Is casting the same thing as converting?

问题:

In Jesse Liberty's Learning C# book, he says "Objects of one type can be converted into objects of another type. This is called casting."

If you investigate the IL generated from the code below, you can clearly see that the casted assignment isn't doing the same thing as the converted assignment. In the former, you can see the boxing/unboxing occurring; in the latter you can see a call to a convert method.

I know in the end it may be just a silly semantic difference--but is casting just another word for converting. I don't mean to be snarky, but I'm not interested in anyone's gut feeling on this--opinions don't count here! Can anyone point to a definitive reference that confirms or denies if casting and converting are the same thing?

    object x;
    int y;

    x = 4;

    y = ( int )x;

    y = Convert.ToInt32( x );

Thank you

rp

Note added after Matt's comment about explicit/implicit:

I don't think implicit/explicit is the difference. In the code I posted, the change is explicit in both cases. An implicit conversion is what occurs when you assign a short to an int.

Note to Sklivvz:

I wanted confirmation that my suspicion of the looseness of Jesse Liberty's (otherwise usually lucid and clear) language was correct. I thought that Jesse Liberty was being a little loose with his language. I understand that casting is routed in object hierarchy--i.e., you can't cast from an integer to a string but you could cast from custom exception derived from System.Exception to a System.Exception.

It's interesting, though, that when you do try to cast from an int to a string the compiler tells you that it couldn't "convert" the value. Maybe Jesse is more correct than I thought!

回答1:

The simple answer is: it depends.

For value types, casting will involve genuinely converting it to a different type. For instance:

float f = 1.5f;
int i = (int) f; // Conversion

When the casting expression unboxes, the result (assuming it works) is usually just a copy of what was in the box, with the same type. There are exceptions, however - you can unbox from a boxed int to an enum (with an underlying type of int) and vice versa; likewise you can unbox from a boxed int to a Nullable<int>.

When the casting expression is from one reference type to another and no user-defined conversion is involved, there's no conversion as far as the object itself is concerned - only the type of the reference "changes" - and that's really only the way that the value is regarded, rather than the reference itself (which will be the same bits as before). For example:

object o = "hello";
string x = (string) o; // No data is "converted"; x and o refer to the same object

When user-defined conversions get involved, this usually entails returning a different object/value. For example, you could define a conversion to string for your own type - and this would certainly not be the same data as your own object. (It might be an existing string referred to from your object already, of course.) In my experience user-defined conversions usually exist between value types rather than reference types, so this is rarely an issue.

All of these count as conversions in terms of the specification - but they don't all count as converting an object into an object of a different type. I suspect this is a case of Jesse Liberty being loose with terminology - I've noticed that in Programming C# 3.0, which I've just been reading.

Does that cover everything?

回答2:

Absolutely not!

Convert tries to get you an Int32 via "any means possible". Cast does nothing of the sort. With cast you are telling the compiler to treat the object as Int, without conversion.

You should always use cast when you know (by design) that the object is an Int32 or another class that has an casting operator to Int32 (like float, for example).

Convert should be used with String, or with other classes.

Try this

static void Main(string[] args)
{
    long l = long.MaxValue;

    Console.WriteLine(l);

    byte b = (byte) l;

    Console.WriteLine(b);

    b = Convert.ToByte(l);

    Console.WriteLine(b);

}

Result:

9223372036854775807

255

Unhandled Exception:

System.OverflowException: Value is greater than Byte.MaxValue or less than Byte.MinValue at System.Convert.ToByte (Int64 value) [0x00000] at Test.Main (System.String[] args) [0x00019] in /home/marco/develop/test/Exceptions.cs:15

回答3:

The best explanation that I've seen can be seen below, followed by a link to the source:

"... The truth is a bit more complex than that. .NET provides three methods of getting from point A to point B, as it were.

First, there is the implicit cast. This is the cast that doesn't require you to do anything more than an assignment:

int i = 5;
double d = i;

These are also called "widening conversions" and .NET allows you to perform them without any cast operator because you could never lose any information doing it: the possible range of valid values of a double encompasses the range of valid values for an int and then some, so you're never going to do this assignment and then discover to your horror that the runtime dropped a few digits off your int value. For reference types, the rule behind an implicit cast is that the cast could never throw an InvalidCastException: it is clear to the compiler that the cast is always valid.

You can make new implicit cast operators for your own types (which means that you can make implicit casts that break all of the rules, if you're stupid about it). The basic rule of thumb is that an implicit cast can never include the possibility of losing information in the transition.

Note that the underlying representation did change in this conversion: a double is represented completely differently from an int.

The second kind of conversion is an explicit cast. An explicit cast is required wherever there is the possibility of losing information, or there is a possibility that the cast might not be valid and thus throw an InvalidCastException:

double d = 1.5;
int i = (int)d;

Here you are obviously going to lose information: i will be 1 after the cast, so the 0.5 gets lost. This is also known as a "narrowing" conversion, and the compiler requires that you include an explicit cast (int) to indicate that yes, you know that information may be lost, but you don't care.

Similarly, with reference types the compiler requires explicit casts in situations in which the cast may not be valid at run time, as a signal that yes, you know there's a risk, but you know what you're doing.

The third kind of conversion is one that involves such a radical change in representation that the designers didn't provide even an explicit cast: they make you call a method in order to do the conversion:

string s = "15";
int i = Convert.ToInt32(s);

Note that there is nothing that absolutely requires a method call here. Implicit and explicit casts are method calls too (that's how you make your own). The designers could quite easily have created an explicit cast operator that converted a string to an int. The requirement that you call a method is a stylistic choice rather than a fundamental requirement of the language.

The stylistic reasoning goes something like this: String-to-int is a complicated conversion with lots of opportunity for things going horribly wrong:

string s = "The quick brown fox";
int i = Convert.ToInt32(s);

As such, the method call gives you documentation to read, and a broad hint that this is something more than just a quick cast.

When designing your own types (particularly your own value types), you may decide to create cast operators and conversion functions. The lines dividing "implicit cast", "explicit cast", and "conversion function" territory are a bit blurry, so different people may make different decisions as to what should be what. Just try to keep in mind information loss, and potential for exceptions and invalid data, and that should help you decide."

• Bruce Wood, November 16th 2005

http://bytes.com/forum/post1068532-4.html

回答4:

Casting involves References

List<int> myList = new List<int>();
//up-cast
IEnumerable<int> myEnumerable = (IEnumerable<int>) myList;
//down-cast
List<int> myOtherList = (List<int>) myEnumerable;

Notice that operations against myList, such as adding an element, are reflected in myEnumerable and myOtherList. This is because they are all references (of varying types) to the same instance.

Up-casting is safe. Down-casting can generate run-time errors if the programmer has made a mistake in the type. Safe down-casting is beyond the scope of this answer.

Converting involves Instances

List<int> myList = new List<int>();
int[] myArray = myList.ToArray();

myList is used to produce myArray. This is a non-destructive conversion (myList works perfectly fine after this operation). Also notice that operations against myList, such as adding an element, are not reflected in myArray. This is because they are completely seperate instances.

decimal w = 1.1m;
int x = (int)w;

There are operations using the cast syntax in C# that are actually conversions.

回答5:

Semantics aside, a quick test shows they are NOT equivalent !
They do the task differently (or perhaps, they do different tasks).

x=-2.5 (int)x=-2 Convert.ToInt32(x)=-2
x=-1.5 (int)x=-1 Convert.ToInt32(x)=-2
x=-0.5 (int)x= 0 Convert.ToInt32(x)= 0
x= 0.5 (int)x= 0 Convert.ToInt32(x)= 0
x= 1.5 (int)x= 1 Convert.ToInt32(x)= 2
x= 2.5 (int)x= 2 Convert.ToInt32(x)= 2

Notice the x=-1.5 and x=1.5 cases.

回答6:

A cast is telling the compiler/interperter that the object in fact is of that type (or has a basetype/interface of that type). It's a pretty fast thing to do compared to a convert where it's no longer the compiler/interperter doing the job but a function actualling parsing a string and doing math to convert to a number.

回答7:

Casting always means changing the data type of an object. This can be done for instance by converting a float value into an integer value, or by reinterpreting the bits. It is usally a language-supported (read: compiler-supported) operation.

The term "converting" is sometimes used for casting, but it is usually done by some library or your own code and does not necessarily result in the same as casting. For example, if you have an imperial weight value and convert it to metric weight, it may stay the same data type (say, float), but become a different number. Another typical example is converting from degrees to radian.

回答8:

In a language- / framework-agnostic manner of speaking, converting from one type or class to another is known as casting. This is true for .NET as well, as your first four lines show:

object x;
int y;

x = 4;

y = ( int )x;

C and C-like languages (such as C#) use the (newtype)somevar syntax for casting. In VB.NET, for example, there are explicit built-in functions for this. The last line would be written as:

y = CInt(x)

Or, for more complex types:

y = CType(x, newtype)

Where 'C' obviously is short for 'cast'.

.NET also has the Convert() function, however. This isn't a built-in language feature (unlike the above two), but rather one of the framework. This becomes clearer when you use a language that isn't necessarily used together with .NET: they still very likely have their own means of casting, but it's .NET that adds Convert().

As Matt says, the difference in behavior is that Convert() is more explicit. Rather than merely telling the compiler to treat y as an integer equivalent of x, you are specifically telling it to alter x in such a way that is suitable for the integer class, then assign the result to y.

In your particular case, the casting does what is called 'unboxing', whereas Convert() will actually get the integer value. The result will appear the same, but there are subtle differences better explained by Keith.

回答9:

According to Table 1-7 titled "Methods for Explicit Conversion" on page 55 in Chapter 1, Lesson 4 of MCTS Self-Paced Training Kit (Exam 70-536): Microsoft® .NET Framework 2.0—Application Development Foundation, there is certainly a difference between them.

System.Convert is language-independent and converts "Between types that implement the System.IConvertible interface."

(type) cast operator is a C#-specific language feature that converts "Between types that define conversion operators."

Furthermore, when implementing custom conversions, advice differs between them.

Per the section titled How to Implement Conversion in Custom Types on pp. 56-57 in the lesson cited above, conversion operators (casting) are meant for simplifying conversions between numeric types, whereas Convert() enables culture-specific conversions.

Which technique you choose depends on the type of conversion you want to perform:

• Define conversion operators to simplify narrowing and widening conversions between numeric types.

• Implement System.IConvertible to enable conversion through System.Convert. Use this technique to enable culture-specific conversions.

• ...

It should be clearer now that since the cast conversion operator is implemented separately from the IConvertible interface, that Convert() is not necessarily merely another name for casting. (But I can envision where one implementation may refer to the other to ensure consistency).

回答10:

Don't forget the other methods of casting and converting variables: as, Parse, TryParse as well as implicit casting between compatible data types.

This site has a good sample of what the outputs for most of the methods: C# Boxing and Unboxing

So given these sample variables:

int i = 3, x;
long l;
string s = "5";

Basically you can have implicit casting, between two compatible types:

l = i;

Explicit casting using unboxing or the as keyword:

s = (string)i;
//or
s = i as string;

Explicit conversions using methods from System.Convert:

i = System.Convert.ToInt32(s);

Explicit conversions using methods from a defined data type:

i = int.Parse(s);
i = int.TryParse(s, x);

Explicit conversions using methods from an instance of a variable:

s = i.ToString();

I think casting is simply a way of making assignments between two compatible types.

Converting is if you need to explicitly copy a value from one incompatible type to another, and you can't trust evil coercion.

Some good info also on MSDN: Casting and Type Conversions

回答11:

Casting is essentially just telling the runtime to "pretend" the object is the new type. It doesn't actually convert or change the object in any way.

Convert, however, will perform operations to turn one type into another.

As an example:

char caster = '5';
Console.WriteLine((int)caster);

The output of those statements will be 53, because all the runtime did is look at the bit pattern and treat it as an int. What you end up getting is the ascii value of the character 5, rather than the number 5.

If you use Convert.ToInt32(caster) however, you will get 5 because it actually reads the string and modifies it correctly. (Essentially it knows that ASCII value 53 is really the integer value 5.)

回答12:

The difference there is whether the conversion is implicit or explicit. The first one up there is a cast, the second one is a more explicit call to a function that converts. They probably go about doing the same thing in different ways.