Is there was a way to pass a List as an argument to a params parameter? Suppose I have a method like this:
void Foo(params int[] numbers)
{
// ...
}
This way I can call it by passing either an array of ints or ints separated by commas:
int[] numbers = new int[] { 1, 5, 3 };
Foo(numbers);
Foo(1, 5, 3);
I wanted to know if there is a way to also be able to pass a List as an argument (without having to convert it to an array). For example:
List<int> numbersList = new List<int>(numbers);
// This won't compile:
Foo(numbersList);
No
Sadly, that's the answer. No, there is not. Not without converting it to an array (for example with .ToArray()).
You would want to change Foo to accept something other than params int[] to do this.
void Foo(IEnumerable<int> numbers) {
}
This would allow you to pass in either an int[] or a List<int>
To allow both ways, you could do this:
void Foo(params int[] numbers) {
Foo((IEnumerable<int>)numbers);
}
void Foo(IEnumerable<int> numbers) {
//Do the real thing here
}
If able to change the existing code, you could always create an overload that accepts a List and passes it to the params method.
void Foo(params int[] numbers)
{
// ...
}
void Foo(IList<int> numbers) => Foo(numbers.ToArray());
Related
The method PrintTimes(string a, int b) prints the string a, b times (i.e. PrintTimes("test",3) will print testtesttest).
I want to create a method, which will get a params array of strings and a params array of integers. So the call function will ook like this
PrintTimes("A","B","C","D",2,1,3,2);
Or
PrintTimes("A",2,"B",1,"C",3,"D",2)
Both of which will print AABCCCDD
Since there can be only one params parameter in a method, this is impossible. So is there a way to do this?
I know I can create a Class with a string and an int variable, and create a params array for the class. But I'd rather not, since it would involve constructing a new Class for each set
Why not using a class
public class PrintParameter
{
public int Count {get;set;}
public string Content{get;set;}
}
Then
public void PrintTimes(List<PrintParameter> inputs)
{
//for each input print the "Content", "Count" times
}
Or
public void PrintTimes(params PrintParameter[] inputs)
{
//for each input print the "Content", "Count" times
}
If you don't want to define a class you may try something like List<KeyValuePair<string,int>> or other alternatives such as List<Tuple<string,int>> and etc. However the preferred way of doing is to use a class with meaningful properties.
How about just dropping the params keyword and taking arrays instead? That is, make the signature PrintTimes(string[], int[]). PrintTimes(new[]{"A","B","C","D"}, new[]{2,1,3,2}); isn't that much more to write.
There are several ways you can go about solving this problem. While you can only have one params, you can just make both your parameters arrays:
PrintTimes(string[] strings, int[] printCounts)
{
// Assert strings.Length == printCounts.Length
for (int i = 0; i < strings.Length; i++)
{
for (int j = 0; j < printCounts[i]; j++)
{
// Print strings[i]
}
}
}
Then it can be called like this:
int[] numbers = new int[3] {1, 2, 3};
string[] names = new string[3] {"Matt", "Joanne", "Robert"};
PrintTimes(names, numbers);
Following up on Hossein's suggestion, why not something as simple as:
void PrintTimes(List<String> strings, List<Int> count)
(or do it as an array as the comments said) Then inside PrintTimes require the inputs to be the same length or some logical fail when they don't.
As you know C# supports variadic methods through the params keyword:
int Add(params int[] xs) {
return xs.Sum();
}
Which can then be called with any number of arguments you like:
Add(1);
Add(1, 2);
Add(1, 2, 3);
But say I want to call Add using an array of ints1. Is this possible and how (preferably without reflection)? I tried the following but they gave syntax errors (the syntax was pure guessing):
var xs = new[] { 1, 2, 3 };
Add(xs...); // doesn't work; syntax error
Add(params xs); // doesn't work; syntax error
1 My actual use-case is different but I thought this example would be less complicated.
Your method needs a return type:
int Add(params int[] xs) {
return xs.Sum();
}
And to call it with an array you just use the ordinary syntax for method calls:
int[] xs = new[] { 1, 2, 3 };
var result = Add(xs);
The params keyword basically just allows you to take advantage of a little syntactic sugar. It tells the compiler that when it sees
Add(1, 2, 3);
It should convert that to
Add(new int[] { 1, 2, 3});
So to do this from your code, you don't have to do anything special.
int[] parameters = new int[] { ... }
results = Add(parameters);
See the documentation for more details.
As far as I know, you can just call it with an array like you would a normal method:
Add(xs);
Nothing fancy, no params keyword on the method call, no dots.
static void Main(string[] args)
{
int[] tmp = {1, 2};
var sum = Add(tmp);
}
public static int Add(params int[] xs)
{
return xs.Sum();
}
Should work just fine..
If it's anything like Java, you can just call the method with the array as an argument.
This feature is also what makes varargs dangerous, especially if one of the vararg types is also an array...
Can I forward a params parameter to another method?
e.g.,
void MyStringMethod(string format, params object[] list)
{
String.Format(format, list);
}
Works for me.
void Main()
{
Method1("{0},{1},{2},{3}","Can","I","do","this").Dump();
}
String Method1(string format, params object[] list)
{
return String.Format(format,list);
}
returns
Can,I,do,this
yes ... but!
I know this is technically not a different answer, however I've been burned so many times with really hard to debug errors (caused by using params in what looked like really simple scenarios) that I feel compelled to post this comment.
Please be very careful when you use params. Ideally don't use them, unless you feel you absolutely have to, and then make sure you don't use the same data type in front of the params! This can make quickly reading the code (scanning) it and knowing what it will do very difficult, or worse, give you an unexpected and hard to find bug.
For example, in the code below, it's not easy to see what will be printed to the Console. Is the (1,2,3) being passed to WhoKnocked actually being identified as int 1 followed by int[] [2,3] or perhaps int[] [1,2,3]?
void Main()
{
Console.WriteLine(WhoKnocked(1,2,3));
}
public string WhoKnocked(int x, params int[] knocks)
{
return "It's mee!";
}
public string WhoKnocked(params int[] knocks)
{
return "No, it's not, its you!";
}
lastly, here's another example that might give you some surprising results.
void Main()
{
Greet(1,"foo", "bar");
Greet(1, 2, "bar");
Greet(1,"foo", new object());
Greet(1,2,3);
Greet(1,2,3,4);
}
public void Greet(int i, params object[] foo)
{
Console.WriteLine("Number then param array of objects!");
}
public void Greet(int i, int x, params object[] foo)
{
Console.WriteLine("Number, ...nuther number, and finally object[]!");
}
public void Greet(int i, string x, params object[] foo)
{
Console.WriteLine("number, then string, then object[]!");
}
produces the following output
Number, then String, then Object[]!
Number, ...nuther Number, and finally Object[]!
Number, then String, then Object[]!
Number, ...nuther Number, and finally Object[]!
Number, ...nuther Number, and finally Object[]!
The params keyword is just a form of syntactic sugar designed to allow you to make method calls as if they had a dynamic parameter count. All it is really is just a compiler transformation of multiple arguments to an array instantiation. That's all it is, an array.
An array is just another object that could be passed to other methods and whatnot so yes, you can forward that array of you wish.
I believe you can, it is just an array of objects. If you then call another function that expects a param list then you can get unexpected results (depending on what you expect of course:-). Notice in the third case you only get 2 params.
void Test()
{
DoIt(1, 2, 3, 4);
}
private void DoIt(params object[] p)
{
Console.WriteLine(p.Length);
DoIt2(p);
DoIt2(p, 5);
}
private void DoIt2(params object[] p)
{
Console.WriteLine(p.Length);
}
As the title says I need to know if there is a corresponding syntax as java's ... in method parameters, like
void printReport(String header, int... numbers) { //numbers represents varargs
System.out.println(header);
for (int num : numbers) {
System.out.println(num);
}
}
(code courtesy of wikipedia)
Yes you can write something like this:
void PrintReport(string header, params int[] numbers)
{
Console.WriteLine(header);
foreach (int number in numbers)
Console.WriteLine(number);
}
Try using the params keyword, placed before the statement, eg
myFunction(params int[] numbers);
Yes, there is. As Adriano said you can use C# 'params' keyword.
An example is the in link below:
params (C# Reference)
http://msdn.microsoft.com/en-us/library/w5zay9db.aspx
"The params keyword lets you specify a method parameter that takes a variable number of arguments.
You can send a comma-separated list of arguments of the type specified in the parameter declaration, or an array of arguments of the specified type. You also can send no arguments.
No additional parameters are permitted after the params keyword in a method declaration, and only one params keyword is permitted in a method declaration."
You can declare a method to har a variable number of parameters by using the params keyword. Just like when using ... in Java, this will give you an array and let you call the metods with a variable number of parameters:
http://msdn.microsoft.com/en-us/library/w5zay9db(v=vs.71).aspx
This should be
void printReport(String header, params int[] numbers)
I believe you mean params
public void printReport(string header, params int[] list)
{
Console.WriteLine(header);
for (int i = 0 ; i < list.Length; i++)
{
Console.WriteLine(list[i]);
}
Console.WriteLine();
}
You can use params, although this must always come last in the list:
public void PrintReport(string header, params int[] numbers)
{
It is however possible to combine params optional parameters (such as [CallerMemberName]) by using named arguments, which works even if the parameters are of the same type.
Declare the method like this:
public static void PrintReport(
[CallerMemberName] string callerName = "",
[CallerFilePath] string sourceFilePath = "",
params string[] inputStrings)
{
and call it like this:
PrintReport(inputStrings: new[] { "string 1", "string 2" } );
I am new to C# (previously working on C++), I used to pass array to function with specific index. Here is the code in C++,
void MyFunc(int* arr) { /*Do something*/ }
//In other function
int myArray[10];
MyFunc(&myArray[2]);
Can I do something like this in C# .Net* ?*
As Array is Enumerable, you can make use of LINQ function Skip.
void MyFunc( int[] array ){ /*Do something*/ }
int[] myArray = { 0, 1, 2, 3, 4, 5, 6 }
MyFunc( myArray.Skip(2) );
The linq version is my preferred one. However it will prove to be very very inefficient.
You could do
int myArray[10];
int mySlice[8];
Array.Copy(myArray, 2, mySlice, 0);
and pass mySlice to the function
.NET has the System.ArraySegment<T> structure which addresses this exact use-case.
But I’ve never actually seen this structure used in code, with good reasons: it doesn’t work. Notably, it doesn’t implement any interfaces such as IEnumerable<T>. The Linq solution (= using Skip) is consequently the best alternative.
Probably the most simple way to do this is:
public void MyFunction(ref int[] data,int index)
{
data[index]=10;
}
And call it by this way:
int[] array= { 1, 2, 3, 4, 5 };
Myfunction(ref array,2);
foreach(int num in array)
Console.WriteLine(num);
This will print 1,2,10,4,5
public void MyFunc(ref int[] arr)
{
// Do something
}
int[] myArray = .... ;
MyFunc(ref myArray);
See here for more information on ref!