number of arguments as an array. -> Option B

4. Quick Check:

   params = variable arguments [OK]

Hint: Remember: params means flexible number of inputs [OK]

Common Mistakes:

• Thinking params changes method speed
• Confusing params with async keyword
• Believing params allows multiple return values

2. Which of the following is the correct syntax to declare a method using params in C#?

easy

A. public void PrintNumbers(params int numbers[]) { }

B. public void PrintNumbers(int params[] numbers) { }

C. public void PrintNumbers(int[] params numbers) { }

D. public void PrintNumbers(params int[] numbers) { }

Solution

1. Step 1: Recall correct params syntax

   The params keyword comes before the array type and parameter name: params int[] numbers.

2. Step 2: Identify incorrect placements

   Options B, C, and D place params incorrectly or use invalid syntax for arrays in C#.

3. Final Answer:

   public void PrintNumbers(params int[] numbers) { } -> Option D

4. Quick Check:

   params before array type [OK]

Hint: Put params right before the array type [OK]

Common Mistakes:

• Placing params after the type or variable name
• Using square brackets incorrectly
• Confusing params with pointer syntax

3. What is the output of this C# code?

public class Program {
  public static void PrintAll(params string[] words) {
    foreach (var word in words) {
      Console.Write(word + " ");
    }
  }
  public static void Main() {
    PrintAll("hello", "world", "!");
  }
}

medium

A. hello world !

B. hello world!

C. hello, world, !

D. Compilation error

Solution

1. Step 1: Understand method call with params

   The method PrintAll accepts any number of strings as an array and prints each with a space after it.

2. Step 2: Trace the output

   It prints "hello ", then "world ", then "! ", resulting in "hello world ! " with spaces after each word.

3. Final Answer:

   hello world ! -> Option A

4. Quick Check:

   Params prints each argument with space [OK]

Hint: Params passes arguments as array, loop prints each [OK]

Common Mistakes:

• Missing spaces between words
• Expecting commas instead of spaces
• Thinking it causes compile errors

4. Identify the error in this method declaration:

public void Show(params int[] numbers, string label) {
  Console.WriteLine(label);
}

medium

A. The 'params' parameter must be the last parameter in the method.

B. The 'params' keyword cannot be used with arrays.

C. The method must return a value when using 'params'.

D. The 'params' keyword requires the parameter to be optional.

Solution

1. Step 1: Check parameter order rules for params

   The params parameter must be the last one in the method signature.

2. Step 2: Identify the error

   Here, params int[] numbers is before string label, which is invalid.

3. Final Answer:

   The 'params' parameter must be the last parameter in the method. -> Option A

4. Quick Check:

   params must be last [OK]

Hint: Params must always be last parameter [OK]

Common Mistakes:

• Placing params before other parameters
• Thinking params can be anywhere
• Confusing params with optional parameters

5. You want to create a method SumAll that sums any number of integers passed to it. Which of these method declarations correctly uses params and returns the sum?

hard

A. public int SumAll(params int numbers) { return numbers.Length; }

B. public int SumAll(int[] numbers, params int n) { return numbers.Sum() + n; }

C. public int SumAll(params int[] numbers) { int sum = 0; foreach(var n in numbers) sum += n; return sum; }

D. public int SumAll(params int[] numbers) { return numbers[0]; }

Solution

1. Step 1: Check correct use of params and summing logic

   public int SumAll(params int[] numbers) { int sum = 0; foreach(var n in numbers) sum += n; return sum; } correctly declares params int[] numbers and sums all elements in a loop.

2. Step 2: Evaluate other options

   public int SumAll(int[] numbers, params int n) { return numbers.Sum() + n; } has invalid params int n after array; public int SumAll(params int numbers) { return numbers.Length; } uses params int numbers which is invalid; public int SumAll(params int[] numbers) { return numbers[0]; } returns only first element, not sum.

3. Final Answer:

   public int SumAll(params int[] numbers) { int sum = 0; foreach(var n in numbers) sum += n; return sum; } -> Option C

4. Quick Check:

   Params array summed with loop [OK]

Hint: Params must be array; sum with loop or LINQ [OK]

Common Mistakes:

• Using params with single int instead of array
• Placing params parameter incorrectly
• Returning only first element instead of sum