Custom collections - how we are able to get foreach and linq features without implementing any methods ?

Question

Custom collections - how we are able to get foreach and linq features without implementing any methods ?

Ravi R 1

Using a custom class Customer.
I have not implemented iEumerable.
Also i have not overridden the methods MoveNext(), Reset() of iEnumerator.

But i am able to use foreach to iterate through customers collection. And able to query using linq where clause.

I am not able to understand how we are able to get these foreach and linq features without implementing any methods in the below example.

Please help me to understand how this works.

public class Customer
{
public string Name { get; set; }
public string City { get; set; }
public string Mobile { get; set; }

// in main method
Customer[] customers = new Customer[]
{
new Customer {Name="Raj Kumar", City="Bangalore", Mobile="9599999123"},
new Customer { Name = "Sudhir Wadje", City = "Bhubaneswar", Mobile = "9488888321" },
new Customer { Name = "Anil", City = "Delhi", Mobile = "8077777987" }
};

        IEnumerable<Customer> result = from cust in customers where (cust.City.StartsWith("B")) select cust;
        foreach (Customer customer in result)
        {
            Console.WriteLine(customer.City);
        }

1 answer

Your answer

Answer 1

Bonnie DeWitt 811

It's because your customers variable is an array of Customer. It's the array that has all those IEnumerable methods. It would be the same for a List also.

Ravi R 1 Reputation point

2021-08-10T00:32:37.093+00:00

Thanks BonnieDeWitt for your quick response. I tried checking array class but could not find these methods overridden.
Could find array is implemented with iEnumerable but no methods have been overridden. Without overriding the methods how we are able to get the functionality. Please help to understand.
Bonnie DeWitt 811 Reputation points

2021-08-10T01:28:16.623+00:00

I'm sorry, I don't know where you're looking at this information. Since an array does implement IEnumerable, I don't really understand what methods there are to be overridden? The array class itself takes care of all that.
Timon Yang-MSFT 9,606 Reputation points

2021-08-10T03:11:45.587+00:00
@Ravi R
The only method in IEnumerable is:

IEnumerator GetEnumerator();

All methods in IEnumerator are overridden in Array class.
Please check their source code in Reference Source
Bonnie DeWitt 811 Reputation points

2021-08-10T04:55:39.003+00:00

My first answer about the array implementing IEnumberable was partly correct, but I didn't go far enough in my explanation. I'll try again:

Here are some quotes from two pieces of documentation:

From this link: https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/arrays/

Array types are reference types derived from the abstract base type Array. All arrays implement IList, and IEnumerable. You can use foreach iteration arrays in C#, since single-dimension arrays also implement IList<T>, and IEnumerable<T>.

And, from here: https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/statements/iteration-statements

The foreach statement executes a statement or a block of statements for each element in an instance of the type that implements the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> interface.

End of quotes.

So, @Ravi R , the IEnumerable that is actually used in your example is the Generic IEnumerable<T>, because you are using LINQ to get the result.
Ravi R 1 Reputation point

2021-08-10T22:33:54.18+00:00
Thanks for your help TimonYang. The link you shared by you https://referencesource.microsoft.com/#mscorlib/system/array.cs,156e066ecc4ccedf is fine where the GetEnumerator() method has been overridden. I could understand. But when i try to verify the same in visual studio i do not see the method implementation.

Please refer the below code and the steps i followed:

int[] nums = { 10, 20, 30, 40 };
var enumerator = nums.GetEnumerator();
while (enumerator.MoveNext())
{
Console.WriteLine(enumerator.Current);
}

In the above code, enumerator is System.Collections.IEnumerator type.

Please Go to the definition of GetEnumerator() in visual studio. Right click on GetEnunerator() method and click on Go to Definition.

It is defined in array class as follows:

public abstract class Array : ICloneable, IList, ICollection, IEnumerable, IStructuralComparable, IStructuralEquatable
{
....

public IEnumerator GetEnumerator(); ....

}

Array is an abstract class and GetEnumerator() has only the definition no implementation.
Ravi R 1 Reputation point

2021-08-10T22:38:48.98+00:00
Thanks BonnieDeWitt for your help. Hope i need to explain my question in a better way.

Please refer the below code and the steps i followed:

int[] nums = { 10, 20, 30, 40 };
var enumerator = nums.GetEnumerator();
while (enumerator.MoveNext())
{
Console.WriteLine(enumerator.Current);
}

In the above code, enumerator is System.Collections.IEnumerator type.

Please Go to the definition of GetEnumerator() in visual studio. Right click on GetEnunerator() method and click on Go to Definition.

It is defined in array class as follows:

public abstract class Array : ICloneable, IList, ICollection, IEnumerable, IStructuralComparable, IStructuralEquatable
{
....

public IEnumerator GetEnumerator(); ....

}

Array is an abstract class and GetEnumerator() has only the definition no implementation.

Please let me know how we are able to get the functionality of MoveNext() without overriding GetEnumerator().
Bonnie DeWitt 811 Reputation points

2021-08-10T23:01:45.86+00:00
Originally you were talking about foreach. I didn't know you actually wanted to know about using an enumerator.

You have to cast enumerator.Current, @Ravi R .

For your num[] array, it would be like this:

Console.WriteLine((int)enumerator.Current);

And for your original Customer[] array example, it would be like this:

Console.WriteLine(((Customer)enumerator.Current).City);

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Custom collections - how we are able to get foreach and linq features without implementing any methods ?

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