Common string methods in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Common string methods

Start with a string

↓

Call a string method

↓

Method processes string

↓

Returns new string or value

↓

Use or display result

↓

End

This flow shows how you start with a string, call a method on it, get a result, and then use that result.

Execution Sample

C Sharp (C#)

string text = "Hello World";
string upper = text.ToUpper();
int length = text.Length;
string replaced = text.Replace("World", "C#");

This code shows common string methods: ToUpper, Length, and Replace.

Execution Table

Step	Code Line	Variable	Operation	Result
1	string text = "Hello World";	text	Assign string	"Hello World"
2	string upper = text.ToUpper();	upper	Convert to uppercase	"HELLO WORLD"
3	int length = text.Length;	length	Get string length	11
4	string replaced = text.Replace("World", "C#");	replaced	Replace substring	"Hello C#"

💡 All string methods executed, variables assigned with expected results.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
text	null	"Hello World"	"Hello World"	"Hello World"	"Hello World"
upper	null	null	"HELLO WORLD"	"HELLO WORLD"	"HELLO WORLD"
length	null	null	null	11	11
replaced	null	null	null	null	"Hello C#"

Key Moments - 2 Insights

Why does the original string 'text' not change after calling ToUpper or Replace?

Strings in C# are immutable, so methods like ToUpper and Replace return new strings without changing the original. See execution_table steps 2 and 4 where 'text' stays the same.

What does the Length property return?

Length returns the number of characters in the string. In step 3, 'length' is 11 because "Hello World" has 11 characters including the space.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the value of 'upper'?

A "hello world"

B "HELLO WORLD"

C "Hello World"

D null

💡 Hint

Check the 'Result' column at step 2 in the execution_table.

At which step does the variable 'length' get assigned a value?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for 'length' variable assignment in the execution_table.

If we changed the Replace method to replace "Hello" with "Hi", what would 'replaced' be at step 4?

A "Hi World"

B "Hello C#"

C "Hi C#"

D "Hello World"

💡 Hint

Replace changes only the specified substring. See step 4 for how Replace works.

Concept Snapshot

Common string methods in C#:
- ToUpper(): returns uppercase version
- Length: property for string length
- Replace(old, new): replaces substring
Strings are immutable; methods return new strings.

Full Transcript

This lesson shows common string methods in C#. We start with a string variable 'text' assigned "Hello World". Then we call ToUpper() to get 'upper' as "HELLO WORLD". Next, we get the length of 'text' which is 11. Finally, we replace "World" with "C#" to get 'replaced' as "Hello C#". The original string 'text' does not change because strings are immutable in C#. Each method returns a new string or value. This helps beginners see how string methods work step-by-step.

Practice

1. Which C# string method returns a new string with all characters converted to uppercase?

easy

A. Substring()

B. Trim()

C. Contains()

D. ToUpper()

Solution

1. Step 1: Understand the purpose of ToUpper()

   The ToUpper() method converts all letters in a string to uppercase and returns a new string.

2. Step 2: Compare with other methods

   Trim() removes spaces, Contains() checks for substring presence, Substring() extracts part of the string.

3. Final Answer:

   ToUpper() -> Option D

4. Quick Check:

   Uppercase conversion = ToUpper() [OK]

Hint: Uppercase all letters? Use ToUpper() method [OK]

Common Mistakes:

• Confusing ToUpper() with ToLower()
• Using Trim() to change case
• Thinking Contains() changes text

2. Which of the following is the correct syntax to check if the string text contains the word "hello"?

easy

A. Contains(text, "hello")

B. text.Contains = "hello"

C. text.Contains("hello")

D. text.Has("hello")

Solution

1. Step 1: Recall the correct method call syntax

   In C#, to check if a string contains another, use the instance method with parentheses: text.Contains("hello").

2. Step 2: Identify incorrect syntax

   Assignments (=) or wrong method names like Has() are invalid for this check.

3. Final Answer:

   text.Contains("hello") -> Option C

4. Quick Check:

   Method call with parentheses = Contains() [OK]

Hint: Use text.Contains("word") with parentheses [OK]

Common Mistakes:

• Using assignment instead of method call
• Wrong method name like Has()
• Passing parameters incorrectly

3. What is the output of the following code?

string s = "  Hello World  ";
string result = s.Trim().Substring(0, 5);
Console.WriteLine(result);

medium

A. " Hel"

B. "Hello"

C. "Hello "

D. "Hello World"

Solution

1. Step 1: Apply Trim() method

   Trim() removes spaces at the start and end, so " Hello World " becomes "Hello World".

2. Step 2: Apply Substring(0, 5)

   Substring(0, 5) extracts characters from index 0 to 4, which is "Hello".

3. Final Answer:

   "Hello" -> Option B

4. Quick Check:

   Trim + Substring(0,5) = "Hello" [OK]

Hint: Trim removes spaces; Substring extracts exact part [OK]

Common Mistakes:

• Not trimming before substring
• Counting spaces in substring
• Confusing substring length

4. Identify the error in this code snippet:

string s = "Example";
if(s.Contains("ex"))
{
    Console.WriteLine("Found");
}

medium

A. Contains() is case-sensitive, so "ex" won't match "Ex"

B. Missing semicolon after if statement

C. Contains() method does not exist for strings

D. Console.WriteLine syntax is incorrect

Solution

1. Step 1: Check Contains() behavior

   Contains() is case-sensitive, so "ex" does not match "Ex" in "Example".

2. Step 2: Verify syntax correctness

   Semicolons and method calls are correct; no syntax errors present.

3. Final Answer:

   Contains() is case-sensitive, so "ex" won't match "Ex" -> Option A

4. Quick Check:

   Contains() case matters = true [OK]

Hint: Remember Contains() is case-sensitive by default [OK]

Common Mistakes:

• Assuming Contains() ignores case
• Looking for syntax errors that don't exist
• Confusing method availability

5. You want to extract the domain name from an email string email = "[e-mail]". Which code correctly extracts "example" using common string methods?

hard

A. string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0];

B. string domain = email.Trim().ToUpper();

C. string domain = email.Contains("@example");

D. string domain = email.Substring(0, email.IndexOf('.'));

Solution

1. Step 1: Find position after '@'

   email.IndexOf('@') finds the '@' position; adding 1 moves to start of domain.

2. Step 2: Extract substring from domain start and split by '.'

   Substring gets "example.com", then Split('.')[0] gets "example".

3. Step 3: Check other options

   B changes case, C returns bool, D extracts wrong part before '@'.

4. Final Answer:

   string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0]; -> Option A

5. Quick Check:

   Extract domain by substring + split = string domain = email.Substring(email.IndexOf('@') + 1).Split('.')[0]; [OK]

Hint: Use IndexOf('@') + Substring + Split('.') to get domain [OK]

Common Mistakes:

• Using Contains() instead of extracting
• Trimming or changing case wrongly
• Extracting before '@' instead of after