String concatenation behavior in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of concatenating strings and integers

What is the output of the following C# code?

C Sharp (C#)

string result = "Age: " + 20 + 5;
Console.WriteLine(result);

A Age: 205

B 25

C Compilation error

D Age: 25

💡 Hint

Remember how C# evaluates string concatenation with numbers from left to right.

In C#, when concatenating strings and numbers, the expression is evaluated left to right. "Age: " + 20 becomes "Age: 20" (string), then + 5 concatenates "5" resulting in "Age: 205".

❓ Predict Output

intermediate

Concatenation with parentheses changing order

What is the output of this C# code snippet?

C Sharp (C#)

string result = "Age: " + (20 + 5);
Console.WriteLine(result);

A Age: 25

B Age: 205

C 25

D Compilation error

💡 Hint

Parentheses change the order of operations.

The parentheses cause 20 + 5 to be evaluated first as integer addition (25), then concatenated with the string "Age: ", resulting in "Age: 25".

❓ Predict Output

advanced

Concatenation with null strings

What will be printed by this C# code?

C Sharp (C#)

string s1 = null;
string s2 = "Hello";
string result = s1 + s2;
Console.WriteLine(result == null ? "null" : result);

A null

B Compilation error

C Hello

D nullHello

💡 Hint

Check how null strings behave when concatenated with non-null strings.

In C#, null strings are treated as empty strings when concatenated. So null + "Hello" results in "Hello".

❓ Predict Output

advanced

Concatenation with StringBuilder and + operator

What is the output of this C# code?

C Sharp (C#)

var sb = new System.Text.StringBuilder();
sb.Append("Hi");
string result = sb + " there";
Console.WriteLine(result);

A Hi there

B Runtime error

C Compilation error

D System.Text.StringBuilder there

💡 Hint

Consider how + operator works with StringBuilder and string.

The + operator with StringBuilder and string calls ToString() on StringBuilder, then concatenates. So sb.ToString() is "Hi", concatenated with " there" results in "Hi there". But since sb is an object, the expression sb + " there" is interpreted as object + string, which calls ToString() on sb, resulting in "Hi" + " there" = "Hi there". However, in C# + operator is not defined for StringBuilder and string, so it calls ToString() on sb. So the output is "Hi there".

❓ Predict Output

expert

Concatenation with overloaded + operator in custom class

Given the following class and code, what is the output?

C Sharp (C#)

public class MyString {
    public string Value;
    public MyString(string val) { Value = val; }
    public static string operator +(MyString a, string b) => a.Value + b;
}

var ms = new MyString("Hello");
string result = ms + " World";
Console.WriteLine(result);

A Compilation error

B Hello World

C HelloWorld

D Runtime error

💡 Hint

Look at how the + operator is overloaded in the custom class.

The + operator is overloaded to concatenate the Value field with the string b, so ms + " World" returns "Hello World".

Practice

1. What does the + operator do when used between two strings in C#?

easy

A. It multiplies the two strings.

B. It subtracts the second string from the first.

C. It joins the two strings into one longer string.

D. It compares the two strings for equality.

Solution

1. Step 1: Understand the + operator with strings

   In C#, the + operator combines two strings by joining them end to end.

2. Step 2: Check other options

   Subtracting, multiplying, or comparing strings are not done with +. Those operations use other operators or methods.

3. Final Answer:

   It joins the two strings into one longer string. -> Option C

4. Quick Check:

   String + String = Joined String [OK]

Hint: Remember: + joins strings like glue [OK]

Common Mistakes:

• Thinking + subtracts or multiplies strings
• Confusing + with comparison operators
• Assuming + works only with numbers

2. Which of the following is the correct syntax to concatenate two strings str1 and str2 in C#?

easy

A. string result = str1 * str2;

B. string result = str1 + str2;

C. string result = str1 - str2;

D. string result = str1 / str2;

Solution

1. Step 1: Identify valid operators for string concatenation

   The + operator is used to join strings in C#. Other arithmetic operators like -, *, / are invalid for strings.

2. Step 2: Confirm syntax correctness

   The statement string result = str1 + str2; correctly concatenates and assigns the result.

3. Final Answer:

   string result = str1 + str2; -> Option B

4. Quick Check:

   Use + for string join [OK]

Hint: Use + to join strings, not arithmetic signs [OK]

Common Mistakes:

• Using - or * instead of + for strings
• Missing semicolon at end
• Trying to divide or multiply strings

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

string a = "Hello";
string b = "World";
string c = a + ", " + b + "!";
Console.WriteLine(c);

medium

A. Hello, World!

B. Hello World!

C. Hello,World!

D. HelloWorld!

Solution

1. Step 1: Analyze string concatenation parts

   The code joins "Hello", ", ", "World", and "!" in order, so the result is "Hello, World!".

2. Step 2: Confirm output of Console.WriteLine

   Console.WriteLine prints the combined string exactly as concatenated.

3. Final Answer:

   Hello, World! -> Option A

4. Quick Check:

   Strings + punctuation join as typed [OK]

Hint: Watch spaces and punctuation in concatenation [OK]

Common Mistakes:

• Missing spaces or commas in output
• Ignoring punctuation strings
• Confusing concatenation order

4. Identify the error in this code snippet:

string first = "Good";
string second = "Morning";
string message = first + second;
message += 5;
Console.WriteLine(message);

medium

A. Missing space between first and second strings.

B. Cannot add integer 5 to a string using += operator.

C. Variable 'message' is not declared.

D. Console.WriteLine syntax is incorrect.

Solution

1. Step 1: Check string concatenation and += usage

   The code concatenates "Good" and "Morning" without space, so message becomes "GoodMorning". Adding 5 converts 5 to string "5" and appends it, resulting in "GoodMorning5".

2. Step 2: Identify the main issue

   The code runs without error, but the missing space between words is a logical mistake causing output to look wrong.

3. Final Answer:

   Missing space between first and second strings. -> Option A

4. Quick Check:

   Check spaces when joining strings [OK]

Hint: Add spaces explicitly when joining words [OK]

Common Mistakes:

• Assuming += 5 causes error
• Ignoring missing spaces in output
• Thinking Console.WriteLine is wrong

5. Given a list of words string[] words = {"apple", "banana", "cherry"};, which code correctly concatenates them into a single comma-separated string using string.Concat?

hard

A. string result = string.Concat(words + ",");

B. string result = string.Concat(words[0] + words[1] + words[2]);

C. string result = string.Concat(words);

D. string result = string.Concat(words[0], ",", words[1], ",", words[2]);

Solution

1. Step 1: Understand string.Concat usage

   string.Concat can join multiple strings passed as arguments. To add commas, they must be separate arguments.

2. Step 2: Analyze each option

   string result = string.Concat(words[0], ",", words[1], ",", words[2]); passes each word and commas separately, correctly joining as "apple,banana,cherry". string result = string.Concat(words + ","); tries to add comma to array, invalid. string result = string.Concat(words); joins words without commas. string result = string.Concat(words[0] + words[1] + words[2]); concatenates words without commas inside Concat.

3. Final Answer:

   string result = string.Concat(words[0], ",", words[1], ",", words[2]); -> Option D

4. Quick Check:

   Pass each string and separator separately to Concat [OK]

Hint: Pass each string and comma separately to string.Concat [OK]

Common Mistakes:

• Trying to add commas inside array
• Passing array directly to Concat without separators
• Using + inside Concat argument incorrectly