What's the Difference Between Dynamic (C# 4) and Var

What's the difference between dynamic (C# 4) and var?

var is static typed - the compiler and runtime know the type - they just save you some typing... the following are 100% identical:

var s = "abc";
Console.WriteLine(s.Length);

and

string s = "abc";
Console.WriteLine(s.Length);

All that happened was that the compiler figured out that s must be a string (from the initializer). In both cases, it knows (in the IL) that s.Length means the (instance) string.Length property.

dynamic is a very different beast; it is most similar to object, but with dynamic dispatch:

dynamic s = "abc";
Console.WriteLine(s.Length);

Here, s is typed as dynamic. It doesn't know about string.Length, because it doesn't know anything about s at compile time. For example, the following would compile (but not run) too:

dynamic s = "abc";
Console.WriteLine(s.FlibbleBananaSnowball);

At runtime (only), it would check for the FlibbleBananaSnowball property - fail to find it, and explode in a shower of sparks.

With dynamic, properties / methods / operators / etc are resolved at runtime, based on the actual object. Very handy for talking to COM (which can have runtime-only properties), the DLR, or other dynamic systems, like javascript.

dynamic vs var in C#

var means the static type is inferred - in your case it's exactly equivalent to

A a1 = new A();

All the binding is still done entirely statically. If you look at the generated code, it will be exactly the same as with the above declaration.

dynamic means that all any expression using a2 is bound at execution time rather than at compile-time, so it can behave dynamically. The compiler won't check whether the Foo method exists - the behaviour is determined at execution time. Indeed, if the object implements IDynamicMetaObjectProvider it could decide what to do with the call at execution time, responding to any method call (or other kind of use) - in other words, there doesn't have to be a "real" method called Foo at all.

If you look at the generated code in the dynamic situation, you'll find all kinds of weird and wonderful stuff going on to do with call sites and binders.

Difference between Object, Dynamic and Var

Everything is Object because it is a base type for every type in .net environment. Every type inherit from Object in a moment, a simple int variable can be boxed to an object and unboxed as well. For example:

object a = 10; // int
object b = new Customer(); // customer object
object c = new Product(); // product object
object d = "Jon"; // string
object e = new { Name = "Felipe", Age = 20 }; // anonymous type

It is the most abstraction for any type and it is a reference type. If you want to get the real type, you need to unbox it (using a conversaion strategy such as methods, casts, etc):

object a = "Some Text";
string text = a.ToString();

// call a string method
text = text.ToUpper();

object i = 10; // declared as object but instance of int
int intValue = (int) i; //declare as an int ... typed

Dynamic is an implementation of a dynamic aspect in C#, it is not strongly typed. For example:

dynamic a = new Class();
a.Age = 18;
a.Name = "Jon";
a.Product = new Product();

string name a.Name; // read a string
int age = a.Age; // read an int
string productName = a.Product.Name; // read a property
a.Product.MoveStock(-1); // call a method from Product property.

var is just a keyword of the C# language that allows you define any object of a type since you initialize it with a value and it will determinate the type from this value, for example:

var a = 10; // int
var b = 10d; // double
var c = "text"; // string
var d = 10m; // decimal
var p = new Product(); // Product type

The compiler will check the type of the value you have defined and set it on the object.

Difference between Dynamic, Var and Object

Here is some differences. Hope this will help to you.

For better understand please refer this

What is the practical difference between dynamic and T in C#

Short answer is that generic type T must be known at compile time, but dynamic is inferred at runtime.

Does the new 'dynamic' C# 4.0 keyword deprecate the 'var' keyword?

No, they're very different.

var means "infer the type of the variable at compile-time" - but it's still entirely statically bound.

dynamic means "assume I can do anything I want with this variable" - i.e. the compiler doesn't know what operations are available, and the DLR will work out what the calls really mean at execution time.

I expect to use dynamic very rarely - only when I truly want dynamic behaviour:

  • var lets you catch typos etc at compile-time
  • statically bound code is always going to run faster than dynamically bound code (even if the difference becomes reasonably small)
  • statically bound code gives more compile-time support beyond just errors: you can find call hierarchies, refactoring will work better, Intellisense is available etc

Difference between var and object in C#

Beginning in Visual C# 3.0, variables that are declared at method scope can have an implicit type var. An implicitly typed local variable is strongly typed just as if you had declared the type yourself, but the compiler determines the type. The following two declarations of i are functionally equivalent:

var i = 10; //implicitly typed
int i = 10; //explicitly typed

var isn't object

You should definitely read this : C# 3.0 - Var Isn't Object

Why would you need to use Dynamic data type over Var in C#?

Dynamic was primarily introduced for COM+ interopability where you where in situations where the types returned from the COM interface where not fixed. This can also be commonly seen in when interoping with other non typed languages like javascript (the code in the View in a ASP.NET MVC app is likely the most common place you will see it used).

Other than that most uses of dynamic you will see people use it for can be better handled by refactoring their code to provide a interface that represents a object.



Related Topics



Leave a reply



Submit