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Boxing a value type in .NET to make it a reference type

, 30 Sep 2011 CPOL
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How to wrap a value type so it can be used in positions where a reference type is required
Sometimes, you want to pass or store data which is in a value type in a context where only a reference type makes sense. This is either because you want to be able to modify the value from another location (as in this question[^]), or because you want to pass it as a type parameter to a generic class or method which has a where T: class (or where T: new()) restriction.

You can write a boxing class which can be used in such scenarios:
public class Box<T> where T:struct {
 public T Value { get; set; }

 public Box() : this(default(T)) {} // so it can be created from new()
 public Box(T value) { this.Value = value; }
 public static implicit operator T(Box<T> box) { return box.Value; }
 public static implicit operator Box<T>(T value) { return new Box<T>(value); }

Usage: If you want to use it as a 'pointer', you need to store the reference and pass it explicitly:
Box<int> refint = 5;
ModifyingClass instance = new ModifyingClass(refint);
Debug.Assert(6 == refint);

// where
class ModifyingClass {
 Box<int> target;
 public ModifyingClass(Box<int> i) { target = i; }
 public void Adjust() { target.Value++; }

To pass it as a prototype to a generic class, simply do
SomeRestrictedGeneric<Box<T>> variable = new SomeRestrictedGeneric<Box<T>>()


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

United Kingdom United Kingdom
I'm a recent graduate (MSci) from the University of Cambridge, no longer studying Geology. Programming is a hobby so I get to write all the cool things and not all the boring things Smile | :) . However I now have a job in which I have to do a bit of work with a computer too.

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Comments and Discussions

GeneralWhat is the advantage of your solution in opposite to just m... Pin
Klaus Luedenscheidt30-Sep-11 19:28
memberKlaus Luedenscheidt30-Sep-11 19:28 
GeneralRe: You can't save a reference passed as a ref parameter for lat... Pin
BobJanova1-Oct-11 1:39
memberBobJanova1-Oct-11 1:39 
GeneralReason for my vote of 5 Excellent explanation from the stand... Pin
SAKryukov29-Sep-11 20:22
mvpSAKryukov29-Sep-11 20:22 
GeneralRe: Some poorly written generic containers have a class or new()... Pin
BobJanova30-Sep-11 3:34
memberBobJanova30-Sep-11 3:34 
Some poorly written generic containers have a class or new() constraint. We had one in our application which initially made me research this, and although we are allowed to change that code (the constraint was not necessary once you know about default(T)) so primitive types were allowed, it is easy to imagine a situation where someone might not be able to.

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