Monday, 28 October 2013

Difference Between Var and Dynamic Keyword in C#

Many people have expressed confusion around the difference between var and dynamic in C#.  For both of them, the type is inferred rather than explicitly declared. 
dynamic test = 1;
var test2 = 2;
If I hover my mouse over the “var” in the code above, IntelliSense will show me that the compiler has correctly inferred that it is an Int32.  If I hover over “dynamic”, it will continue to be typed as “dynamic” since dynamic types aren’t resolved until runtime. 
However, var is statically typed, and dynamic is not. 
// Can a dynamic change type?  
dynamic test = 1;
test = "i'm a string now";  // compiles and runs just fine
var test2 = 2;
test2 = "i'm a string now"; // will give compile error
This is one of the key differences between dynamic and var.  A var is an implicitly typed variable that is inferred by the compiler, but it is just as strongly typed as if you had explicitly typed it yourself using “int test2 = 2;”.  A dynamic variable bypasses all compile-time type checking and resolves everything at runtime.  
I’ll comment out the last line in the code above to get the code to compile, and add some code to verify the types of the variables. 
// Can a dynamic change type?  
dynamic test = 1;
Console.WriteLine("Dynamic as " + test.GetType() + ": " + test);
test = "i'm a string now";  // compiles and run just fine
Console.WriteLine("Dynamic as " + test.GetType() + ": " + test);
var test2 = 2;
//test2 = "i'm a string now"; // will give compile error
Console.WriteLine("Var as " + test2.GetType() + ": " + test2);
This produces the following output:
Dynamic as System.Int32: 1
Dynamic as System.String: i'm a string now
Var as System.Int32: 2

Object Dynamic Var
Can able to store any kind of value, because object is the base class of all type in .net framework. Can able to store any type of the variable, similar to old VB language variable. Can able to store any type of value but it require to initialize at the time of declaration.

Compiler has little information about the type

Compiler doesn't have any information about the this type of variable.

It's compiler safe i.e compiler has all information about the stored value, so that it doesn't cause any issue at run-time.

Object type can be passed as function argument and function also can return object type Dynamic type can be passed as function argument and function also can return object type 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.

Require to cast object variable to original type before using it. So this assigning to object type and converting to original type called as Boxing and Un-Boxing for value type and for the reference type its casting of types. It's actually increasing the overhead when we do this both operation.
Allows to perform operation of given type once it get cast any user defined or primitive data type.
Casting is not require but you need to know the property and methods related to stored type No need to cast because compiler has all information to perform operation.
Cause the problem at run time if the stored value is not get converted to underlying data type.

Cause problem if the wrong method or property accessed because all the information about stored value is get resolve only at run time

Doesn't cause problem because compiler has all info about stored value.
Useful when doesn't have more information about the data type. Useful when coding using reflection or dynamic language support or with the COM objects, because we require to write less amount of code. Useful when getting result out of the linq queries. In 3.5 framework it introduce to support linq feature.

No comments:

Post a Comment