Format specifiers for numbers and dates in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the format specifier "C" do when formatting numbers in C#?

The "C" specifier formats a number as currency, adding the currency symbol and two decimal places by default.

beginner

How do you format a DateTime to show only the year and month in C#?

Use the format specifier "yyyy-MM" with DateTime.ToString(), which shows the full year and two-digit month.

intermediate

What is the difference between "D" and "d" format specifiers for dates in C#?

"D" shows the long date pattern (e.g., "Monday, June 5, 2023"), while "d" shows the short date pattern (e.g., "6/5/2023").

beginner

How does the "N" format specifier format numbers in C#?

"N" formats numbers with thousands separators and two decimal places by default (e.g., "1,234.56").

beginner

What does the format specifier "F" do when formatting numbers?

"F" formats a number as a fixed-point number with a default of two decimal places (e.g., "1234.56"). You can specify decimals like "F3" for three decimals.

Which format specifier would you use to display a number as currency in C#?

A "F"

B "N"

C "D"

D "C"

The "C" specifier formats numbers as currency, adding the currency symbol and decimals.

What does the "d" format specifier show when formatting a DateTime?

A Short date pattern (e.g., 6/5/2023)

B Long date pattern (e.g., Monday, June 5, 2023)

C Time only

D Full date and time

"d" shows the short date pattern, which is a concise date format.

How would you format a number with thousands separators and two decimals in C#?

A "C"

B "F"

C "N"

D "P"

"N" adds thousands separators and formats with two decimals by default.

Which format specifier shows the full year and month in a DateTime string?

A "yyyy-MM"

B "d"

C "D"

D "F"

Using "yyyy-MM" custom format shows the full year and two-digit month.

What does the "F3" format specifier do when formatting a number?

A Formats as currency with 3 decimals

B Formats as fixed-point with 3 decimals

C Formats as number with thousands separators

D Formats as percentage with 3 decimals

"F3" formats a number as fixed-point with exactly 3 decimal places.

Explain how to format a number as currency and as a fixed-point number in C#.

Think about how money and precise decimals look.

Describe the difference between the "D" and "d" date format specifiers in C#.

One is detailed, the other is brief.

Practice

1. Which format specifier in C# is used to display a number as currency with the correct symbol and two decimal places?
decimal price = 123.456m;

easy

A. "N"

B. "C"

C. "P"

D. "D"

Solution

1. Step 1: Understand currency format specifier

   The "C" format specifier converts a number to a currency string with the currency symbol and two decimals by default.

2. Step 2: Compare other specifiers

   "N" formats as number with commas, "P" as percentage, "D" as decimal integer. Only "C" fits currency.

3. Final Answer:

   "C" -> Option B

4. Quick Check:

   Currency format = "C" [OK]

Hint: Currency format specifier is always "C" [OK]

Common Mistakes:

• Using "N" which formats number with commas but no currency symbol
• Using "P" which formats as percentage
• Using "D" which is for integers only

2. Which of the following is the correct syntax to format a DateTime variable date to show only the year as four digits in C#?

easy

A. date.ToString("yy")

B. date.ToString("Year")

C. date.ToString("YYYY")

D. date.ToString("yyyy")

Solution

1. Step 1: Identify correct year format specifier

   In C#, "yyyy" outputs the full four-digit year, while "yy" outputs two digits.

2. Step 2: Check case sensitivity and keywords

   "YYYY" is invalid (case sensitive), and "Year" is not a recognized format string.

3. Final Answer:

   date.ToString("yyyy") -> Option D

4. Quick Check:

   Year four-digit format = "yyyy" [OK]

Hint: Use lowercase "yyyy" for full year, uppercase is invalid [OK]

Common Mistakes:

• Using uppercase "YYYY" which is invalid
• Using "yy" which shows only two digits
• Using non-format strings like "Year"

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

double value = 0.7567;
string result = value.ToString("P1");
Console.WriteLine(result);

medium

A. 75.7%

B. 0.8%

C. 0.76%

D. 75.67%

Solution

1. Step 1: Understand "P1" format specifier

   "P" formats the number as a percentage by multiplying by 100 and adding "%". "1" means one decimal place.

2. Step 2: Calculate output

   0.7567 * 100 = 75.67, rounded to one decimal place is 75.7, so output is "75.7%".

3. Final Answer:

   75.7% -> Option A

4. Quick Check:

   Percentage with 1 decimal = 75.7% [OK]

Hint: Percentage multiplies by 100 and adds % [OK]

Common Mistakes:

• Not multiplying by 100 before formatting
• Rounding incorrectly to 0.8%
• Confusing decimal places with digits

4. The following code produces unexpected output. What is the problem?

DateTime dt = DateTime.Now;
string formatted = dt.ToString("MM-dd-yyyyy");
Console.WriteLine(formatted);

medium

A. Incorrect use of uppercase 'MM' for month

B. Missing escape characters for dashes

C. Too many 'y' characters in the format string

D. DateTime cannot be formatted with ToString

Solution

1. Step 1: Check the format string for errors

   The format string uses "yyyyy" which causes the fifth 'y' to be treated as a literal 'y', producing unexpected output; year specifier should be 4 'y' characters for full year.

2. Step 2: Validate other parts

   Dashes do not need escaping, "MM" is correct for month, and DateTime supports ToString formatting.

3. Final Answer:

   Too many 'y' characters in the format string -> Option C

4. Quick Check:

   Year format max 4 'y' characters [OK]

Hint: Year format uses max 4 'y' characters [OK]

Common Mistakes:

• Thinking dashes need escaping
• Believing ToString can't format DateTime
• Confusing month format with day format

5. You have a list of decimal prices and want to display each price formatted as currency with two decimals and a comma as thousand separator. Which code snippet correctly formats and prints the list?

List<decimal> prices = new() { 1234.5m, 98765.432m, 12m };

hard

A. foreach(var p in prices) Console.WriteLine(p.ToString("C2"));

B. foreach(var p in prices) Console.WriteLine(p.ToString("N"));

C. foreach(var p in prices) Console.WriteLine(p.ToString("F"));

D. foreach(var p in prices) Console.WriteLine(p.ToString("D2"));

Solution

1. Step 1: Understand required format

   We want currency format with two decimals and thousand separators. "C2" means currency with 2 decimals.

2. Step 2: Check other format specifiers

   "N" is number with commas but no currency symbol, "F" is fixed-point without commas, "D2" is decimal integer with 2 digits (not for decimals).

3. Final Answer:

   foreach(var p in prices) Console.WriteLine(p.ToString("C2")); -> Option A

4. Quick Check:

   Currency with 2 decimals = "C2" [OK]

Hint: Use "C2" for currency with 2 decimals and commas [OK]

Common Mistakes:

• Using "N" which lacks currency symbol
• Using "F" which lacks thousand separators
• Using "D2" which is for integers only