In .NET, there are two categories of types, reference types and value types.
Structs are value types and classes are reference types.
The general difference is that a reference type lives on the heap, and a value type lives inline, that is, wherever it is your variable or field is defined.
A variable containing a value type contains the entire value type value. For a struct, that means that the variable contains the entire struct, with all its fields.
A variable containing a reference type contains a pointer, or a reference to somewhere else in memory where the actual value resides.
This has one benefit, to begin with:
*value type*s always contains a value
reference types can contain a null-reference, meaning that they don't refer to anything at all at the moment
Internally, *reference type*s are implemented as pointers, and knowing that, and knowing how variable assignment works, there are other behavioral patterns:
copying the contents of a value type variable into another variable, copies the entire contents into the new variable, making the two distinct. In other words, after the copy, changes to one won't affect the other
copying the contents of a reference type variable into another variable, copies the reference, which means you now have two references to the same somewhere else storage of the actual data. In other words, after the copy, changing the data in one reference will appear to affect the other as well, but only because you're really just looking at the same data both places
When you declare variables or fields, here's how the two types differ:
variable: value type lives on the stack, reference type lives on the stack as a pointer to somewhere in heap memory where the actual memory lives
class/struct-field: value type lives inside the class, reference type lives inside the class as a pointer to somewhere in heap memory where the actual memory lives.
Classes can be nothing - the reference can point to a null.
Structs are the actual value - they can be empty but never null. For this reason structs always have a default constructor with no parameters - they need a 'starting value.
For Example:
struct MyStruct
{
string MyProperty { get; set; }
}
void ChangeMyStruct(MyStruct input)
{
input.MyProperty = "new value";
}
...
// Create value type
MyStruct testStruct = new MyStruct { MyProperty = "initial value" };
ChangeMyStruct(testStruct);
// Value of testStruct.MyProperty is still "initial value"
// the method changed a new copy of the structure.
*********For a class this would be different*******
class MyClass
{
string MyProperty { get; set; }
}
void ChangeMyClass(MyClass input)
{
input.MyProperty = "new value";
}
...
// Create reference type
MyClass testClass = new MyClass { MyProperty = "initial value" };
ChangeMyClass(testClass);
// Value of testClass.MyProperty is now "new value"
// - the method changed the instance passed.
