变量已分配但从未使用过

Question

int i, j;
i = j = 1;

j is highlighted by VS 2010 with warning:

The variable is assigned but never used

Why i is "used" and j - is not?

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An addition with cooperation with Daniel:

int i, j, k, l, m;
i = j = k = l = m = 1;

Only m is highlighted.

Answer 1

I think it's a bug, It should be in reverse order, = operator is a right precedent operator according to Microsoft documentation. So when we have i = j = 1 it should parse it as i = (j = 1) in this case value of j used to initialize i so the compiler should say i initiated but never used, not j .

Answer 2

Technically this should be the case for both i and j

EDIT:

I have again checked the code

int ii, jj;
ii = jj = 1;

using Reflector to generate IL I found

.maxstack 2
.locals init (
    [0] int32 ii,
    [1] int32 jj)
L_0000: nop 
L_0001: ldc.i4.1 //pushes the integer value of 1 onto the evaluation stack
L_0002: dup //copies the current topmost value on the evaluation stack, and then pushes the copy
L_0003: stloc.1 
L_0004: stloc.0 
L_0005: ret 

From this, it would make it seem that 1 is assigned to ii, and then ii is copied to jj.

Answer 3

This is a limitation of the Visual Studio C# compiler. We can't answer because we haven't implemented it.

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Original answer

i is probably used later in your function, while j isn't.

To remove the "probably", you should post the whole function. ( Update: This is not true. The whole code inside main is what the OP posted.)

Answer 4

我已经用Resharper尝试了它，它足够聪明，可以正确地给出所有变量的警告。

Answer 5

Actually, it is intended behaviour for i to not be highlighted as a warning, because it is initialised from the value of a variable. This warning is only ever given when the variable is assigned from a compile-time constant.

Try out the following code:

int i = 0;
string s = "string";
string t = "another string";
string u = t;
var v = new string('v', 1);
var y = new XElement("hello");

Only i and s are given warnings. According to this post , this is intended (although for a rather questionable reason IMO).

So really the mystery here is why there are any warnings at all!

Answer 6

I tried this on Visual Studio 2010 Professional and got no such warning. What flavour of VS2010 are you using? I use Professional at work and Express at home, and find the Express version less accurate with warnings.