Flags attribute and bitwise enums in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of combined flags with bitwise OR

What is the output of this C# code using a Flags enum?

C Sharp (C#)

using System;

[Flags]
public enum Permissions
{
    None = 0,
    Read = 1,
    Write = 2,
    Execute = 4
}

class Program
{
    static void Main()
    {
        Permissions userPerms = Permissions.Read | Permissions.Write;
        Console.WriteLine(userPerms);
    }
}

A "Read"

B "Read, Write"

C "3"

D "Write"

💡 Hint

Look at how Flags attribute changes enum ToString output when combined with bitwise OR.

The Flags attribute allows combined enum values to be displayed as a comma-separated list of names. Here, Read (1) OR Write (2) equals 3, but ToString shows "Read, Write".

❓ Predict Output

intermediate

Check if a flag is set using bitwise AND

What will this program print when checking if the Write flag is set?

C Sharp (C#)

using System;

[Flags]
public enum Access
{
    None = 0,
    Read = 1,
    Write = 2,
    Delete = 4
}

class Program
{
    static void Main()
    {
        Access rights = Access.Read | Access.Delete;
        bool canWrite = (rights & Access.Write) == Access.Write;
        Console.WriteLine(canWrite);
    }
}

A Runtime exception

B True

C Compile error

D False

💡 Hint

Bitwise AND with a flag returns the flag if set, otherwise zero.

The rights variable has Read and Delete flags set, but not Write. So (rights & Access.Write) equals 0, which is not equal to Access.Write (2), so canWrite is false.

❓ Predict Output

advanced

Output of combined flags with overlapping values

What is the output of this code with overlapping enum values and Flags attribute?

C Sharp (C#)

using System;

[Flags]
public enum Options
{
    None = 0,
    Alpha = 1,
    Beta = 2,
    Gamma = 3
}

class Program
{
    static void Main()
    {
        Options opt = Options.Alpha | Options.Beta;
        Console.WriteLine(opt);
    }
}

A "Gamma"

B "Alpha"

C "Alpha, Beta"

D "3"

💡 Hint

Check the numeric values and how bitwise OR combines them.

Alpha is 1, Beta is 2, so Alpha | Beta is 3. Gamma is also 3. The Flags attribute ToString returns the exact matching name if found, so it prints "Gamma".

❓ Predict Output

advanced

Count how many flags are set in a Flags enum

What is the output of this program that counts set flags?

C Sharp (C#)

using System;

[Flags]
public enum Features
{
    None = 0,
    FeatureA = 1,
    FeatureB = 2,
    FeatureC = 4,
    FeatureD = 8
}

class Program
{
    static void Main()
    {
        Features enabled = Features.FeatureA | Features.FeatureC | Features.FeatureD;
        int count = 0;
        foreach (Features f in Enum.GetValues(typeof(Features)))
        {
            if (f != Features.None && (enabled & f) == f)
                count++;
        }
        Console.WriteLine(count);
    }
}

A 3

B 4

C 2

D 1

💡 Hint

Count how many individual flags are included in the combined value.

FeatureA, FeatureC, and FeatureD are set, so count is 3.

🧠 Conceptual

expert

Why use powers of two in Flags enums?

Why must each flag in a Flags enum have a value that is a power of two?

A To make the enum values easier to read as decimal numbers

B To prevent the enum from being used in switch statements

C To ensure each flag represents a unique bit so they can be combined without overlap

D To allow the enum to be sorted alphabetically by value

💡 Hint

Think about how bits combine in binary when using bitwise OR.

Using powers of two ensures each flag corresponds to a unique bit position. This allows combining flags with bitwise OR without losing information or causing overlap.

Practice

1. What is the main purpose of the [Flags] attribute in C# enums?

easy

A. To allow combining multiple enum values using bitwise operations

B. To make enum values readonly

C. To automatically assign sequential integer values

D. To restrict enum values to a single choice only

Solution

1. Step 1: Understand the role of [Flags]

   The [Flags] attribute marks an enum to be treated as a set of bit fields, allowing multiple values to be combined.

2. Step 2: Compare with other options

   Options B, C, and D describe unrelated enum behaviors. Only To allow combining multiple enum values using bitwise operations correctly describes the bitwise combination feature.

3. Final Answer:

   To allow combining multiple enum values using bitwise operations -> Option A

4. Quick Check:

   [Flags] enables bitwise combination [OK]

Hint: Remember: [Flags] means combine enum values with bits [OK]

Common Mistakes:

• Thinking [Flags] restricts enum to one value
• Confusing [Flags] with readonly or sequential enums
• Assuming [Flags] changes enum underlying type

2. Which of the following is the correct way to define a bitwise enum with the [Flags] attribute?

easy

A. [Flags]
public enum Colors { Red = 0, Green = 1, Blue = 2 }

B. [Flags]
public enum Colors { Red = 1, Green = 2, Blue = 4 }

C. public enum Colors { Red = 1, Green = 2, Blue = 3 }

D. [Flags]
public enum Colors { Red = 1, Green = 1, Blue = 2 }

Solution

1. Step 1: Check enum values for bitwise uniqueness

   Bitwise enums require values as powers of two (1, 2, 4, 8...) to avoid overlap.

2. Step 2: Validate each option

   [Flags] public enum Colors { Red = 1, Green = 2, Blue = 4 } uses 1, 2, 4 which are correct powers of two. [Flags] public enum Colors { Red = 0, Green = 1, Blue = 2 } starts with 0 which is valid for None but not typical for flags representing colors. public enum Colors { Red = 1, Green = 2, Blue = 3 } uses 3 which overlaps bits. [Flags] public enum Colors { Red = 1, Green = 1, Blue = 2 } repeats 1 twice, causing overlap.

3. Final Answer:

   [Flags] public enum Colors { Red = 1, Green = 2, Blue = 4 } -> Option B

4. Quick Check:

   Enum values must be powers of two [OK]

Hint: Use powers of two for flags enum values [OK]

Common Mistakes:

• Assigning zero to a flag value
• Using overlapping values like 3 or repeated numbers
• Forgetting the [Flags] attribute

3. Given the enum and code below, what is the output?

[Flags]
public enum Permissions { Read = 1, Write = 2, Execute = 4 }

Permissions userPerms = Permissions.Read | Permissions.Execute;
bool canWrite = (userPerms & Permissions.Write) == Permissions.Write;
Console.WriteLine(canWrite);

medium

A. True

B. Compile-time error

C. Runtime exception

D. False

Solution

1. Step 1: Understand the combined flags

   The variable userPerms combines Read (1) and Execute (4) using bitwise OR, resulting in 5.

2. Step 2: Check if Write flag is set

   The expression (userPerms & Permissions.Write) checks if Write (2) is set in 5. Since 5 & 2 = 0, it is not set, so canWrite is false.

3. Final Answer:

   False -> Option D

4. Quick Check:

   5 & 2 == 0 means Write not set [OK]

Hint: Check flags with AND (&) and compare to flag value [OK]

Common Mistakes:

• Assuming OR sets all flags including Write
• Confusing bitwise AND with OR
• Expecting True because Write is in enum

4. Identify the error in the following code snippet using a [Flags] enum:

[Flags]
public enum Options { None = 0, First = 1, Second = 2, Third = 2 }

Options selected = Options.First | Options.Third;
Console.WriteLine(selected);

medium

A. Duplicate enum values cause overlapping flags

B. Missing [Flags] attribute

C. Bitwise OR operator is incorrect

D. Enum values must start from 1, not 0

Solution

1. Step 1: Check enum value assignments

   Both Second and Third are assigned the value 2, causing overlap in flags.

2. Step 2: Understand impact of duplicate values

   Duplicate values mean Options.Third is not unique, so bitwise operations may behave unexpectedly.

3. Final Answer:

   Duplicate enum values cause overlapping flags -> Option A

4. Quick Check:

   Enum values must be unique powers of two [OK]

Hint: Check for unique powers of two in enum values [OK]

Common Mistakes:

• Ignoring duplicate values in flags enums
• Thinking 0 is invalid for None
• Assuming bitwise OR is wrong syntax

5. You have the following [Flags] enum:

[Flags]
public enum Features { None = 0, FeatureA = 1, FeatureB = 2, FeatureC = 4, FeatureD = 8 }

How would you write code to remove FeatureB from a variable currentFeatures that may contain multiple features combined?

hard

A. currentFeatures ^= Features.FeatureB;

B. currentFeatures |= Features.FeatureB;

C. currentFeatures &= ~Features.FeatureB;

D. currentFeatures = Features.FeatureB;

Solution

1. Step 1: Understand how to remove a flag

   To remove a flag, use bitwise AND with the complement (NOT) of the flag.

2. Step 2: Apply the operation to remove FeatureB

   The expression currentFeatures &= ~Features.FeatureB; clears the FeatureB bit while keeping others intact.

3. Final Answer:

   currentFeatures &= ~Features.FeatureB; -> Option C

4. Quick Check:

   Remove flag with AND and NOT [OK]

Hint: Remove flags with AND and NOT (~) operator [OK]

Common Mistakes:

• Using OR (|) to remove flags instead of AND (&)
• Assigning flag directly, overwriting all others
• Using XOR (^) which toggles flags instead of removing