Why string type allows to use as constant?

Question

Because the string gets initialise in heap so how they can use as constant. how does the compiler know the size of the string? and what is string table ? is that used for calculating the string length.

if I used the string constant many places in the application, does it increase the memory consumption?

Constants in C# simply compile verbatim. So, when you have `const string something = "testing";` and then you write `Console.WriteLine(something)`, the compiler replaces it with `Console.WriteLine("testing")`. Strings are interned in C#, so no, duplicate constant strings will not take more memory. See https://stackoverflow.com/questions/16636363/string-caching-memory-optimization-and-re-use — Rob, Jul 14 '17 at 04:22
Thanks, @Rob, but why they allow to use as constant because no reference types have used as constant. — Ankit Rana, Jul 14 '17 at 04:26
They're immutable. And constants can be numbers, boolean values, strings, or a null reference. — , Jul 14 '17 at 04:26
@AnkitRana Strings are a special type of reference type, in that they can be constructed at compile time, unlike other reference types. Hence, they can be marked `constant`. — Rob, Jul 14 '17 at 04:27

Theodoros Chatzigiannakis · Accepted Answer · 2017-07-14T06:03:13.613

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The .NET team could have gone ahead and allowed arbitrary expressions to be used as constants — e.g. new Vector2(0, 0) if the Vector2 constructor was known to have no external side effects and the type was known to be a struct or otherwise immutable. But they simply didn't take the time to do this, perhaps because figuring out these requirements is extra work for the compilers (remember, C# has no immutable or pure keywords yet).

The string type is special to the compiler and the runtime: it was designed from the start to be immutable and its constructors have no externally observable side-effects, so the creators of the .NET runtime baked this knowledge into the compiler. That's why string has literals and gets special treatment.

Still, they probably wanted to avoid teaching the infrastructure too many special types — just a handful of fundamental types, namely primitives and strings. DateTime simply wasn't deemed important enough to be included.

edited Jul 14 '17 at 06:03

answered Jul 14 '17 at 05:00

Theodoros Chatzigiannakis

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Thanks for you reply, does use float as constant increase memory consumption ? – Ankit Rana Jul 14 '17 at 17:52
@AnkitRana Is there any reason why you are asking about `float` specifically? The same should hold for all kinds of constants. – Theodoros Chatzigiannakis Jul 15 '17 at 04:52
please see this https://stackoverflow.com/questions/1819117/c-sharp-do-const-fields-use-less-memory – Ankit Rana Jul 15 '17 at 16:58

score 3 · Answer 2 · edited Jun 20 '20 at 09:12

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Constants can be numbers, Boolean values, strings, or a null reference.

A constant expression is an expression that can be fully evaluated at compile time. Therefore, the only possible values for constants of reference types are string and a null reference.

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/const

Strings can be fully evaluated at compile time. DateTimes cannot.

edited Jun 20 '20 at 09:12

Community

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answered Jul 14 '17 at 04:29

so how they find the string length in compile time? – Ankit Rana Jul 14 '17 at 04:31
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It's a string literal. The compiler is able to count the characters. It's the same way the string length is calculated at runtime. But this doesn't have anything to do with your original question. – Jul 14 '17 at 04:32

Joe Phillips · Answer 3 · 2017-07-14T04:52:03.397

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Because a const string is not treated as a reference, it becomes a literal.

edited Jul 14 '17 at 04:52

answered Jul 14 '17 at 04:35

Joe Phillips

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Why string type allows to use as constant?

3 Answers3