SelectMany for flattening in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this SelectMany example?

Consider the following C# code using LINQ's SelectMany method. What will be printed to the console?

C Sharp (C#)

var numbers = new List<List<int>> {
    new List<int> {1, 2},
    new List<int> {3, 4},
    new List<int> {5}
};

var flat = numbers.SelectMany(x => x);
foreach(var num in flat)
{
    Console.Write(num + " ");
}

A 1 3 5

B System.Collections.Generic.IEnumerable`1[System.Int32]

C 1 2 3 4 5

D 1 2 3 4

💡 Hint

SelectMany flattens nested collections into one sequence.

SelectMany takes each inner list and combines all elements into a single sequence. So all numbers from all inner lists are printed in order.

❓ Predict Output

intermediate

What does this SelectMany with index produce?

Look at this C# code using SelectMany with an index parameter. What is the output?

C Sharp (C#)

var data = new List<string[]> {
    new string[] {"a", "b"},
    new string[] {"c"},
    new string[] {"d", "e", "f"}
};

var result = data.SelectMany((arr, i) => arr.Select(s => s + i));
foreach(var s in result)
{
    Console.Write(s + " ");
}

A a0 b0 c1 d2 e2 f2

B a b c d e f

C a1 b1 c2 d3 e4 f5

D a0 b1 c2 d3 e4 f5

💡 Hint

The index i is the position of the outer array in the list.

SelectMany passes the index of each inner array. Each string is concatenated with that index, so first array elements get '0', second '1', third '2'.

🔧 Debug

advanced

Why does this SelectMany code throw an exception?

This code throws an exception at runtime. What is the cause?

C Sharp (C#)

var list = new List<List<int>> {
    new List<int> {1, 2},
    null,
    new List<int> {3}
};

var flat = list.SelectMany(x => x);
foreach(var n in flat)
{
    Console.Write(n + " ");
}

A Output: 1 2 3

B InvalidOperationException because SelectMany expects non-empty lists

C Compile-time error due to null in list

D NullReferenceException because one inner list is null

💡 Hint

Check what happens when SelectMany tries to iterate a null collection.

SelectMany tries to iterate each inner list. When it reaches the null element, it throws NullReferenceException because you cannot iterate null.

📝 Syntax

advanced

Which option correctly uses SelectMany to flatten and filter?

You want to flatten a list of integer arrays and keep only even numbers. Which code snippet does this correctly?

A var result = list.SelectMany(x => x.Filter(n => n % 2 == 0));

B var result = list.SelectMany(x => x.Where(n => n % 2 == 0));

C var result = list.SelectMany(x => x.Select(n => n % 2 == 0));

D var result = list.SelectMany(x => n => n % 2 == 0);

💡 Hint

Use Where inside SelectMany to filter elements.

Option B correctly uses Where to filter even numbers inside each inner collection before flattening. Others have syntax errors or wrong methods.

🚀 Application

expert

How many items are in the flattened list?

Given this nested list, how many integers will be in the flattened list after using SelectMany?

C Sharp (C#)

var nested = new List<List<int>> {
    new List<int> {1, 2, 3},
    new List<int> {},
    new List<int> {4, 5},
    new List<int> {6}
};

var flat = nested.SelectMany(x => x).ToList();
int count = flat.Count;

A 6

B 5

C 7

D 4

💡 Hint

Count all elements in all inner lists combined.

The inner lists have 3 + 0 + 2 + 1 elements = 6 total elements after flattening.