c# - List<Object> vs List<dynamic>

Question

I need to create a heterogeneous List of objects (custom classes). My first thought was to create a List<ISomeMarkerInterface> but I quickly learned that this is not what I want. My next thought was List<dynamic> and this didn't seem to be a bad idea. However, I was doing some research and came across this article about boxing and unboxing and in the example, they're doing basically what I want using List<Object>.

Aside from the fact that dynamic will be evaluated at runtime and Object at compile-time, what is the difference between List<dynamic> and List<Object>? Aren't they essentially the same thing?

Answer 1

There are 3 "general" types (although not all are real types) in C#: object, var and dynamic.

Object

An actual type, like any other type, with one special rule: if a type doesn't inherit, it inherits from object. From this, it follows that all types inherit from object, directly or indirectly.

Emphasis: object is a type. An object can be of type object, and the type has its methods, like ToString(). Since everything inherits from object, everything can be upcast into object. When you assign an object to an object reference, you are doing upcasting just like when you assign an Elephant type object to an Animal reference where Elephant inherits from Animal.

SomeType x = new SomeType();
object obj = x;
obj.DoSomething();

• obj is treated as being of type object at compile time, and will be of type object at runtime (which is logical, since it is an actual type - obj is declared as object so can only be of that type)
• obj.DoSomething() will cause a compile-time error, as object does not have this method, regardless of whether SomeType has it.

Var

This is not an actual type, it is merely shorthand for "compiler, figure out the type for me based on the right side of the assignment".

SomeType x = new SomeType();
var obj = x;
obj.DoSomething();

• obj is treated as being of type SomeType at compile time, and will be of type SomeType at runtime, just as if you had written "SomeType" instead of "var".
• if SomeType has a method DoSomething(), this code will work
• if SomeType doesn't have the method, the code will cause a compile-time error

Dynamic

This is a type that tells the compiler to disable compile-time type checking on the variable. An object is treated as having the type dynamic at compile-time and run-time.

SomeType x = new SomeType();
dynamic obj = x;
obj.DoSomething();

• obj is of type dynamic at compile and run time
• if SomeType has a method DoSomething(), this code will work
• if SomeType doesn't have the method, the code will compile, but throw an exception at run-time

• note that dynamic can cause exceptions very easily if used carelessly:

  public void f(dynamic x)
  { 
      x.DoSomething();
  }
  

This will throw an exception if x is of a type that doesn't have the DoSomething method, but it will still be possible to call it and pass any object as the parameter without a compile-time error, causing an error that only shows itself at run-time, and possibly only in specific circumstances - a potential bug. So if you use dynamic in any kind of public interface of a class, you should always manually type-check at runtime using reflection, carefully deal with exceptions, or not do it in the first place.

Note: the object being referred to never changes its type, of course. While obj may be object, the x that it refers to is still SomeType.