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John Simmons / outlaw programmer wrote: I did, and it aborts in the debugger.
That sounds strange...
Aborts? What is that exactly? An exception?
I tried the code both in VS 2005 (C# 2.0) and VS 2008 (C# 3.0), and I can't get it to fail.
John Simmons / outlaw programmer wrote: The only way it would work is to cast using Convert.ToIn32
That doesn't make sense. Why would it not be possible to unbox an int?
Despite everything, the person most likely to be fooling you next is yourself.
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Thanks Guffa,
1.
I have tried that it works because b will be unboxed to a temporary int, then int will be converted to long -- which are all legal, right?
2.
When we unbox a value type, we could only unbox to the exact original type, any other types allowed?
regards,
George
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George_George wrote: I have tried that it works because b will be unboxed to a temporary int, then int will be converted to long -- which are all legal, right?
Yes. You can use implicit conversion from int to long:
long l = (int)b;
Or you can use explicit conversion from int to long:
long l = (long)(int)b;
George_George wrote: When we unbox a value type, we could only unbox to the exact original type, any other types allowed?
You can unbox to the actual type, or any (non-abstract) base type.
Despite everything, the person most likely to be fooling you next is yourself.
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Sorry for my limited knowledge, Guffa!
I have tried to write some code to test your idea "You can unbox to the actual type, or any (non-abstract) base type.", I tried to write a struct, and write another struct to derives from it. It seems in C# it is not supported to have a struct inherits from another struct?
regards,
George
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It's perfectly valid to cast an object to long.
Sure, in trivial cases like this, the compiler could do data-flow analysis; see that b is a boxed int and report a failure when unboxing it to long.
But do you want to make a certain kind of data-flow analysis mandatory for C# compilers?
Either you need to make the C# specifications extremely complex, or you need to make a C# compiler that violates the specification (since it would reject your program that is specified to fail at runtime, not at compile time).
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Thanks Daniel,
It is ok for me the compiler does not check the data flow. My question is, I want to know what C# compiler check for type conversion (even if it is simple and can not find all the issues until runtime, I want to know the rules). Any ideas?
regards,
George
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Part of the confusion here is that "(Type)expression" can have many meanings:
- type conversion
- unboxing
- boxing
- cast
What it means depends on the type of "expression" (the source type) and the type in the parenthesis (the target type).
If there is a conversion operator that converts from source type to target type, then that is used.
So "(long)an_int_variable" works because there is an conversion operator that sign-extends a 32-bit int to a 64-bit long.
Otherwise, if the source type is System.Object (or an interface) and the target type, then unboxing is used. Unboxing "unpacks" the value from box object. The box object must have exactly the correct type for this to work, otherwise you get an InvalidCastException at runtime.
If the source type is a value type and the target type is System.Object (or an interface implemented by that value type), then boxing is used. This means that a new object is created and the value is copied into it.
Otherwise, it's a cast; which basically means that a pointer is re-interpreted to point to a different object type. In this case, no conversion and no object creation happens.
You get a compile time error only if the compiler can see, by looking at source type and target type ONLY, that the conversion/cast would ALWAYS fail.
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Thanks Daniel,
Great reply! Two more comments,
1.
What is the differences between conversion and cast?
2.
When we unbox an object, it must be the same type of the original value type or could be different in some situations?
regards,
George
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Here are some examples:
int -> long: compiles, can never fail at runtime
int -> object -> long: compiles (int->object is valid boxing instruction, object->long is valid unboxing instruction), but always fails at runtime because a box containing int cannot be unboxed to long
int -> object -> int -> long: compiles and works at runtime
int -> IComparable -> int: same as int -> object -> int
int -> string: fails with compile error, there's no conversion operator int->string (ToString is not a conversion operator), and there's no way this cast could succeed
int -> object -> string: compiles (int->object is valid boxing instruction, object->string is valid cast), but always fails at run time because a boxed int cannot be cast to string.
long -> int: compiles, but might fail at runtime when the long is too large and integer overflow checking is enabled
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Thanks Daniel,
1.
Daniel Grunwald wrote: overflow checking is enabled
Enable this in compile or runtime? How to enable it? (I did this in unmanaged C++ before, but never did this in C# because I think C# will always check overflow.)
2.
Previously, I think object is the base type for int/long/... such primitive types. But now I think we should not say object is the base type for int/long/... such primitive types, it should be called boxing to object, right?
3.
int --> string does not work, because we can only box to object? Not box to other types?
regards,
George
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What you did is perfectly okay with the compiler (sometimes you can't rely on automated systems - you simply have to "do it right"). If you had used the correct casting mechanism, it would have been fine. I almost never use (long)varName to cast. Instead, I use Convert.ToIn32(varName), which in your case would have yielded the expected value (100).
One other thing to remember - ALWAYS put a try/catch block around a Convert method, even if you simply eat the exception. You don't want your app crashing ungracefully.
"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001
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Thanks John,
Good to learn Convert class from you. But should we use Convert.ToInt64 to convert to a long? 
regards,
George
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No. Int32 is a long (in the traditional sense of the type).
"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001
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John Simmons / outlaw programmer wrote: No. Int32 is a long (in the traditional sense of the type).
But we are not using the "traditional sense of the type". We are using C#, where the types are defined as:
int = System.Int32<br />
long = System.Int64
MSDN Library: Built-In Types[^]
Despite everything, the person most likely to be fooling you next is yourself.
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Good to learn, thanks Guffa!
regards,
George
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Compiler doesn't look at its value. Compiler just look at b's type. 'b' is casting to long, b is an object; So it allows to do it as it doesn't look at b's value.
Charith Jayasundara
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Thanks Charith,
I am wondering what are the documented rules for compiler what to check and what not to check?
regards,
George
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Hello Everyone...
I'm using this Barcode control form Codeproject Articles
http://www.codeproject.com/KB/miscctrl/barcodectl.aspx[^]
it is perfect, but I have faced a small problem with it, I've send a thread to the owner of this control but it seems like he is not available anymore, so I thought I might get help here...
My problem is that all the numbers in this control display only in English, no matter what I do...
I'm developing an Arabic application using "ar-SA" culture but still I see the numbers only in English format...
How can I solve this?
Please help...
Thanks in advanced...
Waleed
Thanks
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Hi Guys. I am using DateTime.Today.ToString("ddMMyyyyhhmmss") as part of my file name. I get the correct date and hour returned but the minute and seconds seem to default to 0000. Any idea why?
StreamWriter sw = new StreamWriter(@"C:\SOLZ_PROV_" + DateTime.Today.ToString("ddMMyyyyhhmmss") + ".txt");
Excellence is doing ordinary things extraordinarily well.
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Ah, yet again I have answered my Own Question.
DateTime.Now.ToString("ddMMyyyyhhmmss") seems to work just fine.
Excellence is doing ordinary things extraordinarily well.
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Hi again,
four remarks this time:
1.
if you were to change the order to "yyyyMMddhhmmss" then the alphabetical order
would be the same as the chronological order, which makes life easier.
2.
you can embed some more characters in the date format string, the ToString() does not
mind about some characters such as an underscore. it just copies them to the output, e.g. I often use "_yyyyMMdd_hhmmss" to get a filename that is more readable.
3.
you don't have to try and do everything in a single line; splitting a statement in two
or three smaller statements yields more readable code, and facilitates debugging, since
now you can set breakpoints on the parts, and watch intermediate results.
4.
if your code is executed repeatedly, you might generate the same filename more than once,
since a modern CPU can do a lot in one second. Even adding milliseconds may be insufficient
since the milliseconds don't get incremented by one, as you might expect; instead they
increase by a larger number but at a lower frequency. For more on this, read my timers
article. A possible remedy is to insert a Thread.Sleep() so the time advances before
you have a chance to generate yet another filename (assuming single-thread use here).
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Thanks Luc. In South Africa we use "ddMMyyyy" as date format. I wa using DateTime.Today which consistantly returned the correct date but the time as 120000 which is probably the start of the day. When I changed to DateTime.Now it returned the time correctly.
I expect to only create one file per day so I should be OK but I wanted to add the date time to show when the report was generated.
Luc Pattyn wrote: 3.
you don't have to try and do everything in a single line; splitting a statement in two
or three smaller statements yields more readable code, and facilitates debugging, since
now you can set breakpoints on the parts, and watch intermediate results.
I will take this advice and use it. I usually do that.
Thanks again mate. Much appreciated. I try and learn everyday and browsing the posts and Boards helps loads.
Excellence is doing ordinary things extraordinarily well.
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It takes a little time to create a file, but the time it takes is enough to pretty much guarantee a unique name every time, especially if your code is performing exception handling (which it should), and properly closing the file (which it should). If you include the millisecond portion of the date time, you should be good to go. Besides, if you think there's a chance of it, the simple solution is to insert the line Thread.Sleep(25); just before you create the file. Doing so guarantees a unique filename using the date/time naming method, and it will introduce unnoticeable latency in your code.
If you don't really care what the file name is as long as it's unique, use a GUID for the name. A GUID is guaranteed to be unique no matter how many you create in a given second.
"Why don't you tie a kerosene-soaked rag around your ankles so the ants won't climb up and eat your candy ass..." - Dale Earnhardt, 1997
-----
"...the staggering layers of obscenity in your statement make it a work of art on so many levels." - Jason Jystad, 10/26/2001
modified on Sunday, May 4, 2008 10:04 AM
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Kwagga wrote: In South Africa we use "ddMMyyyy" as date format
Well, get out there and start teaching people about ISO 8601[^]
Think globally, act locally. 
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You're welcome.
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