C# | Copying the Collection<T> elements to an array - GeeksforGeeks

01 Feb, 2019
Collection<T>.CopyTo(T[], Int32) method is used to copy the entire Collection<T> to a compatible one-dimensional Array, starting at the specified index of the target array.
Syntax:
public void CopyTo (T[] array, int index);

Parameters:
array : The one-dimensional Array that is the destination of the elements copied from Collection<T>. The Array must have zero-based indexing.
index : The zero-based index in array at which copying begins.
Exceptions:
ArgumentNullException : If the array is null.
ArgumentOutOfRangeException : If the index is less than zero.
ArgumentException : If the number of elements in the source Collection<T> is greater than the available space from index to the end of the destination array.
Below given are some examples to understand the implementation in a better way:
Example 1:
// C# code to copy the entire Collection// to a compatible one-dimensional Array,// starting at the specified index of// the target arrayusing System;using System.Collections.Generic;using System.Collections.ObjectModel;  class GFG {      // Driver code    public static void Main()    {        // Creating a collection of strings        Collection<string> myColl = new Collection<string>();          myColl.Add("A");        myColl.Add("B");        myColl.Add("C");        myColl.Add("D");        myColl.Add("E");          // Creating a string array        string[] myArr = new string[myColl.Count];          // Copying the entire Collection to a        // compatible one-dimensional Array,        // starting at the specified index        // of the target array        myColl.CopyTo(myArr, 0);          // Displaying the elements in myArr        foreach(string str in myArr)        {            Console.WriteLine(str);        }    }}
Output:
A
B
C
D
E

Example 2:
// C# code to copy the entire Collection// to a compatible one-dimensional Array,// starting at the specified index of// the target arrayusing System;using System.Collections.Generic;using System.Collections.ObjectModel;  class GFG {      // Driver code    public static void Main()    {        // Creating a collection of ints        Collection<int> myColl = new Collection<int>();          myColl.Add(2);        myColl.Add(3);        myColl.Add(4);        myColl.Add(5);          // Creating an integer array        int[] myArr = new int[myColl.Count];          // Copying the entire Collection to a        // compatible one-dimensional Array,        // starting at the specified index        // of the target array        // This should raise "ArgumentOutOfRangeException"        // as index is less than 0        myColl.CopyTo(myArr, -5);          // Displaying the elements in myArr        foreach(int i in myArr)        {            Console.WriteLine(i);        }    }}
Runtime Error:
Unhandled Exception:System.ArgumentOutOfRangeException: Value has to be >= 0.Parameter name: destinationIndex
Note:
This method uses Array.Copy to copy the elements.
The elements are copied to the Array in the same order in which the enumerator iterates through the Collection<T>.
This method is an O(n) operation, where n is Count.
Reference:
https://docs.microsoft.com/en-us/dotnet/api/system.collections.objectmodel.collection-1.copyto?view=netframework-4.7.2
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