C＃ - 这个声明的字符串是否被视为const？

Question

Help me settle an argument here.

Is this:

SqlCommand cmd = new SqlCommand( "sql cmd", conn);

treated exactly the same as this:

const string s = "sql cmd";
SqlCommand cmd = new SqlCommand( s, conn);

Ie. does it make a difference if I state specifically that the string s is a const.

And, if it is not treated in the same way, why not?

Answer 1

In the latter snippet, it's not that the string is const - it's that the variable is const. This is not quite the same as const in C++. (Strings are always immutable in .NET.)

And yes, the two snippets do the same thing. The only difference is that in the first form you'll have a metadata entry for s as well, and if the variable is declared at the type level (instead of being a local variable) then other methods could use it too. Of course, due to string interning if you use "sql cmd" elsewhere you'll still only have a single string object in memory... but if you look at the type with reflection you'll find the const as a field in the metadata with the second snippet if it's declared as a constant field, and if it's just a local variable it'll be in the PDB file if you build one.

Answer 2

The value of a const always gets burned directly into the caller, so yes they are identical.

Additionally, the compiler interns strings found in source code - a const is helpful if you are using the same string multiple times (purely from a maintenance angle - the result is the same either way).

Answer 3

SqlCommand的构造函数不会“看到”任何差异，因此将以相同的方式行事。

Answer 4

是的，那些完全一样。

Answer 5

Yes, feel free to use Reflector to look at the assembly, const strings will be replaced with literal strings on compilation. I also have a blog post about this to safe you the work of using Reflector :)

Answer 6

I'm not 100% sure about this, but I'd bet it is the same.

const will just make sure you don't reassing the variable, but it's something that can be done at compile time.

Morover, strings are inmutable, so I don't think it will make any difference declaring the variable or not.

However, the definite prove would be studing the IL code generated in both cases.