What's the difference between struct and class in .NET?

Question

What\'s the difference between struct and class in .NET?

Answer 1

1. Events declared in a class have their += and -= access automatically locked via a lock(this) to make them thread safe (static events are locked on the typeof the class). Events declared in a struct do not have their += and -= access automatically locked. A lock(this) for a struct would not work since you can only lock on a reference type expression.

2. Creating a struct instance cannot cause a garbage collection (unless the constructor directly or indirectly creates a reference type instance) whereas creating a reference type instance can cause garbage collection.

3. A struct always has a built-in public default constructor.

   class DefaultConstructor
   {
       static void Eg()
       {
           Direct     yes = new   Direct(); // Always compiles OK
           InDirect maybe = new InDirect(); // Compiles if constructor exists and is accessible
           //...
       }
   }
   

   This means that a struct is always instantiable whereas a class might not be since all its constructors could be private.

   class NonInstantiable
   {
       private NonInstantiable() // OK
       {
       }
   }
   
   struct Direct
   {
       private Direct() // Compile-time error
       {
       }
   }
   

4. A struct cannot have a destructor. A destructor is just an override of object.Finalize in disguise, and structs, being value types, are not subject to garbage collection.

   struct Direct
   {
       ~Direct() {} // Compile-time error
   }
   class InDirect
   {
       ~InDirect() {} // Compiles OK
   }
   
   And the CIL for ~Indirect() looks like this:
   
   .method family hidebysig virtual instance void
           Finalize() cil managed
   {
     // ...
   } // end of method Indirect::Finalize
   

5. A struct is implicitly sealed, a class isn't.
   A struct can't be abstract, a class can.
   A struct can't call : base() in its constructor whereas a class with no explicit base class can.
   A struct can't extend another class, a class can.
   A struct can't declare protected members (for example, fields, nested types) a class can.
   A struct can't declare abstract function members, an abstract class can.
   A struct can't declare virtual function members, a class can.
   A struct can't declare sealed function members, a class can.
   A struct can't declare override function members, a class can.
   The one exception to this rule is that a struct can override the virtual methods of System.Object, viz, Equals(), and GetHashCode(), and ToString().