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What is the difference between const and readonly?
are these interchangeable? can you show me code on how you would apply the two?
asked Jun 17, 2010 at 20:45
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1Duplicate of stackoverflow.com/questions/55984/…– Eric LippertCommented Jun 17, 2010 at 20:48
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1And stackoverflow.com/questions/755685/c-static-readonly-vs-const– Eric LippertCommented Jun 17, 2010 at 20:49
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1And stackoverflow.com/questions/555534/…– Eric LippertCommented Jun 17, 2010 at 20:50
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1And stackoverflow.com/questions/410723/…– Eric LippertCommented Jun 17, 2010 at 20:51
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@Eric, the problem with such questions is that it is easier/faster to answer them than to search for dupes. But thumbs up for searching the dupes.– LuceroCommented Jun 17, 2010 at 20:52
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3 Answers
No, they aren't.
A const field is literal value embedded in the assembly.
Only primitive values (strings and numbers) can be const, and they are evaluated at compile time.
When you reference a const field, the compiler embeds the literal value of the field. Therefore, if use use a const from another assembly, and the other assembly is recompiled with a different value, your assembly will only use the new value if you recompile it against the new version.
A readonly field is a normal field that cannot be changed outside the constructor.
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answer this one: stackoverflow.com/questions/2624796/… Commented Jun 20, 2010 at 20:23
Const can't perform evaluations whereas readonly can on initialization. (ie you could read in a value for a readonly variable from a config file or based on some other parameter that is known at runtime, const can only be set to something known at compile time)
answered Jun 17, 2010 at 20:49
A member of any type can be readonly. It simply means the member cannot be reassigned after the construction of the containing class; i.e., it cannot be set to a new object with the = operator. Mutable classes such as collections can still be modified with respect to their members; it's just that, if you have a readonly member that is a collection, it cannot be assigned to an entirely new collection after construction.
A const is not so different from a literal (like 5): it represents an unchanging value and thus only really makes sense in the context of specifying a value (as opposed to an object).
