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

Last Updated : 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 array 
using 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 array 
using 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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