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

C# Problem Overview

I had read a ton of articles about that new keyword that is shipping with C# v4, but I couldn't make out the difference between a "dynamic" and "var".

This article made me think about it, but I still can't see any difference.

Is it that you can use "var" only as a local variable, but dynamic as both local and global?

Could you show some code without dynamic keyword and then show the same code with dynamic keyword?

C# Solutions

Solution 1 - C#

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.

Solution 2 - C#

Variables declared with var are implicitly but statically typed. Variables declared with dynamic are dynamically typed. This capability was added to the CLR in order to support dynamic languages like Ruby and Python.

I should add that this means that dynamic declarations are resolved at run-time, var declarations are resolved at compile-time.

Solution 3 - C#

I am going to explain difference between dynamic and var.

dynamic d1;
d1 = 1;
d1 = "http://mycodelogic.com";

This will work. compiler can re-create the type of dynamic variable.
first it create type as integer and after that compiler will recreate type as string
but in case of var

var v1;  // Compiler will throw error because we have to initialized at the time of declaration  
var v2 = 1; // Compiler will create v1 as **integer**
v2 = "Suneel Gupta"; // Compiler will throw error because, compiler will not recreate the type of variable

When using the ‘var’ keyword, the type is decided by the compiler at compile time, whereas when using the ‘dynamic’ keyword, the type is decided by the runtime.
‘var’ keyword, a strongly implicitly typed local variable for which the compiler is able to determine the type from the initialization expression - very useful when doing LINQ programming.
Compiler doesn't have any information about the dynamic type of variable. so compiler will not show any intelligence .
compiler has all information about the stored value of var type so compiler will show intelligence.
dynamic type can be passed as function argument and function also can return object type
But
var type can not be passed as function argument and function can not return object type. This type of variable can work in the scope where it defined.

Solution 4 - C#

var implies that static type checking (early binding) is applied. dynamic implies that dynamic type checking (late binding) is applied. In terms of the code, condsider the following:

class Junk
{
    public void Hello()
    {
        Console.WriteLine("Hello");
    }
}

class Program
{
    static void Main(String[] args)
    {
        var a = new Junk();
        dynamic b = new Junk();

        a.Hello();

        b.Hello();
    }
}

If you compile this and inspect the results with ILSpy, you will find that the compiler has added some late binding code which will handle the call to Hello() from b, whereas becuase early binding was applied to a, a is able to call Hello() directly.

e.g. (ILSpy disassembly)

using System;
namespace ConsoleApplication1
{
	internal class Junk
	{
		public void Hello()
		{
			Console.WriteLine("Hello");
		}
	}
}

using Microsoft.CSharp.RuntimeBinder;
using System;
using System.Runtime.CompilerServices;
namespace ConsoleApplication1
{
	internal class Program
	{
		[CompilerGenerated]
		private static class <Main>o__SiteContainer0
		{
			public static CallSite<Action<CallSite, object>> <>p__Site1;
		}
		private static void Main(string[] args)
		{
			Junk a = new Junk();      //NOTE: Compiler converted var to Junk
			object b = new Junk();    //NOTE: Compiler converted dynamic to object
			a.Hello();  //Already Junk so just call the method.

                          //NOTE: Runtime binding (late binding) implementation added by compiler.
			if (Program.<Main>o__SiteContainer0.<>p__Site1 == null)
			{
				Program.<Main>o__SiteContainer0.<>p__Site1 = CallSite<Action<CallSite, object>>.Create(Binder.InvokeMember(CSharpBinderFlags.ResultDiscarded, "Hello", null, typeof(Program), new CSharpArgumentInfo[]
				{
					CSharpArgumentInfo.Create(CSharpArgumentInfoFlags.None, null)
				}));
			}
			Program.<Main>o__SiteContainer0.<>p__Site1.Target(Program.<Main>o__SiteContainer0.<>p__Site1, b);
		}
	}
}

The best thing you can do to discover the difference is to write yourself a little console app like this one, and test it yourself with ILSpy.

Solution 5 - C#

One big difference - you can have a dynamic return type.

dynamic Foo(int x)
{
    dynamic result;

    if (x < 5)
      result = x;
    else
      result = x.ToString();

    return result;
}

Solution 6 - C#

Here is simple example which demonstrates difference between Dynamic (4.0) and Var

dynamic  di = 20;
dynamic ds = "sadlfk";
var vi = 10;
var vsTemp= "sdklf";

Console.WriteLine(di.GetType().ToString());          //Prints System.Int32
Console.WriteLine(ds.GetType().ToString());          //Prints System.String
Console.WriteLine(vi.GetType().ToString());          //Prints System.Int32
Console.WriteLine(vsTemp.GetType().ToString());      //Prints System.String

**ds = 12;**   //ds is treated as string until this stmt now assigning integer.

Console.WriteLine(ds.GetType().ToString());          **//Prints System.Int32**

**vs = 12**; //*Gives compile time error* - Here is the difference between Var and Dynamic. var is compile time bound variable.

Shiva Mamidi

Solution 7 - C#

var is just a shorthand for a normal type declaration, where you let the compiler guess the correct type.

dynamic is a new (static) type, where all checks are done at runtime, not by the compiler.

Solution 8 - C#

The type of a variable declared with var is determined by the compiler, it is a shortcut to specifying the type's name, nothing more.

However dynamic is determined at runtime, the compiler has no idea of the actual type, and all method/field/property accesses with that variable will be worked out at runtime.

Solution 9 - C#

This is a nice youtube video which talks about var VS Dynamic with practical demonstration.

Below is a more detailed explanation with snapshot.

Var is early binded (statically checked) while dynamic is late binded (dynamically evaluated).

Var keyword looks at your right hand side data and then during compile time it decides the left hand data type.In other words var keyword just saves you typing lot of things. Have a look at the below image where when we have given string data and x variable shows string data type in my tool tip.

enter image description here

On the other hand dynamic keyword is for completely different purpose. Dynamic objects are evaluated during runtime. For instance in the below code the "Length" property exists or not is evaluated during runtime.I have purposely typed a small "l" , so this program compiled fine but when it actually executed it throwed up a error when the "length" property was called ( SMALL "l").

enter image description here

Solution 10 - C#

dynamic variable and var variable both can store any type of value but its required to initialize 'var' at the time of declaration.

Compiler doesn't have any information about the 'dynamic' type of variable. var is compiler safe i.e compiler has all information about the stored value, so that it doesn't cause any issue at run-time.

Dynamic type can be passed as function argument and function also can return it. Var type can not be passed as function argument and function can not return object type. This type of variable can work in the scope where it defined.

In case of dynamic Casting is not require but you need to know the property and methods related to stored type , while for var No need to cast because compiler has all information to perform operation.

dynamic: Useful when coding using reflection or dynamic language support or with the COM objects, because we require to write less amount of code.

var: Useful when getting result out of the linq queries. In 3.5 framework it introduce to support linq feature.

Reference : Counsellingbyabhi

Solution 11 - C#

Here are the differences

var is statically typed (compile time), dynamic is dynamically typed (run time)
A variable declared as var can only be used locally , dynamic variables can be passed in as params to function (function signature can define a param as dynamic but not var).
with dynamic the resolution of the properties happens at runtime and thats not the case with var which means at compile time any variable declared as dynamic can call a method which may or maynot exist and so the compiler would not throw an error.
Type casting with var not possible but with dynamic its possible (you can cast an object as dynamic but not as var).

Arun Vijayraghavan

Solution 12 - C#

Var and dynamic define type.
var at the compile time while dynamic are at run time.
in the var declaration and initialization both are mandatory like constant variable while
in dynamic initialization can be at run time like readonly variables.
in var type whatever type are decided at the time initialization can not change next but
dynamic can adopt any type even user define datatype also.

Solution 13 - C#

Do not confuse dynamic and var. Declaring a local variable using var is just a syntactical shortcut that has the compiler infer the specific data type from an expression. The var keyword can be used only for declaring local variables inside a method while the dynamic keyword can be used for local variables, fields, and arguments. You cannot cast an expression to var, but you can cast an expression to dynamic. You must explicitly initialize a variable declared using var while you do not have to initialize a variable declared with dynamic.

Solution 14 - C#

The Var(Implicit typed local variable) keyword is used to define local variables.In case of Var , the underlying data type is determined at compile time itself based on the initial assignment.Once the initial assignment has been made with Var type , then it will become strongly typed.If you try to store any incompatible value with the Var type it will result in compile time error.

Example:

Var strNameList=new List<string>(); By using this statement we can store list of names in the string format. 
strNameList.add("Senthil");
strNameList.add("Vignesh");

strNameList.add(45); // This statement will cause the compile time error.

But in Dynamic type, the underlying type is determined only at run time.Dynamic data type is not checked at compile time and also it is not strongly typed.We can assign any initial value for dynamic type and then it can be reassigned to any new value during its life time.