What is happening when you use the |= operator in C#?

Question

If I'm using something like this:

xr.Settings.ValidationFlags |= XmlSchemaValidationFlags.ReportValidationWarnings;

What precisely is the |= accomplishing?

Squirrel

Answer 1

|= is a shortcut for OR'ing two values together and assigning the result to the first variable.

xr.Settings.ValidationFlags |= XmlSchemaValidationFlags.ReportValidationWarnings;

Is equivalent to:

xr.Settings.ValidationFlags = xr.Settings.ValidationFlags | XmlSchemaValidationFlags.ReportValidationWarnings;

| is the OR operator in C#, so the code above effectively sets the ReportValidationWarnings flag on the value xr.Settings.ValidationFlags.

Rainproof

How does the comparison after the assignment operator actually occur? I know it's comparing the bits, but I'm having a little difficulty getting my head around it. – Squirrel

It's not comparing bits at all. If you have two bytes 1001 1101 and you OR it with 1100 1011 you would get 1101 1111. If a bit it set in either operand, then it is set in the result. The shorthand of |= just states that the left operand of the OR is the operand to the left of the |=. – Wier

Doesn't the link below indicate that the bitwise comparison is taking place? msdn.microsoft.com/en-us/library/h5f1zzaw%28v=vs.71%29.aspx. I suppose my confusion lies in something like this: bool b = true; b |= false; and yet b still returns true. I feel stupid, but I'm not seeing how that translates into what's happening above. – Squirrel

| means OR; If either parameter is true the result is true. In this case it is a bitwise OR, which means that if any bit is 1 in either parameter, it stays 1. – Incapacious

Answer 2

In this case you are setting XmlSchemaValidationFlags.ReportValidationWarnings flag in your ValidationFlags.

|= is generally bitwise or operator, in case of Flags it's used to set flag.

Tow

Answer 3

This is boolean OR-Equals.

Equivilent to:

xr.Settings.ValidationFlags = xr.Settings.ValidationFlags | XmlSchemaValidationFlags.ReportValidationWarnings;

Which is ensuring that the bits in XmlSchemaValidationFlags.ReportValidationWarnings are set.

Wier

Answer 4

That is a compound assignment. Essentially you are doing:

xr.Settings.ValidationFlags = 
    xr.Settings.ValidationFlags | 
    XmlSchemaValidationFlags.ReportValidationWarnings;

Which will essentially add XmlSchemaValidationFlags.ReportValidationWarnings to the set of flags contained in xr.Settings.ValidationFlags.

Springs

So, why does it do that? I'm having trouble getting my head around the operation. – Squirrel

Answer 5

As others have mentioned, a |= b is shorthand for a = a | b, in much the same way as a += b is short for a = a + b. Now what does the | operator do? It can be overloaded, but the general use of it is bit-wise OR. It's similar to the || operator, but it works on a bit by bit basis (think of each bit as a boolean):

false || true is true

0100 | 0110 is 0111

Then the last thing is that one of the classic ways to pass a bunch of boolean flags is to encode them in an integer. A single 32 bit integer can hold 32 individual flags, one per bit. to set a flag, you set the corresponding bit to 1.

So 0000 has no flags set, while 1001 has flags at position 1 and 4 set.

Then |= is a convenient way to set a specific flag.

int my_flags = 0;

my_flag |= validate_flag;

Tracay

Answer 6

driis' answer is correct.

|= is the same as using the | operator in a conditional test, as in the following examples:

xr.Settings.ValidationFlags |= XmlSchemaValidationFlags.ReportValidationWarnings

is semantically identical to

xr.Settings.ValidationFlags = xr.Settings.ValidationFlags | XmlSchemaValidationFlags.ReportValidationWarnings

Additional note about | vs ||

One additional tidbit I found to be useful was the difference between the logical OR (|) and the conditional OR (||):

The logical OR will always evaluate both operands, even if the first is true. The conditional OR will only evaluate the second if the first is false, effectively short-circuiting if the execution of the second operand is unnecessary to determine the final result.

Ancestress