I noticed that in C# that the + operator takes a string on the left and an object on the right:
string output = "" + numbers.ElementAt(0);
//vs
string output = numbers.ElementAt(0).ToString();
Why is the default behavior to accept an object on the right and not a string? When I look other + operators for say int, it is strongly typed for int/int instead of int/object.
I know in C# all objects have the ToString() method, is this just a way to remove the need to call .ToString() in the first example above?
Why is the default behavior to accept an object on the right and not a string?
Well, first off, there is an overload that accepts two strings, and if you pass two strings that's the overload that will be used, so it is the default behavior. But if you pass a string and a non-string, then the (string, object) overload will be used.
As for why that overload is there, it's because the C# language designers felt that it was entirely sensible to concat a string to any object, and had a reasonable implementation for doing so for any type of object.
When I look other + operators for say int, it is strongly typed for int/int instead of int/object.
How would you implement a operator +(int, object) overload? I don't see any good implementation here, unlike with string concatenation.
I know in C# all objects have the ToString() method, is this just a way to remove the need to call .ToString() in the first example above?
It actually works if the object is null, whereas your solution does not. If you would account for that then yes, that is essentially all that the overload is doing.
is this just a way to remove the need to call .ToString() in the first example above
Yes.
String concatenation automatically transforms non-strings into strings.
Source: C# Language Specification: Addition operator, MSDN.
I have a function that casts a double on string values.
string variable = "5.00";
double varDouble = (double)variable;
A code change was checked in and the project builds with the error: System.InvalidCastException: Specified cast is not valid.
However, after doing the following...
string variable = "5.00";
double varDouble = Convert.ToDouble(variable);
...the project builds without any errors.
What is the difference between casting and using the Convert.To() method? Why does casting throw an Exception and using the Convert.To() does not?
Even if you may see them somehow as equivalent they're completely different in purpose. Let's first try to define what a cast is:
Casting is the action of changing an entity of one data type into another.
It's a little bit generic and it's somehow equivalent to a conversion because a cast often has the same syntax of a conversion so the question should be when a cast (implicit or explicit) is allowed by the language and when do you have to use a (more) explicit conversion?
Let me first draw a simple line between them. Formally (even if equivalent for language syntax) a cast will change the type while a conversion will/may change the value (eventually together with the type). Also a cast is reversible while a conversion may not be.
This topic is pretty vast so let's try to narrow it a little bit by excluding custom cast operators from the game.
Implicit casts
In C# a cast is implicit when you won't lose any information (please note that this check is performed with types and not with their actual values).
Primitive types
For example:
int tinyInteger = 10;
long bigInteger = tinyInteger;
float tinyReal = 10.0f;
double bigReal = tinyReal;
These casts are implicit because during the conversion you won't lose any information (you just make the type wider). Vice versa implicit cast isn't allowed because, regardless of their actual values (because they can be checked only at run-time), during the conversion you may lose some information. For example this code won't compile because a double may contain (and actually it does) a value not representable with a float:
// won't compile!
double bigReal = Double.MaxValue;
float tinyReal = bigReal;
Objects
In case of an object (a pointer to) the cast is always implicit when the compiler can be sure that the source type is a derived class (or it implements) the type of the target class, for example:
string text = "123";
IFormattable formattable = text;
NotSupportedException derivedException = new NotSupportedException();
Exception baseException = derivedException;
In this case the compiler knows that string implements IFormattable and that NotSupportedException is (derives from) Exception so the cast is implicit. No information is lost because objects don't change their types (this is different with structs and primitive types because with a cast you create a new object of another type), what changes is your view of them.
Explicit casts
A cast is explicit when the conversion isn't done implicitly by the compiler and then you must use the cast operator. Usually it means that:
You may lose information or data so you have to be aware of it.
The conversion may fail (because you can't convert one type to the other) so, again, you must be aware of what you're doing.
Primitive types
An explicit cast is required for primitive types when during the conversion you may lose some data, for example:
double precise = Math.Cos(Math.PI * 1.23456) / Math.Sin(1.23456);
float coarse = (float)precise;
float epsilon = (float)Double.Epsilon;
In both examples, even if the values fall within the float range, you'll lose information (in this case precision) so the conversion must be explicit. Now try this:
float max = (float)Double.MaxValue;
This conversion will fail so, again, it must be explicit so you're aware of it and you may do a check (in the example the value is constant but it may come from some run-time computations or I/O). Back to your example:
// won't compile!
string text = "123";
double value = (double)text;
This won't compile because the compiler can't convert text to numbers. Text may contain any characters, not numbers only and this is too much, in C#, even for an explicit cast (but it may be allowed in another language).
Objects
Conversions from pointers (to objects) may fail if the types are unrelated, for example this code won't compile (because the compiler knows there is no possible conversion):
// won't compile!
string text = (string)AppDomain.Current;
Exception exception = (Exception)"abc";
This code will compile but it may fail at run-time (it depends on the effective type of casted objects) with an InvalidCastException:
object obj = GetNextObjectFromInput();
string text = (string)obj;
obj = GetNextObjectFromInput();
Exception exception = (Exception)obj;
Conversions
So, finally, if casts are conversions then why do we need classes like Convert? Ignoring the subtle differences that come from Convert implementation and IConvertible implementations actually because in C# with a cast you say to the compiler:
trust me, this type is that type even if you can't know it now, let me do it and you'll see.
-or-
don't worry, I don't care if something will be lost in this conversion.
For anything else a more explicit operation is needed (think about implications of easy casts, that's why C++ introduced long, verbose and explicit syntax for them). This may involve a complex operation (for string -> double conversion a parsing will be needed). A conversion to string, for example, is always possible (via ToString() method) but it may mean something different from what you expect so it must be more explicit than a cast (more you write, more you think about what you're doing).
This conversion can be done inside the object (using known IL instructions for that), using custom conversion operators (defined in the class to cast) or more complex mechanisms (TypeConverters or class methods, for example). You're not aware of what will happen to do that but you're aware it may fail (that's why IMO when a more controlled conversion is possible you should use it). In your case the conversion simply will parse the string to produce a double:
double value = Double.Parse(aStringVariable);
Of course this may fail so if you do it you should always catch the exception it may throw (FormatException). It's out of topic here but when a TryParse is available then you should use it (because semantically you say it may not be a number and it's even faster...to fail).
Conversions in .NET can come from a lot of places, TypeConverter, implicit/explicit casts with user defined conversion operators, implementation of IConvertible and parsing methods (did I forget something?). Take a look on MSDN for more details about them.
To finish this long answer just few words about user defined conversion operators. It's just sugar to let the programmer use a cast to convert one type to another. It's a method inside a class (the one that will be casted) that says "hey, if he/she wants to convert this type to that type then I can do it". For example:
float? maybe = 10; // Equals to Nullable<float> maybe = 10;
float sure1 = (float)maybe; // With cast
float sure2 = maybe.Value; // Without cast
In this case it's explicit because it may fail but this is let to the implementation (even if there are guidelines about this). Imagine you write a custom string class like this:
EasyString text = "123"; // Implicit from string
double value = (string)text; // Explicit to double
In your implementation you may decide to "make programmer's life easier" and to expose this conversion via a cast (remember it's just a shortcut to write less). Some language may even allow this:
double value = "123";
Allowing implicit conversion to any type (check will be done at run-time). With proper options this can be done, for example, in VB.NET. It's just a different philosophy.
What can I do with them?
So the final question is when you should use one or another. Let's see when you can use an explicit cast:
Conversions between base types.
Conversions from object to any other type (this may include unboxing too).
Conversions from a derived class to a base class (or to an implemented interface).
Conversions from one type to another via custom conversion operators.
Only the first conversion can be done with Convert so for the others you have no choice and you need to use an explicit cast.
Let's see now when you can use Convert:
Conversions from any base type to another base type (with some limitations, see MSDN).
Conversions from any type that implements IConvertible to any other (supported) type.
Conversions from/to a byte array to/from a string.
Conclusions
IMO Convert should be used each time you know a conversion may fail (because of the format, because of the range or because it may be unsupported), even if the same conversion can be done with a cast (unless something else is available). It makes clear to who will read your code what's your intent and that it may fail (simplifying debug).
For everything else you need to use a cast, no choice, but if another better method is available then I suggest you use it. In your example a conversion from string to double is something that (especially if text comes from user) very often will fail so you should make it as much explicit as possible (moreover you get more control over it), for example using a TryParse method.
Edit: what's the difference between them?
According to updated question and keeping what I wrote before (about when you can use a cast compared to when you can/have to use Convert) then last point to clarify is if there are difference between them (moreover Convert uses IConvertible and IFormattable interfaces so it can perform operations not allowed with casts).
Short answer is yes, they behave differently. I see the Convert class like a helper methods class so often it provides some benefit or slightly different behaviors. For example:
double real = 1.6;
int castedInteger = (int)real; // 1
int convertedInteger = Convert.ToInt32(real); // 2
Pretty different, right? The cast truncates (it's what we all expect) but Convert performs a rounding to nearest integer (and this may not be expected if you're not aware of it). Each conversion method introduces differences so a general rule can't be applied and they must be seen case by case...19 base types to convert to every other type...list can be pretty long, much better to consult MSDN case by case!
Casting is a way of telling the compiler, "I know that you think that this variable is a Bar, but I happen to know more than you; the object is actually a Foo, so let me treat it as if it were a Foo from now on." Then, at runtime, if the actual object turned out to really be a Foo then your code works, if it turns out that the object was not a Foo at all, then you get an exception. (Specifically an System.InvalidCastException.)
Converting on the other hand is a way of saying, "If you give me an object of type Bar I can create a brand new Foo object that represents what is in that Bar object. I won't change the original object, it won't treat the original object differently, it will create something new that is just based on some other value. As to how it will do that, it could be anything. In the case of Convert.ToDouble it will end up calling Double.Parse which has all sorts of complex logic for determining what types of strings represent what numeric values. You could write your own conversion method that mapped strings to doubles differently (perhaps to support some entirely different convention for displaying numbers, such as roman numerals or whatever). A conversion could do anything, but the idea is that you're not really asking the compiler to do anything for you; you are the one writing the code to determine how to create the new object because the compiler, without your help, has no way of knowing how to map (as an example) a string to a double.
So, when do you convert, and when do you cast? In both cases we have some variable of a type, let's say A, and we want to have a variable of type B. If our A object really, actually, under the hood, is a B, then we cast. If it's not really a B, then we need to Convert it, and define how the program is supposed to get a B from an A.
From MSDN:
Explicit conversions (casts): Explicit conversions require a cast operator. Casting is required when information might be lost in the conversion, or when the conversion might not succeed for other reasons. Typical examples include numeric conversion to a type that has less precision or a smaller range, and conversion of a base-class instance to a derived class.
Consider the following example:
double a = 2548.3;
int b;
b = (int)a; //2548 --> information (.3) lost in the conversion
And also:
A cast is a way of explicitly informing the compiler that you intend to make the conversion and that you are aware that data loss might occur.
You could use System.Convert class when you want to convert between non-compatible types. The main difference between casting and convert is compile and run-time. The type conversion exceptions are appeared at run-time , i.e a type cast that fails at run-time will cause an InvalidCastException to be thrown.
Conclusion: In casting you are telling to the compiler that a is really type b and if so the project builds without any errors like this example:
double s = 2;
int a = (int) s;
But in conversion you're saying to the compiler there is a way to create a new object from a of type b, please do it and project builds without any errors but as I said if type cast fails at run-time, it will cause an InvalidCastException to be thrown.
For example the code below is never compile because compiler detect that cannot cast expression of type DateTime to type int:
DateTime s = DateTime.Now;
int a = (int)(s);
But this one is compiled successfully:
DateTime s = DateTime.Now;
int a = Convert.ToInt32(s);
But at run-time you will get InvalidCastException which says:
Invalid cast from 'DateTime' to 'Int32'.
In your example you are attempting to cast a string to a double (non integral type).
An explicit conversion is required for it to work.
And i must point that you could have used Convert.ToDouble instead of Convert.ToInt64 as you can lose the fractional parts of the double value when you convert to an int.
if your variable has the value "5.25" varDouble would have been 5.00 (loss of 0.25 because of the Conversion to Int64)
To answer your question about casting vs converting.
Your cast (an explicit cast) doesn't meet the requirements for an explicit cast. the value you are trying to cast with the cast operator is invalid (i.e non integral).
Visit this MSDN Page for the rules of casting / conversions
The Convert.Double method actually just internally calls the Double.Parse(string) method.
Neither the String type nor the Double type define an explicit/implicit conversion between the two types, so casting will always fail.
The Double.Parse method will look at each character in the string and build a numeric value based on the values of the characters in the string. If any of the characters are invalid, the Parse method fails (causing the Convert.Double method to fail as well).
Casting does not involve any conversion, i.e. the internal representation of a value is not changed. Example:
object o = "Hello"; // o is typed as object and contains a string.
string s = (string)o; // This works only if o really contains a string or null.
You can convert a double to string like this
double d = 5;
string s = d.ToString(); // -> "5"
// Or by specifying a format
string formatted = d.ToString("N2"); // -> "5.00"
You can convert a string to a double in several ways (here just two of them):
string s = "5";
double d = Double.Parse(s); // Throws an exception if s does not contain a valid number
Or the safe way
string s = "5";
double d;
if (Double.TryParse(s, out d)) {
Console.WriteLine("OK. Result = {0}", d);
} else {
Console.WriteLine("oops!");
}
double varDouble = (double)variable assumes that variable is already a double. If variable isn't a double (it's a string) then this will fail.
double varDouble = Convert.ToDouble(variable) does like it says - it converts. If it can parse or otherwise extract a double from variable then it will.
I second using Double.Parse or Double.TryParse because it more clearly indicates what's supposed to be happening. You're starting with a string and expecting it to be convertible to a double. If there's any doubt, use TryParse.
If variable is a method argument, change the type to double. Make the caller responsible for providing the correct type. That way the compiler does the work for you.
string variable = "5.00";
double varDouble = (double)variable;
Above conversion is simply not allowed by language. Here is a list of explicit casts for numerical types: http://msdn.microsoft.com/en-us/library/yht2cx7b.aspx As you can see, even not every numerical type could be converted to another numerical type
Some more info about casting here
And how does this differ to Convert.ToDouble()?
When you cast a type, data structure is not changed. Well, in case of numerical values conversion it you may loose few bits or get few additional 0 bits. But you are still working with a number. You are just changing an amount of memory taken by that number. That is safe enough for compiler do everything needed.
But when you are trying to cast string to a number, you can't do that because it is not enough to change amount of memory taken by variable. For instance, 5.00 as a string is a sequence of "numbers":53(5) 46(.) 48(0) 48(0) - that is for ASCII, but string will contain something similar. If compiler will just take first N (4 for double? not sure) bytes from a string - that piece will contain completely different double number.
At the same time Convert.ToDouble() run special algorithm which will take each symbol of a string, figure out digit which it represents and make a double number for you, if string represents a number.
Languages like PHP will, roughly speaking, call Convert.ToDouble for you in background. But C#, like a statically typed language, will not do that for you. This allows you to be sure that any operation is type safe and you will not get something unexpected doing something like:
double d = (double)"zzzz"
Casting a string to a double like that is not allowed C# which is why you get an Exception, you need to have the string converted (MSDN doc that shows acceptable conversion paths). This is simply because a string isn't necessarily going to contain numeric data, but the various numeric types will (barring null values). A Convert will run a method that will check the string to see if it can be turned into a numeric value. If it can, then it will return that value. If it can't, it'll throw an exception.
To convert it, you have several options. You used the Convert method in your question, there's Parse which is largely similar to Convert, but you should also look at TryParse which would allow you to do:
string variable = "5.00";
double varDouble;
if (Double.TryParse(variable, out varDouble)) {
//Code that runs if the conversion succeeded.
} else {
//Code that runs if the conversion failed.
}
This avoids the possible exception should you try to Convert or Parse a non-numerical string.
The most important difference is that if type casting is used and the conversion fails
(say we are converting a very big float value to int ) no exception will be thrown and the minimum value an int can hold will be shown.
But in case of using Convert , an exception will be thrown for such scenarios.
This is super dumb, but I've googled and checked references and I just cannot find an answer... Why can an int or float etc be added as part of a string without converstion but not on it's own? that is:
while this work fine:
string st = "" + 12;
this doesn't (of course):
string st = 12;
Where is the magic here? I know it works I just want to know WHY it works and how I control HOW the conversion is done?
In the first statement, the left operand for the + is a string, and as such + becomes the concatenation operator. The compiler finds the overload for that operator which takes a string and an arbitrary value as operands. This converts both operands to strings (using ToString()) then joins them.
The second statement does not work because there's no implicit cast from int to string.
You can control how the conversion is done by using parentheses to change the order of operations (semi-effective) or by writing code to handle the conversions pre-emptively.
This is how string concatenation is designed to work, and as BoltClock's answer noted, the compiler is using the + as the string concatenation operator. From the C# Language Specification on string concatenation, we find that:
Any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object.
String concats in .NET ultimately resolve to calls to one of the overloads of the static String.Concat methods. This is an optimization to reduce the number of temporary strings that would otherwise be created when mutliple concatenations occur in a single statement.
In short the reason this works is because a number of the String.Concat overloads will accept object in the argument list and since an int, float etc. are in essence objects they can be passed to the Concat overload that accepts one or more object parameters. Internally Concat of basically does a .ToString() on the incomming object therefore turning your int into it's string representation.
In your specific example
string st = "" + 12;
The compiler will reconize that the first string is empty and simply call the String.Concat(object) overload. Which will convert the integer 12 to a string and assign it to st.
This overload is called because the integer can be implicitly boxed to fit into the object type and therefore satisfy the method overload selection.
Because the compiler will call .ToString() on all objects if one of the parameters is a string.
This is working because of the operator overload of the operator + on the string datatype.
Because the first is an expression and there C# makes an implicit conversion.
The second is an assignement with a static value, and the static value has no methods to be called.
As long as the value to be asigned is a variable or expression return there will be a toSting method to call.
It's because the compiler translates the addition to a call to String.Concat().
Internally, all operands are boxed (if necessary) and passed to the String.Concat method (which of course calls ToString() on all arguments).
I am going through a book on C# and have come across something that I can't seem to look up, because I don't know what it is called, or trying to search for something by description.
Could some explain to me what is going on, or the meaning behind the (type) that comes before a reference to an object as in (int)objectname.variablename?
It seems like casting to me.
EDIT:
Since most of you are going off 'My' reference to casting when I am only guessing, and needed more code, I am including the code that I am reviewing that has brought on this question. I am questioning the (int) in the (int)numericupDown1.Value;
private void numericUpDown1_ValueChanged(object sender, EventArgs e)
{
dinnerParty.NumberOfPeople = (int)numericUpDown1.Value;
DisplayDinnerPartyCost();
}
It is casting, it is trying to cast variablename into type (type) at runtime
It is casting(converting) the numericUpDown1.Value to an int value. This can also be done using
Convert.toInt32(numericUpDown1.Value)
I'm assuming that dinnerParty.NumberOfPeople is an integer value and therefore if you didn't cast it would throw an error if the value supplied was not an integer. For example if the Value supplied was a double or a string it would throw an error, with casting it would convert the double say 20.5 to 20 and it would be accepted. For the string it would depend on if the string contained a number in it. If the string was "12" then using the convert method mentioned above would convert it to the integer 12.
You're right. It is casting.
Without knowing the types involved it's hard to say exactly what is happening.
Casting will explicitly invoke the conversion operator from one type to another:
http://msdn.microsoft.com/en-us/library/ms173105(VS.80).aspx
It could however be unboxing or explicit boxing. Boxing/Unboxing is an expensive way of storing value types on the heap.
http://msdn.microsoft.com/en-us/library/yz2be5wk.aspx
As Eric Lippert (one of the designers of C#) mentioned in a comment elsewhere, this is Conversion - converting from one type to another.
It's not Casting. Casting is a leftover concept from C, where sometimes the compiler didn't know what type something was. The cast was a message to the compiler how to interpret the variable.
This could be a very basic question for prodigies.But I have a doubt to handle it.
During Conversion we are using :
int.Parse(someThing)
Convert.ToInt32 or Convert.ToString()....
(Int32)(someValue)
even we use "as"
What are the restrictions for using each?
int.Parse assumes a string as a parameter and as such is only suitable for converting raw string representations to integers
Convert.ToInt32() will attempt to convert pretty much any object to an integer representation. Where the representation isn't a valid int (i.e. using a float with the value 55.3 or a string containing a word) this will raise a FormatException. If the integer is too large to fit in an int then an OverflowException will occur.
(int) is a direct cast. It's basically saying "I know that this object is really an integer, treat it as such". If the object actually isnt an integer you'll get an invalid cast exception.
Finally, as behaves the same as a direct cast except where the object is not of the correct type it will assign null. Not sure how this applies to int with it being a non-nullable type but certainly usable with
int? myInt = someVar as int?;
Convert.ToNnnn has the restrictions stipulated by its overloads; for instance, you cannot call Convert.ToMyCustomType (since that method does not exist). You can pass pretty much anything into Convert.ToString (or Convert.ToInt32 or any other of the Convert.ToNnnn methods) but the result may not always make sense. Also, if the method fails to perform the conversion it may throw an InvalidCastException.
The regular type case ((int)someValue) can be used in all cases where there is an explicit conversion available. If you try to perform an illegal cast you will get an exception thrown at you.
The as keyword can be used to cast type to another (reference) type (and it will return null if it's not possible). The as keyword cannot be used with value types (such as Int32, Point or DateTime).
In my own code I typically use a mixture of them, but in general I follow the following scheme:
If I want to convert a string to a numeric type, I usually use the TryParse provided by that numeric type.
If I want to cast a class instance to a base class or interface, I usually use the as keyword.
In other cases I use regular type casting within try/catch blocks.
Convert.ToInt32 can convert from various types (DateTime, Decimal, etc) back to an integer, while int.Parse() only converts from a string.
If you're only parsing strings back to integers, Convert.ToInt32 is just an alias for int.Parse, so you might as well use int.Parse instead.
Regarding casting, you cannot cast a string to an integer, you have to convert it like mentioned above.
Consider that you are working with a database, and you have a column that is inefficiently declared as varchar while it stores integer data. You cannot do (int)dr["column"] when using a SqlDataReader for instance, you would have to use int.Parse(dr["column"].ToString()) instead. If the column was of the int column type in the database, you could then have used the cast: (int)dr["column"].