This operator allows you to assign a value to a nullable type if the retrieved value is in fact null.

In C# the value can be directly referenced to a variable, so there is no need of pointer. Use of pointer sometime crashes the application. But C# supports pointer, that means we can use pointer in C#.The use of pointer in C# is defined as a unsafe code. So if we want to use pointer in C# the pointer must be present in the body of unsafe declaration. But pointer does not come under garbage collection.

Var word was introduced with C#3.5(specifically for LINQ) while dynamic is introduced in C#4.0.
Variables declared with var keyword will get compiled and you can have all its related methods by intellisense while variables declared with dynamic keyword will never get compiled. All the exceptions related to dynamic type variables can only be caught at runtime.

Anonymous methods are another way to declare delegates with inline code except named methods.

Optional parameter allows omitting arguments to function while named parameters allow passing arguments by parameter name.

Parse method is used to parse any value to specified data type.
TryParse is a good method if the string you are converting to an integer is not always numeric. The TryParse method returns a boolean to denote whether the conversion has been successfull or not, and returns the converted value through an out parameter.

String Class
Once the string object is created, its length and content cannot be modified.
Slower
StringBuilder
Even after object is created, it can be able to modify length and content.
Faster

The sealed modifier is used to prevent derivation from a class. A compile-time error occurs if a sealed class is specified as the base class of another class. (A sealed class cannot also be an abstract class).

Convariance : Convert from Wider(Double) to Narrow(Float).
Contravariance: Convert from Narrow(Float) to Wider(Double).

C# 2.0-Generics (.NET Framework support was added, and C# benefited from this); iterator pattern (the yield keyword); anonymous methods (the delegate keyword), nullable types, and the null coalescing operator .
C# 3.0-Anonymous types, extension methods, object initializers,collection initializers, implicitly typed local variables (var keyword),lambda expressions (=>), and the LINQ query expression pattern.
C# 4.0-Optional Parameters and Named Arguments, Dynamic typing (dynamic type), improved COM-Interop, and Contra and Co-Variance.