Major differences between C# 3.0 and C# 4.0 .Net

Introduced in C# 3.0

Introduced in C# 4.0

Statically typed – This means the type of variable declared is decided by the compiler at compile time.

Dynamically typed - This means the type of variable declared is decided by the compiler at runtime time.

Need to initialize at the time of declaration.

e.g., var str=”I am a string”;

Looking at the value assigned to the variable str, the compiler will treat the variable str as string.

No need to initialize at the time of declaration.

e.g., dynamic str; 

str=”I am a string”; //Works fine and compiles

str=2; //Works fine and compiles

Errors are caught at compile time.

Since the compiler knows about the type and the methods and properties of the type at the compile time itself

Errors are caught at runtime 

Since the compiler comes to about the type and the methods and properties of the type at the run time.

Visual Studio shows intellisense since the type of variable assigned is known to compiler.

Intellisense is not available since the type and its related methods and properties can be known at run time only

e.g., var obj1;

will  throw a compile error since the variable is not initialized. The compiler needs that this variable should be initialized so that it can infer a type from the value.

e.g., dynamic obj1; 

will compile;

e.g. var obj1=1;

will compile 

var obj1=” I am a string”;

will throw error since the compiler has already decided that the type of obj1 is System.Int32 when the value 1 was assigned to it. Now assigning a string value to it violates the type safety.

e.g. dynamic obj1=1;

will compile and run 

dynamic obj1=” I am a string”; 

will compile and run since the compiler creates the type for obj1 as System.Int32 and then recreates the type as string when the value “I am a string” was assigned to it.

This code will work fine.