StringBuilder methods and performance in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of StringBuilder Append and Insert

What is the output of this C# code snippet?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder("Hello");
        sb.Append(" World");
        sb.Insert(5, ",");
        Console.WriteLine(sb.ToString());
    }
}

A Hello, World

B Hello World,

C Hello World

D Hello,World

💡 Hint

Remember that Insert adds characters at the specified index without removing existing ones.

The StringBuilder starts with "Hello". Append adds " World" making "Hello World". Insert at index 5 adds a comma after "Hello", resulting in "Hello, World".

❓ Predict Output

intermediate

StringBuilder Replace method effect

What will be printed by this code?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder("abracadabra");
        sb.Replace("a", "o", 3, 5);
        Console.WriteLine(sb.ToString());
    }
}

A abrocodobro

B abrocodobra

C obrocodobro

D obrocadobra

💡 Hint

Replace only affects the substring starting at index 3 for length 5.

Replace("a", "o", 3, 5) changes 'a' to 'o' only in substring from index 3 to 7. The original string is "abracadabra". The substring "acada" (index 3 to 7) has 'a's replaced with 'o's resulting in "ocodo". So the final string is "abrocodobra".

❓ Predict Output

advanced

Performance difference between string concatenation and StringBuilder

What is the expected output of this code snippet?

C Sharp (C#)

using System;
using System.Text;
using System.Diagnostics;

class Program {
    static void Main() {
        int n = 10000;
        Stopwatch sw = new Stopwatch();

        sw.Start();
        string s = "";
        for (int i = 0; i < n; i++) {
            s += i.ToString();
        }
        sw.Stop();
        Console.WriteLine("String concat time: " + sw.ElapsedMilliseconds + "ms");

        sw.Reset();
        sw.Start();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < n; i++) {
            sb.Append(i.ToString());
        }
        string result = sb.ToString();
        sw.Stop();
        Console.WriteLine("StringBuilder time: " + sw.ElapsedMilliseconds + "ms");
    }
}

A String concat time: much greater than StringBuilder time

B String concat time: much less than StringBuilder time

C Both times are about the same

D Code throws an exception due to memory overflow

💡 Hint

String concatenation creates many intermediate strings, while StringBuilder modifies in place.

String concatenation inside a loop is slow because it creates new strings each time. StringBuilder is optimized for many appends and is much faster.

❓ Predict Output

advanced

Effect of Capacity and Length on StringBuilder

What will be the output of this code?

C Sharp (C#)

using System;
using System.Text;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder(5);
        Console.WriteLine(sb.Capacity);
        sb.Append("HelloWorld");
        Console.WriteLine(sb.Capacity);
        sb.Length = 5;
        Console.WriteLine(sb.ToString());
    }
}

A

5
20
HelloWorld

B

5
10
Hello

C

5
16
Hello

D

5
16
HelloWorld

💡 Hint

Capacity grows automatically when exceeded. Length truncates the string.

Initial capacity is 5. After appending "HelloWorld" (10 chars), capacity grows to 16 (at least double the previous capacity). Setting Length to 5 truncates to "Hello".

❓ Predict Output

expert

StringBuilder Thread Safety and Exception

What happens when this code runs?

C Sharp (C#)

using System;
using System.Text;
using System.Threading.Tasks;

class Program {
    static void Main() {
        StringBuilder sb = new StringBuilder();
        Parallel.For(0, 1000, i => {
            sb.Append(i);
        });
        Console.WriteLine(sb.Length);
    }
}

A Prints a number close to 2893 without error

B Throws a System.ArgumentOutOfRangeException

C Throws a System.IndexOutOfRangeException

D Throws a System.InvalidOperationException

💡 Hint

StringBuilder is not thread-safe for concurrent writes.

StringBuilder is not safe to use from multiple threads without synchronization. Concurrent Append calls cause InvalidOperationException.