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==
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Instead of coding:
if (btnPause.Content == "Pause")
try writing it this way round
if ("Pause" == btnPause.Content)
then when you inadvertently miss one of the = signs the compiler will give you a much more useful message.
It's time for a new signature.
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Richard MacCutchan wrote: the compiler will give you a much more useful message.
I don't know about that, you'd get "The left-hand side of an assignment must be a variable, property or indexer", which is true of course but it tells you to fix the wrong thing, it's saying that you should change the LHS into a variable. It's also saying that it's an assignment, but that just adds to the confusion because "there is no assignment, so the error makes no sense". OTOH, the "Cannot implicitly convert [type] to bool" is entirely clear, and since it's pointing to an expression in an if it tells you "you stuck something into an if that is not a bool" which is exactly what the problem is.
Or maybe I'm just used to that kind of error..
edit: I did not vote "bad answer" by the way, so don't get mad at me please
modified on Saturday, August 7, 2010 10:34 AM
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it really should say
if() expects a boolean and "btnPause.Content" isn't a boolean nor convertible to one
and it could add
there is an assignment in your expression; did you intend to test for equality? if so use ==
Why can't error messages be very specific, after all the parser is specific when it checks things.
harold aptroot wrote: I did not vote "bad answer"
Neither did I, although I didn't like it much; the user should not change his habits just because the compiler lacks proper error reporting. The suggestion may be OK for C/C++, but doesn't help much for C# code.
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 suitably chastised ...
It's time for a new signature.
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I thought it was a good idea, though I could never do that myself.
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Here's a radical idea: why not throw out the '==' and '===' confusion and make the compiler use the single '=' as either an assignment or a comparative operator depending on the context, like VB does?
Oh yeah, because we don't want C# to be like VB with all the stigma associated with such simpleton languages. Sorry, I forgot.
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I don't care much what symbol(s) get used for operators, I started out in Fortran which used .EQ. for equality testing.
However if assignment and equality test operators coincide, some semantics get lost, as in a=b==c versus a=b=c; there is only so much context analysis can do.
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You joke, but there's a lot of truth to that. you need the braces { and the weird 'for' syntax, the ++/-- operators and == for equality. it means it's a real programming language. Microsoft could have solved all of the technical problems without C#, by simply releasing .NET with only VB as the language, and it would simply have been the new version of VB, and the rest of us would be dealing with CString and MFC message maps. But now we get to implement IDispoable!
ken@kasajian.com / www.kasajian.com
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Nope.
Firstly, there is no === in C#.
Secondly the 'inconvenience' of == is outweighed by its benefit. For example:
while((line = reader.ReadLine()) != null)
This would be more difficult to read (albeit probably less confusing) without the explicit identity equality operator:
line = reader.ReadLine();
while(line != null)
{
line = reader.ReadLine();
}
The second block has absolutely no intent locality (keeping the same ideas in the same place in code).
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Chineese Proverb]
Jonathan C Dickinson (C# Software Engineer)
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Jonathan C Dickinson wrote: This would be more difficult to read (albeit probably less confusing) without the explicit identity equality operator:
Except your example uses the inequality operator. 
It's time for a new signature.
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I was talking hypothetically, i.e. if '=' was the identity equality operator; expressions like the one I used wouldn't work.
while((line == reader.ReadLine()) != null) { }
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Chineese Proverb]
Jonathan C Dickinson (C# Software Engineer)
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Jonathan C Dickinson wrote: if '=' was the identity equality operator; expressions like the one I used wouldn't work.
I don't see that that follows, since the compiler would still recognise != as the not equals operator.
It's time for a new signature.
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Hurm... The example doesn't need a == operator because it is demonstrating what is possible when a = and a == are distinct. With == around; = becomes more versatile. Because = is more versatile the example I gave is possible.
If there was only = (and no ==) the example I gave simply wouldn't work (you would get an warning saying that a boolean is never null). I think the mathematical term for this kind of 'proof' is proof by contradiction.
The compiler would recognize != as the not equals operator, BUT it would recognize the = as identity equality and not assignment. Thus the AST would look (where the VB-route is taken) something like this:
WHILESTMT(BOOLEXPR(BOOLEXPR("line", Operator.IdentityEquality, "reader.ReadLine"), Operator.IdentityInequality, NULL))
As opposed to (and why my example works):
WHILESTMT(BOOLEXPR(BOOLEXPR("line", Operator.Assign, "reader.ReadLine"), Operator.IdentityInequality, NULL))
More simply, the following expression results in a boolean type (and boolean value) in VB:
a = b
In C# is results in the type of 'a' (and the value contained by 'a'). Which is why these statements are possible:
int0 = int1 = int2 = int3 = int4 = 0; // Set all to 0.
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Chineese Proverb]
Jonathan C Dickinson (C# Software Engineer)
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It's time for a new signature.
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*shrug*
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Chineese Proverb]
Jonathan C Dickinson (C# Software Engineer)
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I don't like this way of working personally, I think the semantics are less clear and it always feels like cutting the suit to match the cloth to me. I vaguely remember it was a good idea to use the 2nd form in C++, the compiler wouldn't throw an error otherwise, and would happily set btnPause.Content to "Pause"
I haven't voted (and won't ) vote on your response - I don't think there is anything wrong with what you said, it is just a different opinion about how to work.
ragnaroknrol The Internet is For Porn[^]
Pete o'Hanlon: If it wasn't insulting tools, I'd say you were dumber than a bag of spanners.
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I have to say I tend to use the alternate form somewhat less these days, but it was offered to me years ago as a good way of catching such mistakes, especially in the days before C++ when I was writing pure C.
And as for the down voting, well frankly my dear ... I just wish these jerks would have the guts to add a reasoned and useful argument as the rest of you have done.
It's time for a new signature.
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The basic problem is that it only works when comparing an Lvalue and an Rvalue; many times that is not the case, so you need to think about what you are doing and recognize the situation. And if you can do that, then chances are you won't make that mistake anyway.
When it comes right down to it; anyone who is prone to that kind of mistake is also unlikely to remember the rule.
By the way... HP C V7.3-009 on OpenVMS Alpha V8.3 says:
JB> cc aa.c /warn=(enable=(check),verbose)
if ( result = 5 )
....^
%CC-I-CONTROLASSIGN, In this statement, the assignment expression "result=5" is used as the controlling expression of an if, while o
r for statement.
at line number 13 in file MY$ROOT:[000000]AA.C;3
Description: A common user mistake is to accidentally use assignment operator "=" instead of the equality operator "==" in an expres
sion that controls a transfer. For example saying if (a = b) instead of if (a == b). While using the assignment operator is valid,
it is often not what was intended. When this message is enabled, the compiler will detect these cases at compile-time. This can oft
en avoid long debugging sessions needed to find the bug in the user's program.
User Action: Make sure that the assignment operator is what is expected.
if ( 5 = result )
.........^
%CC-E-NEEDLVALUE, In this statement, "5" is not an lvalue, but occurs in a context that requires one.
at line number 18 in file MY$ROOT:[000000]AA.C;3
Description: An expression that must be an lvalue was not an lvalue. For example, the operand of the address-of operator must be an
lvalue.
User Action: Modify the expression so that it is an lvalue.
Edit:
C:\>cc aa
Borland C++ 5.5 for Win32 Copyright (c) 1993, 2000 Borland
aa.c:
Warning W8060 aa.c 14: Possibly incorrect assignment in function main
C:\>\mingw\bin\gcc -Wall aa.c
aa.c: In function `main':
aa.c:14: warning: suggest parentheses around assignment used as truth value
modified on Saturday, August 7, 2010 11:40 PM
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Right.
I've seen some C compilers (don't recall which, I've used so many for all kinds of embedded systems) also warning about the probably mistaken assignment in an if statement.
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I thought I had seen something like that in my days as a UNIX programmmer, it must have been gcc and possibly Sun' own C/C++ compiler.
It's time for a new signature.
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Yeugh! That looks too much like C++ for my liking. The alternate compiler error is just as confusing; and the alternate code is more confusing to read.
The reason they did things the second way in C++ is because:
if (btnPause->Content = "Pause")
And:
if ("Pause" = btnPause->Content)
We don't need to resort to such language hacks in C#. Please, for the kittens!
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Chineese Proverb]
Jonathan C Dickinson (C# Software Engineer)
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The point isn't that the compiler gives a more useful message (as has been pointed out), the point is it will give an error message even if the type in question in fact has a conversion-to-bool operator!
Whether or not you cannot figure out what the actual cause of the resulting message is, is an entirely different question. I'd suppose everyone intelligent enough to put literals on the left hand side of a comparison will recognise what it means when the compiler states he wants an lvalue! At least I am 
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I agree, but it seems this construct is no longer 'in vogue'. Being a sheep I'll just follow the herd.
It's time for a new signature.
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