Combining flags

Question

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.NET3.5

I have a problem.
Sometimes in .net when we use a function like

search_text(text, from_position, options)

we are forced to use multiple options in the "options" field like match case, search direction, etc.
How do we combine options in the flags?
I've used the AND operator:

search_text("aditya", 0, search.matchcase And search.dir_up)

But it does not work. Why is this?

Posted 5-Dec-09 20:49pm

amit_upadhyay

Updated 19-Dec-09 8:25am

0x3c0

v3

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4 solutions

Solution 2

Also, assuming you are working in C# (like your tag indicates), the "And" operator you are using is not valid. It is in VB, but not C#. In C#, the AND operator is "&" and the OR operator is "|". Those are arithmetic operations. Logical AND is "&&" and logical OR is "||". For enums, you want "|". For example:

C#

search_text("aditya", 0, search.matchcase | search.dir_up);

Posted 5-Dec-09 21:13pm

AspDotNetDev

Solution 3

In addition to the correct answer you already have, your 'option' enum should always have a value that equals 0

[Flags]
public enum MyOptions
{
    None = 0,
    // ...
}

in case no options are selected - this also serves as the default value!

Posted 19-Dec-09 8:54am

DaveyM69

Updated 19-Dec-09 8:55am

v2

Comments

Sergey Alexandrovich Kryukov 21-Dec-10 19:02pm

It's good to have, but no, this is not required because every enum value is compatible with 0.
This is legal:
MyOption option;
//...
if (option == 0) {/* ... */}

DaveyM69 21-Dec-10 19:28pm

Not required, but highly recommended!

Solution 4

To specify multiple options with a single value, you need to bitwise add the options (having orthogonal values) by means of the OR operator.
:)

Posted 19-Dec-09 13:30pm

CPallini

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This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

AspDotNetDev · Accepted Answer · 2009-12-05T20:58:00

Assuming that "option" is an enum, use the "Or" operator. The enum will have to be defined in a manner which makes it suitable for this type of operation. For example, this would be suitable:

C#

[Flags]
public enum SomeOptions
{
	FirstOption = 1,
	SecondOption = 2,
	ThirdOption = 4,
	FourthOption = 8,
	FifthOption = 16
	// Each time the number doubles.
}

This would not be suitable:

C#

public enum SomeOptions
{
	FirstOption = 1,
	SecondOption = 2,
	ThirdOption = 3,
	FourthOption = 4,
	FifthOption = 5
	// Just add 1 to the number each time.
}

Using the "Flags" attribute just tells the compiler that you will be using this enum in the scenario you have described (combining various options). By doubling the numbers (1, 2, 4, 8, ...), you ensure that using the OR operator on them will produce valid results. The reason for that is how numbers are stored in binary. 1 is stored as "0001", 2 as "0010", 4 as "0100", 8 as "1000", and so on. If you combine 1 and 4, you get "0101" (i.e., the binary digits don't overlap).

Combining flags

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Combining flags

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