C#
Flags attribute
Learn to create bit-based combinations with [Flags].
Bit fields with [Flags]
The [Flags] attribute lets an enum represent combinations of bit fields. A single variable can then hold several values at once. It is ideal for file permissions, option flags, or day combinations.
Declaration: [Flags] enum Perm { Read = 1, Write = 2, Exec = 4 }. Make sure the values are powers of two, so each flag occupies its own bit and bitwise operations can combine them. The [Flags] attribute only prettifies the ToString output; it does not change behaviour.
Combining with bitwise OR
To combine two or more flags, use the bitwise OR operator |. The result is a new value that holds the bits of each flag. This is the standard way to represent multiple permissions or options in a single variable.
For example, Perm p = Perm.Read | Perm.Write; assigns both read and write permissions to p. (int)p gives 3. Thanks to [Flags], calling p.ToString() returns a readable result like "Read, Write".
Querying with HasFlag
To check whether a specific flag is set in a combined flag value, use the HasFlag method. It returns a bool that is true when the queried flag is set.
For example, p.HasFlag(Perm.Read) returns true if p includes the read flag. Alternatively, you can write (p & Perm.Read) != 0, which uses bitwise AND to check the flag. Both approaches are common.
Exercise Combine permissions and print
Define a [Flags] enum called Perm with members Read = 1, Write = 2, Exec = 4. In Main, combine the two flags as Perm p = Perm.Read | Perm.Write; and print (int)p with Console.WriteLine.
Show example solution
Define [Flags] enum Perm { Read = 1, Write = 2, Exec = 4 }, then add Perm p = Perm.Read | Perm.Write; and Console.WriteLine((int)p);
Example solution:
using System;
[Flags]
enum Perm { Read = 1, Write = 2, Exec = 4 }
class Program
{
static void Main()
{
Perm p = Perm.Read | Perm.Write;
Console.WriteLine((int)p);
}
}
Test yourself
-
What does Perm.Read | Perm.Write give?
[Flags] enum Perm { Read = 1, Write = 2, Exec = 4 }
Show answer
3 — Correct! 1 | 2 = 3; bitwise OR combines the flags.
-
Combine the flags
Perm p = Perm.Read ____ Perm.Write;
Show answer
Perm p = Perm.Read | Perm.Write;
-
Match the operation with its result
- Read | Write
- 3
- HasFlag(Read)
- true/false
- Read & Write
- 0
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