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".