The following code sample prints:
T
T[]
T[]
While first two lines are as expected, why compiler selected param array for a regular array?
public class A
{
public void Print<T>(T t)
{
Console.WriteLine("T");
}
public void Print<T>(params T[] t)
{
Console.WriteLine("T[]");
}
}
class Program
{
static void Main(string[] args)
{
A a = new A();
a.Print("string");
a.Print("string","string");
a.Print(new string[] {"a","b"});
}
}
Under the hood
a.Print("string","string");
is just syntactic sugar for
a.Print(new string[]{"string","string"});
EDIT: Like I said, the params keyword only automagically creates the array for you, you tell the compiler: either accept an array of T directly or use the X input params to construct that array.
It addition to what others have said, the params keyword also causes a ParamArrayAttribute to the generated for array parameter. So, this...
public void Print<T>(params T[] t) { }
Is generated by the compiler as...
public void Print<T>([ParamArray] T[] t); { }
It is that attribute which indicates to the compiler and the IDE that the method can be called using simpler syntax...
a.Print("string", "string");
rather than...
a.Print(new string[] { "string", "string" });
I think this actually has more to do with type inference than with the params keyword. The inference engine assumes on the third line that the type of T is string[] and therefore passes it to the first method.
try Console.WriteLine(typeof(T)) if you don't believe me
when having the params keyword, the compiler will do a little bit of translation for the formal function declaration, as well as the actual function call.
Formal function declaration:
Under the hood, the IL will be translated to essentially the same as
public void Print<T>(T[] array);
Except, when compiling, the actual function call will be checked for syntax translation. Meaning,
a.Print("string1", "string2");
Becomes the same IL code as
a.Print(new string[]{"string1", "string2"});
That is why line 2 and 3 are the same output, because under the hood, they got translated to the exact same IL.
Question about why line 3 is not print "T" is because, .NET compiler will always try to find the best overloaded match, and so both line 2 and 3 called to the T[] version instead of the plain T.
Exactly as arul said. If you open up the project in reflector, you'll see the following:
a.Print<string>(new string[] { "string", "string" });
Params allows you to pass multiple objects of the same type. it is a shortcut way of passing array
Related
After learning how to Create a method with infinite parameters, I wonder is it legal to store the parameter array into a array. Will it cause any problem, since I don't see many people use this approach.
Code below :
class Foo
{
private String[] Strings;
public Foo(params String[] strings)
{
Strings = strings;
}
...
}
That's fine - it's just an array.
All the compiler does with a parameter array is convert a call like this:
Foo("x", "y");
into:
Foo(new string[] { "x", "y" });
That's really all there is to it. Anything you'd expect to be appropriate with the second call is fine with a parameter array.
Arrays passed into public methods are rarely suitable to store directly due to all arrays being mutable - but that's a matter of how you handle mutable parameter types rather than being specific to parameter arrays.
I have a method like this:
public void Foo(params string[] args) {
bar(args[0]);
bar(args[1]);
}
The new requirements lead to a change like this:
public void Foo(string baz, params string[] args) {
if("do bar".Equals(baz)) {
bar(args[0]);
bar(args[1]);
}
}
The problem is that even though I've changed the method signature, no compilation errors occur, which is correct of course, but I want there to be compilation errors for every call to Foo method where the argument baz has not been specified. That is, if a call to Foo before the change was this one:
Foo(p1,p2); //where p1 and p2 are strings
it now needs to be this one:
Foo(baz,p1,p2);
If it wouldn't be changed in this way, p1 would be assigned to baz, and the params array args would be of length 1 and an OutOfBounds exception would be thrown.
What's the best way to change the signature and ensure that all the calling code is updated accordingly? (The real scenario is where Foo lives in an assembly shared by many projects automatically built on a build server. A compilation error would thus be an easy way to detect all the code that needs to be touched to accomodate the change.)
Edit:
As Daniel Mann and others pointed out, the example above suggests that I should not use params at all. So I should explain that in my real world example it's not always the case that args needs to have two elements, as far as the logic in Foo is concerned args can contain any number of elements. So let's say this is Foo:
public void Foo(string baz, params string[] args) {
if("do bar".Equals(baz)) {
int x = GetANumberDynamically();
for(int i = 0; i<x; i++)
bar(args[i]);
}
}
Here's the solution. Do not change the former method signature, just add the Obsolete attribute with both arguments specified.
[Obsolete("Use Foo(string, params string[]) version instead of this", true)]
public void Foo(params string[] args) {
bar(args[0]);
bar(args[1]);
}
Then create a new method with a new signature.
public void Foo(string baz, params string[] args) {
if("do bar".Equals(baz)) {
bar(args[0]);
bar(args[1]);
}
}
The second argument in the Obsolete attribute ensures a compilation error. Without it it just causes a compilation warning. More info about the attribute is available on MSDN.
EDIT:
Based on discussion in comments below, Daniel Mann came up with an interesting problem.
That wouldn't solve the problem. What about if you call Foo("a", "b")? In that case, it will still call the non-obsolete method with only two arguments, and cause the same problem.
I would advise to check if there is more then one argument passed through args before calling bar.
The easiest solution is to not use the params keyword if you have required parameters.
Obviously, you're expecting args to contain at least two parameters. It's safe to say that those are required. Why not have a method signature like this?
public void Foo(string baz, string requiredArgument1, string requiredArgument2, params string[] optionalArguments)
That removes the ambiguity: It will always require at least 3 arguments.
Another option I hadn't even thought of for some reason is to use named parameters. Obviously, all of your code would have to explicitly do so, but you could do this:
Foo(baz: "bar", args: new [] {"a", "b", "c"});
Why does execute TestMethod<T>(params T[] input) method instead of execute TestMethod(object input).I am confusing why complier execute generic method.Is there any priority hierarchy on .net framework ?
class TestClass
{
static void Main()
{
TestMethod("Hello World");
}
static void TestMethod(object input)
{
Console.WriteLine("object");
}
static void TestMethod<T>(params T[] input)
{
Console.WriteLine("params T[]");
}
}
The comment on your question contains the link to the overload resolution specs of C# which contains all infos you need to answer the question in depth. It's not the most easy read, though and hard to find the actual path of resolution in your case, so here is what happens, as far as I can tell:
Parameter arrays:
First, we need to look at what the params keyword does: As it is a shortcut for the developers convenience, here is what it is identical to:
static void TestMethod<T>(params T[] input)
gets translated to
static void TestMethod<T>(T[] item)
and calls get translated accordingly:
TestMethod("string2", "string2");
gets translated to:
TestMethod(new string[] { "string1", "string2" );
Resolution:
With this in mind, let's see what the compiler does: The compiler evaluates its options to resolve your call at runtime.
Your call is:
TestMethod("string");
The first option is to translate this call to:
TestMethod(new string[] { "string" });
This would allow to call the generic version, while using string as T, which would result in the 'resolved method signature':
TestMethod(string[] item)
So, the required conversion from actual to required argument type is
string[] to string[]
Option b is to interpret the parameter "string" as what it is in our eyes, as a string. Then it would be feasible to call the non-generic version.
string to object
The conversions are evaluated according to section 7.4.3.4 of the C# spec and to avoid the downgrading of string to object, the first option is chosen.
this is because of overload resolution.
here is a good article
Similar Question:
Generic Overload Resolution
That is the question.
Background: C# Params
In C#, you can declare the last parameter in a method / function as 'params', which must be a single-dimension array, e.g.:
public void SomeMethod(int fixedParam, params string[] variableParams)
{
if (variableParams != null)
{
foreach(var item in variableParams)
{
Console.WriteLine(item);
}
}
}
This then essentially allows syntactic sugar at the call site to implicitly build an array of zero or more elements:
SomeMethod(1234); // << Zero variableParams
SomeMethod(1234, "Foo", "Bar", "Baz"); // << 3 variableParams
It is however still permissable to bypass the sugar and pass an array explicitly:
SomeMethod(1234, new []{"Foo", "Bar", "Baz"});
Yes. In standard C++, you can use va_arg and the ... syntax. See MSDN for details.
For C++/CLI, There is a shortcut for this.
You do this as:
void TheMethod( String^ firstArgument, ... array<Object^>^ variableArgs );
See this blog post for details.
For unmanaged C++ with the same convenient syntax, no.
But there is support for variable argument lists to functions in C++.
Basically you declare a function with the last parameter being an ellipsis (...), and within the body of the function use the va_start()/va_arg() calls to parse out the supplied parameter list.
This mechanism is not type safe, and the caller could pass anything, so you should clearly document the public interface of the function and what you expect to be passed in.
For managed C++ code, see Reed's comments.
Nowadays, with modern C++, you can use modern type-safe practices for variadic functions.
Use either variadic templates or std::initializer_list if all your arguments have the same type
With variadic template, you use recursion to go through a variadic parameter list. Variadic template example:
template<class T>
void MyFoo(T arg)
{
DoSomething(arg);
}
template<class T, class... R>
void MyFoo(T arg, R... rest)
{
DoSomething(arg);
// If "rest" only has one argument, it will call the above function
// Otherwise, it will call this function again, with the first argument
// from "rest" becoming "arg"
MyFoo(rest...);
}
int main()
{
MyFoo(2, 5.f, 'a');
}
This guarantees that if DoSomething, or any other code you run before the recursive call to MyFoo, has an overload for the type of each argument you pass to the function MyFoo, that exact overload will get called.
With std::initializer_list, you use a simple foreach loop to go through the arguments
template<class T>
void MyFoo(std::initializer_list<T> args)
{
for(auto&& arg : args)
{
DoSomething(arg);
}
}
int main()
{
MyFoo({2, 4, 5, 8, 1, 0}); // All the arguments have to have the same type
}
Yes! C++11 and above allows function templates to take a type-safe variable number of arguments, creating what's known as a "parameter pack". You can unpack that into, e.g., a std::array and get something a lot like you are looking for:
struct S {
template <typename... Args>
void SomeMethod(int someParam, Args&&... args) const {
//^ The "&&" here is a "forwarding reference" because Args is a template.
// Next line isn't required but arguably helps with error messages:
static_assert((std::is_convertible_v<Args, std::string_view> && ...), "All args in the pack must convert to string_view.");
// Convert args... to a std::array<std::string_view, N>:
const auto variableParams = std::array<std::string_view, sizeof...(args)>{std::forward<Args>(args)...};
if (not variableParams.empty()) { //< Not needed because it's a nop to iterate over an empty array:
for (const auto& item : variableParams) {
fmt::print("{}\n", item);
}
}
}
};
int main() {
S s;
s.SomeMethod(42, "foo", "bar", "baz");
}
output:
foo
bar
baz
https://godbolt.org/z/PbPb7qTv9
In C++20, you can use concepts to get a better error message and to be more concise:
void SomeMethod(int someParam,
std::convertible_to<std::string_view> auto&&... args) const {
Happy case: https://godbolt.org/z/aTG74Wx7j
Error case: https://godbolt.org/z/jToxYMThs
There is a named parameters library in boost (if I understood correctly what params in C# are). It allows writing functions like this:
int y = lib::f(_name = "bob", _index = 2);
Can't tell anything about if there is a significant overhead involved.
If you go right now and type string.Format into your IDE, you'll see that there are 4 different overloads: one taking a string and object, another taking a string and two objects, then one taking three objects, and finally one that uses params. According to this answer, this is because params generates 'overhead', and some other languages may not support it.
My question is, why can't a method call like this:
void Foo()
{
Bar(1, 2, 3);
}
void Bar(params int[] args)
{
// use args...
}
Be essentially transformed at compile time to
void Foo()
{
Bar(new[] { 1, 2, 3 });
}
void Bar(int[] args)
{
// use args...
}
? Then it wouldn't create any overhead except for the array creation (which was necessary anyway), and would be fully compatible with other languages.
The number of arguments is already known at compile-time, so what's preventing the C# compiler from doing some kind of string substitution and making the first scenario essentially syntactic sugar for the second? Why did we have to implement hidden language features specifically to support variadic arguments?
The title makes an incorrect assumption.
Both a params and a non-params methods take an array; the difference is the compiler will emit the IL to create an array implicitly when making a params method call. An array is passed to both methods, as a single argument.
This can be seen in this .NET Fiddle (view "Tidy Up -> View IL").
using System;
public class Program
{
public static void Main()
{
var a1 = 1;
var a2 = 2;
var a3 = 3;
with_params(a1,a2,a3);
no_params(new [] {a1,a2,a3});
}
public static void with_params(params int[] x) {}
public static void no_params(int[] x) {}
}
In both cases the IL is identical; a new array is created, it is populated, and the array is supplied to the invoked method.
There is an "exception" to this identical IL generation in that the compiler can move out constant-valued arrays when used in the non-parameter form and use 'dup' initialization, as seen here. However, a new array is supplied as the argument in both cases.