Value Types
Value types include the following:
- All numeric data types
- Boolean, Char, and Date
- All structures, even if their members are reference types
- Enumerations, since their underlying type is always SByte, Short, Integer, Long, Byte, UShort, UInteger, or ULong
Reference Types
Reference types include the following:
- String
- All arrays, even if their elements are value types
- Class types, such as Form
- Delegates
Elements That Are Not Types
The following programming elements do not qualify as types, because you cannot specify any of them as a data type for a declared element:
- Namespaces
- Modules
- Events
- Properties and procedures
- Variables, constants, and fields
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:
[edit]
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
nullreference 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.
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