Value vs Reference

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Exam 70-536 Study Guide

1. Types and collections Manage data Value types Reference types Attributes Generic types Exception classes Boxing, Unboxing TypeForwardedTo Value vs Reference Manage collections Generic collections Specialized collections Implement interfaces Events and delegates 2. Process, threading,… 3. Embedding features 4. Serialization, I/O 5. .NET Security 6. Interop., reflection,… 7. Global., drawing, text

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Contents 1 An example 2 Summary

An example

This example shows the difference between how a value type works in memory, and how a reference does. First we define a new Value Type and a new Reference Type.

class ReferenceType
{
   public Int32 X;
}
 
struct ValueType
{
   public Int32 X;
}

Now we want to use these types to show how they work:

ReferenceType ref1 = new ReferenceType();
ValueType val1     = new ValueType();
 
ref1.X = 5;
val1.X = 5;

The layout of the memory at this point looks something like the following:

If we now make copies of these values using the following code:

ReferenceType ref2 = ref1;
ValueType val2 = val1;

The layout of the memory now looks something like the following:

If we now change the value of each of the new copies using the following code:

Ref2.X = 8;
Val2.X = 8;

The layout of the memory now looks something like the following:

Summary

• A variable that is of type value directly contains a value. Assigning a variable of type value to another variable of type value COPIES that value.
• A variable of type reference, points to a place in memory where the actual object is contained. Assigning a variable of type reference to another variable of type reference copies that reference (it tells the new object where the place in memory is), but does not make a copy of the object.

• Value types are stored on the stack.
• Reference types are stored on the heap.

• Value types can not contain the value null. *
• Reference types can contain the value null.

• Value types have a default implied constructor that initializes the default value.
• Reference types default to a null reference in memory.

• Value types derive from System.ValueType.
• Reference types derive from System.Object.

• Value types cannot derive a new type from an existing value type, but they are able to implement interfaces.
• Reference types can derive a new type from an existing reference type as well as being able to implement interfaces.

• Changing the value of one value type does not affect the value of another value type.
• Changing the value of one reference type MAY change the value of another reference type.

• The nullable type (only in .NET 2.0) can be assigned the value null.