C# Difference betwen passing multiple enum values with pipe and ampersand

Question

C# accepts this:

this.MyMethod(enum.Value1 | enum.Value2);

and this:

this.MyMethod(enum.Value1 & enum.Value2);

Whats the difference?

Answer 1

When you do |, you select both. When you do &, you only what overlaps.

Please note that these operators only make sense when you apply the [Flags] attribute to your enum. See http://msdn.microsoft.com/en-us/library/system.flagsattribute.aspx for a complete explanation on this attribute.

As an example, the following enum:

[Flags]
public enum TestEnum
{
    Value1 = 1,
    Value2 = 2,
    Value1And2 = Value1 | Value2
}

And a few test cases:

var testValue = TestEnum.Value1;

Here we test that testValue overlaps with Value1And2 (i.e. is part of):

if ((testValue & TestEnum.Value1And2) != 0)
    Console.WriteLine("testValue is part of Value1And2");

Here we test whether testValue is exactly equal to Value1And2. This is of course not true:

if (testValue == TestEnum.Value1And2)
    Console.WriteLine("testValue is equal to Value1And2"); // Will not display!

Here we test whether the combination of testValue and Value2 is exactly equal to Value1And2:

if ((testValue | TestEnum.Value2) == TestEnum.Value1And2)
    Console.WriteLine("testValue | Value2 is equal to Value1And2");

Answer 2

this.MyMethod(enum.Value1 | enum.Value2);

This will bitwise 'OR' the two enum values together, so if enum.Value is 1 and enum.Value2 is 2, the result will be the enum value for 3 (if it exists, otherwise it will just be integer 3).

this.MyMethod(enum.Value1 & enum.Value2);

This will bitwise 'AND' the two enum values together, so if enum.Value is 1 and enum.Value2 is 3, the result will be the enum value for 1.

Answer 3

One is bitwise-or, the other is bitwise-and. In the former case this means that all the bits that are set in one or the other are set in the result. In the latter case this means that all the bits that are in common and set in both are set in the result. You can read about bitwise operators on Wikipedia.

Example:

enum.Value1 = 7  = 00000111
enum.Value2 = 13 = 00001101

then

enum.Value1 | enum.Value2 = 00000111
                           |00001101
                          = 00001111
                          = 15

and

enum.Value1 & enum.Value2 = 00000111
                           &00001101
                          = 00000101   
                          = 5

Answer 4

This question has a nice explanation: What does the [Flags] Enum Attribute mean in C#?

Answer 5

Enum parameters can be binary numbers, for example

enum WorldSides
{
    North=1,
    West=2,
    South=4,
    East=8
}

WorldSides.North | WorldSides.West = both values -> 3

So, | is used to combine the values.

Use & to strip some part of the value, for example

if (ws & WorldSides.North)
{
     //  ws has north component
}