Important points regarding ref in C#

• ref keyword is used to return more than one values from a function.
• ref keywords need to be defined before you can pass them as a parameter to function
• Called method is not bound to assign any value to the ref parameter since it has already been initialized by calling method

Case Study

We need to find if a number i.e. 4 exist in an array and if exist we would like to change its value to 111

Example using C# 6.0

Consider below extension method which returns index of a number if it’s found in array

public static class NumberExtensions
    {
        public static int FindIndexOfNumber(this int[] numbers, int numberToSearch)
        {
            for (int i = 0; i < numbers.Length; i++)
            {
                if (numbers[i] == numberToSearch)
                {
                    return i;
                }
            }
            throw new ArgumentException(string.Format("Number {0} not found", numberToSearch));
        }
}

And here is called method which finds 4 in array and change its value

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     int[] numbers = new[]{ 1, 2, 3, 4, 5, 6};

            try
            {
                int index = numbers.FindIndexOfNumber(4); //Get Index of number so that we can change value later
                numbers[index] = 111;//change value at returned index
            }
            catch (Exception)
            {
                //Do nothing
            }
            Console.WriteLine("Value at index 3 is {0}", numbers[3]);//Output : Value at index 3 is 111

So as you can see that array is iterated twice, first time while searching the value i.e. inside FindIndexOfNumber method and second time while assigning value i.e.

numbers[index] = 111;

For small arrays, the performance impact is negligible but for very large arrays there will be a significant hit in performance and thus is not a good way of doing this.

Let’s try to create the same example in C# 7 using Locals as Ref feature

We will change our extension method as

public static class NumberExtensions

{

public static ref int FindReferenceOfNumber(this int[] numbers, int numberToSearch)

{

for (int i = 0; i < numbers.Length; i++)

{

if (numbers[i] == numberToSearch)

{

return ref numbers[i];

}

}

throw new ArgumentException(string.Format("Number {0} not found", numberToSearch));

}

}

And our caller method will look like

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private void btnReturnLocalAsRef_Click(object sender, EventArgs e)

{

int[] numbers = new[] { 1, 2, 3, 4, 5, 6 };

 

try

{

int index = numbers.FindReferenceOfNumber(4); //Get Index of number so that we can change value later

index = 111;//No change in value since destination is not a ref type

 

Console.WriteLine("Value at index 3 is {0}", numbers[3]);//Output: Value at index 3 is 4

 

//Correct Usage to store returning refs

ref int refIndex =ref numbers.FindReferenceOfNumber(4); //Get Index of number so that we can change value later

refIndex = 111;

Console.WriteLine("Value at index 3 is {0}", numbers[3]);//Output: Value at index 3 is 111

 

}

catch (Exception)

{

//Do nothing

}

}

A very handy feature that can save you lot of CPU while working with large arrays.

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Some restrictions have been defined by language while returning ref locals

• You can only return refs that are “safe to return”: Ones that were passed to you, and ones that point into fields in objects.
• Ref locals are initialized to a certain storage location, and cannot be mutated to point to another.

All code samples are available at my GitHub profile

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