JavaScript spread syntax in C#

C# 12 has added the spread feature.

int[] row0 = [1, 2, 3];
int[] row1 = [4, 5, 6];
int[] row2 = [7, 8, 9];
int[] single = [..row0, ..row1, ..row2];
foreach (var element in single)
{
    Console.Write($"{element}, ");
}

https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-12

Old answer (pre C# 12)

There isn't a spread option, but some useful alternatives.

1. Method Parameters aren't an array in C# unless you use the params keyword
2. Method Parameters that use the param keyword would have to either:
   1. Share the same type
   2. Have a castable shared type such as double for numerics
   3. Be of type object[] (as object is the root type of everything)

However, having said that, you can get similar functionality with various language features.

Answering your example:

C#

var arr = new []{
   "1",
   "2"//...
};

Console.WriteLine(string.Join(", ", arr));

The link you provide has this example:

Javascript Spread

function sum(x, y, z) {
  return x + y + z;
}

const numbers = [1, 2, 3];

console.log(sum(...numbers));
// expected output: 6

console.log(sum.apply(null, numbers));

Params In C#, with same type

public int Sum(params int[] values)
{
     return values.Sum(); // Using linq here shows part of why this doesn't make sense.
}

var numbers = new int[] {1,2,3};

Console.WriteLine(Sum(numbers));

In C#, with different numeric types, using double

public int Sum(params double[] values)
{
     return values.Sum(); // Using linq here shows part of why this doesn't make sense.
}

var numbers = new double[] {1.5, 2.0, 3.0}; // Double usually doesn't have precision issues with small whole numbers

Console.WriteLine(Sum(numbers));

Reflection In C#, with different numeric types, using object and reflection, this is probably the closest to what you are asking for.

using System;
using System.Reflection;

namespace ReflectionExample
{
    class Program
    {
        static void Main(string[] args)
        {
            var paramSet = new object[] { 1, 2.0, 3L };
            var mi = typeof(Program).GetMethod("Sum", BindingFlags.Public | BindingFlags.Static);
            Console.WriteLine(mi.Invoke(null, paramSet));
        }

        public static int Sum(int x, double y, long z)
        {
            return x + (int)y + (int)z;
        }
    }
}

C# 12 has introduced the spread operator similar to Javascript. It is available with .Net 8

We can write

int[] row0 = [1, 2, 3];
int[] row1 = [4, 5, 6];
int[] row2 = [7, 8, 9];
int[] single = [..row0, ..row1, ..row2];

One trick to get a behavior similar to this (without reflection) is to accept params SomeObject[][] and to also define an implicit operator from SomeObject to SomeObject[]. Now you can pass a mixture of arrays of SomeObject and individual SomeObject elements.

public class Item
{
    public string Text { get; }

    public Item (string text)
    {
        this.Text = text;
    }

    public static implicit operator Item[] (Item one) => new[] { one };
}

public class Print
{
    // Accept a params of arrays of items (but also single items because of implicit cast)

    public static void WriteLine(params Item[][] items)
    {
        Console.WriteLine(string.Join(", ", items.SelectMany(x => x)));
    }
}

public class Test
{
    public void Main()
    {
        var array = new[] { new Item("a1"), new Item("a2"), new Item("a3") };
        Print.WriteLine(new Item("one"), /* ... */ array, new Item("two")); 
    }
}

there is no direct pre-built library in C# to handle what is built into Spread

In order to get that functionality in C#, you need to Reflect the object and get the methods, properties, or fields by their access modifiers.

You'd do something like:

var tempMethods = typeof(myClass).GetMethods();
var tempFields = typeof(myClass).GetFields();
var tempProperties = typeof(myClass).GetProperties();

then iterate through and throw them into your dynamic object:

using System;
using System.Collections.Generic;
using System.Dynamic;

namespace myApp
{
    public class myClass
    {
        public string myProp { get; set; }
        public string myField;
        public string myFunction()
        {
            return "";
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var fields = typeof(myClass).GetFields();
            dynamic EO = new ExpandoObject();
            foreach (int i = 0; i < fields.Length; i++)
            {
                AddProperty(EO, "Language", "lang" + i);
                Console.Write(EO.Language);
            }
        }

        public static void AddProperty(ExpandoObject expando, string propertyName, object propertyValue)
        {
            // ExpandoObject supports IDictionary so we can extend it like this
            var expandoDict = expando as IDictionary<string, object>;
            if (expandoDict.ContainsKey(propertyName))
                expandoDict[propertyName] = propertyValue;
            else
                expandoDict.Add(propertyName, propertyValue);
        }
    }
} 

https://www.oreilly.com/learning/building-c-objects-dynamically

I came here looking for the c# range operator as in numbers[ 1 .. 4]

int[] numbers = new[] { 0, 10, 20, 30, 40, 50 };
int start = 1, amountToTake = 3;
int[] subset = numbers[start..(start + amountToTake)];  // contains 10, 20, 30
numbers[1 .. 4] // returns 10, 20, 30

In linqpad this looks like this