Value Or Reference? A C# Puzzle
Written by Balamurugan Boominathan   
Article Index
Value Or Reference? A C# Puzzle
Solution

Solution

It doesn't work and its all due to the use of a value type within the foreach loop.

Because Product entity has been declared as struct which is value type  you can not directly update the Quantity property of a  Product entity which is being used as the iteration variable.

It even generates a compiler error message:

“Cannot modify p because it is foreach iteration variable.”

This doesn't happen if the Product entity is declared as a Class which is a reference type. In this case

p.Quantity = p.Quantity - Quantity;

will work because it modifies the object that p refers to rather than the value of p.

If you want to use a value type in a loop in this way you have to do the job more explicity and avoid using it as the loop variable.

For example:.

for (int i = 0; i < lstProducts.Count; i++)
{
Product p = lstProducts[i];
if (p.Name == ProductName)
{
p.Quantity = p.Quantity - Quantity;
lstProducts[i] = p;
}
}

In this case we have to retrieve the value type from the collection before we work on it.

Pattern

The only real solution is to avoid using value types within sophisticated data structures. In other words, always prefer a class to a struct. This also avoids the potential problem of some future programmer converting your struct to a class thinking that it doesn't make any difference.

 

Further reading:

Inside C# 4 Data Structs

Value and Reference

 

Banner

 

blog comments powered by Disqus

 

To be informed about new articles on I Programmer, subscribe to the RSS feed, follow us on Google+, Twitter, Linkedin or Facebook or sign up for our weekly newsletter.

More Puzzles

Python
Python Puzzle - Where Did The Time Go

A Python programming puzzle to get you up to speed. This one is all about time keeping, or is it? There are some strange things that go on in Python when you aren't paying attention. 


PHP
The Missing PHP Global

A global variable is a global variable and that's all there is to it. But if you write PHP code to be used by other people, you might want to consider that this isn't always the case. Can you see wher [ ... ]


C++
A problem of order - constructor initialization

Initializers can be used to make constructors more elegant but they also introduce subtle problems. This brainteaser for C++ programmers illustrates a common pitfall.


Other Articles

<ASIN:0321637003>

<ASIN:0596800959>

<ASIN:047043452X>

<ASIN:0123745144>



 
 

   
RSS feed of all content
I Programmer - full contents
Copyright © 2014 i-programmer.info. All Rights Reserved.
Joomla! is Free Software released under the GNU/GPL License.