Reading text files in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to read all lines from a file into a string array.

C Sharp (C#)

string[] lines = System.IO.File.[1]("example.txt");

Drag options to blanks, or click blank then click option'

AGetLines

BReadAllLines

CReadFile

DReadText

Attempts:

3 left

💡 Hint

Common Mistakes

Using a method that does not exist like ReadText or GetLines.

Confusing ReadAllLines with ReadAllText which returns a single string.

The method ReadAllLines reads all lines from a text file and returns them as a string array.

2fill in blank

medium

Complete the code to read the entire content of a file as a single string.

C Sharp (C#)

string content = System.IO.File.[1]("example.txt");

Drag options to blanks, or click blank then click option'

AReadAllText

BReadAllLines

CReadTextFile

DReadFileContent

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAllLines which returns an array of strings.

Using non-existent methods like ReadTextFile.

The method ReadAllText reads the entire file content as one string.

3fill in blank

hard

Fix the error in the code to open a file and read its first line using StreamReader.

C Sharp (C#)

using var reader = new System.IO.StreamReader("example.txt");
string firstLine = reader.[1]();

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AReadAllLines

BRead

CReadToEnd

DReadLine

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAllLines which is not a StreamReader method.

Using ReadToEnd which reads the whole file.

The ReadLine method reads the next line from the StreamReader.

4fill in blank

hard

Fill both blanks to read all lines from a file and print only lines longer than 5 characters.

C Sharp (C#)

string[] lines = System.IO.File.[1]("example.txt");
foreach (var line in lines)
{
    if (line.[2] > 5)
    {
        Console.WriteLine(line);
    }
}

Drag options to blanks, or click blank then click option'

AReadAllLines

BLength

CCount

DReadAllText

Attempts:

3 left

💡 Hint

Common Mistakes

Using ReadAllText which returns a single string, not an array.

Using Count which is not a string property.

ReadAllLines reads all lines into an array. Length gives the number of characters in a string.

5fill in blank

hard

Fill all three blanks to create a dictionary with words as keys and their lengths as values, only for words longer than 3 characters.

C Sharp (C#)

var words = new List<string> { "apple", "cat", "banana", "dog" };
var wordLengths = words.ToDictionary([1] => [2], [3] => [3].Length);
var filtered = wordLengths.Where(kv => kv.Value > 3).ToDictionary(kv => kv.Key, kv => kv.Value);

Drag options to blanks, or click blank then click option'

Aword

Bw

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names for key and value selectors.

Not using the Length property for the value.

We use a lambda with a variable name for the key selector and value selector. Both use the same variable w. The key is w and the value is w.Length.

Practice

1. What does the File.ReadAllLines method do in C#?

easy

A. Reads the entire file content as a single string.

B. Reads all lines from a text file and returns them as a string array.

C. Writes lines to a text file.

D. Deletes the specified text file.

Solution

1. Step 1: Understand the method purpose

   File.ReadAllLines reads a text file and returns each line as an element in a string array.

2. Step 2: Compare with other methods

   File.ReadAllText returns the whole file as one string, not an array of lines.

3. Final Answer:

   Reads all lines from a text file and returns them as a string array. -> Option B

4. Quick Check:

   ReadAllLines = string array [OK]

Hint: ReadAllLines returns array of lines, not one big string [OK]

Common Mistakes:

• Confusing ReadAllLines with ReadAllText
• Thinking it writes to a file
• Assuming it deletes files

2. Which of the following is the correct syntax to read all text from a file named "data.txt" using File.ReadAllText?

easy

A. string content = File.ReadAllText("data.txt");

B. string content = File.ReadAllLines("data.txt");

C. string[] content = File.ReadAllText("data.txt");

D. string[] content = File.ReadAllLines("data.txt");

Solution

1. Step 1: Identify method return types

   File.ReadAllText returns a single string, so the variable must be string.

2. Step 2: Match syntax with variable type

   string content = File.ReadAllText("data.txt"); uses string variable with ReadAllText correctly. Options B and D mismatch method and variable types. string[] content = File.ReadAllText("data.txt"); tries to assign string to string array.

3. Final Answer:

   string content = File.ReadAllText("data.txt"); -> Option A

4. Quick Check:

   ReadAllText returns string, so variable is string [OK]

Hint: Match method return type with variable type [OK]

Common Mistakes:

• Assigning ReadAllText to string array
• Using ReadAllLines but expecting string
• Wrong variable type for method

3. What will be the output of this C# code if the file "example.txt" contains three lines: "Hello", "World", "!"?

string[] lines = File.ReadAllLines("example.txt");
Console.WriteLine(lines.Length);

medium

A. 3

B. Hello World !

C. 1

D. Error at runtime

Solution

1. Step 1: Understand ReadAllLines output

   The method returns an array with one element per line in the file. Here, 3 lines means array length is 3.

2. Step 2: Analyze Console.WriteLine output

   Printing lines.Length outputs the number of lines, which is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Array length = number of lines = 3 [OK]

Hint: ReadAllLines length equals number of lines in file [OK]

Common Mistakes:

• Expecting content printed instead of length
• Confusing ReadAllText with ReadAllLines
• Assuming runtime error without cause

4. Identify the error in this code snippet that reads all lines from "notes.txt":

string[] lines = File.ReadAllLines(notes.txt);
foreach (string line in lines)
{
    Console.WriteLine(line);
}

medium

A. Console.WriteLine cannot print strings.

B. File.ReadAllLines returns a string, not string array.

C. foreach loop syntax is incorrect.

D. Missing quotes around the file name in ReadAllLines.

Solution

1. Step 1: Check file path syntax

   The file name must be a string literal, so it needs quotes: "notes.txt".

2. Step 2: Verify other code parts

   File.ReadAllLines returns string array, foreach syntax is correct, and Console.WriteLine can print strings.

3. Final Answer:

   Missing quotes around the file name in ReadAllLines. -> Option D

4. Quick Check:

   File path must be in quotes [OK]

Hint: File names must be in quotes in method calls [OK]

Common Mistakes:

• Forgetting quotes around file path
• Thinking ReadAllLines returns string
• Misunderstanding foreach syntax

5. You want to read a text file and count how many lines contain the word "error" (case insensitive). Which code snippet correctly does this?

hard

A. var lines = File.ReadAllLines("log.txt");
int count = 0;
foreach(var line in lines) {
if(line.Contains("error")) count++;
}

B. string content = File.ReadAllText("log.txt");
int count = content.Split('\n').Count(line => line.Contains("error"));

C. var lines = File.ReadAllLines("log.txt");
int count = lines.Count(line => line.ToLower().Contains("error"));

D. string[] lines = File.ReadAllText("log.txt").Split('\n');
int count = lines.Count(line => line.Contains("error"));

Solution

1. Step 1: Choose method to read lines

   File.ReadAllLines returns an array of lines, perfect for line-by-line processing.

2. Step 2: Count lines with "error" case-insensitive

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); converts each line to lowercase and checks if it contains "error", then counts matches using LINQ.

3. Step 3: Check other options

   string content = File.ReadAllText("log.txt"); int count = content.Split('\n').Count(line => line.Contains("error")); uses ReadAllText but misses case-insensitive check. var lines = File.ReadAllLines("log.txt"); int count = 0; foreach(var line in lines) { if(line.Contains("error")) count++; } misses case-insensitive check. string[] lines = File.ReadAllText("log.txt").Split('\n'); int count = lines.Count(line => line.Contains("error")); incorrectly assigns ReadAllText to string array without ToLower.

4. Final Answer:

   var lines = File.ReadAllLines("log.txt"); int count = lines.Count(line => line.ToLower().Contains("error")); -> Option C

5. Quick Check:

   Use ReadAllLines + ToLower + Count for case-insensitive search [OK]

Hint: Use ReadAllLines and ToLower for case-insensitive line checks [OK]

Common Mistakes:

• Ignoring case sensitivity
• Using ReadAllText but treating as array
• Not converting lines to lowercase