The default keyword.
The default keyword can be used in the switch statement or in generic code:
•
The switch statement: Specifies the default label.
•
Generic code: Specifies the default value of the type parameter. This will be null for reference types and zero for value types.
-------------------------
Fixed
The fixed statement prevents the garbage collector from relocating a movable variable. The fixed statement is only permitted in an unsafe context. Fixed can also be used to create fixed size buffers.
The fixed statement sets a pointer to a managed variable and "pins" that variable during the execution of the statement. Without fixed, pointers to movable managed variables would be of little use since garbage collection could relocate the variables unpredictably. The C# compiler only lets you assign a pointer to a managed variable in a fixed statement.
----------------------
The extern modifier is used to declare a method that is implemented externally. A common use of the extern modifier is with the DllImport attribute when you are using Interop services to call into unmanaged code. In this case, the method must also be declared as static, as shown in the following example:
[DllImport("avifil32.dll")]
private static extern void AVIFileInit();
The extern keyword can also define an external assembly alias, which makes it possible to reference different versions of the same component from within a single assembly.
----------------
The foreach statement repeats a group of embedded statements for each element in an array or an object collection that implements the System.Collections.IEnumerable or System.Collections.Generic.IEnumerable<T> interface. The foreach statement is used to iterate through the collection to get the information that you want, but can not be used to add or remove items from the source collection to avoid unpredictable side effects. If you need to add or remove items from the source collection, use a for loop.
The embedded statements continue to execute for each element in the array or collection. After the iteration has been completed for all the elements in the collection, control is transferred to the next statement following the foreach block.
At any point within the foreach block, you can break out of the loop by using the break keyword, or step to the next iteration in the loop by using the continue keyword.
A foreach loop can also be exited by the goto, return, or throw statements.
For generic type parameters, the in keyword specifies that the type parameter is contravariant. You can use the in keyword in generic interfaces and delegates.
interface IContravariant<in A> { }
// Extending contravariant interface.
interface IExtContravariant<in A> : IContravariant<A> { }
------------------------
In C#, the new keyword can be used as an operator, a modifier, or a constraint.
new Operator
Used to create objects and invoke constructors.
new Modifier
Used to hide an inherited member from a base class member.
new Constraint
Used to restrict types that might be used as arguments for a type parameter in a generic declaration.
------------------
The readonly keyword is different from the const keyword. A const field can only be initialized at the declaration of the field. A readonly field can be initialized either at the declaration or in a constructor. Therefore, readonly fields can have different values depending on the constructor used. Also, while a const field is a compile-time constant, the readonly field can be used for runtime constants
-------------
The unsafe keyword denotes an unsafe context, which is required for any operation involving pointers.
To compile unsafe code, you must specify the /unsafe compiler option. Unsafe code is not verifiable by the common language runtime.
--------------
The volatile keyword indicates that a field might be modified by multiple threads that are executing at the same time. Fields that are declared volatile are not subject to compiler optimizations that assume access by a single thread. This ensures that the most up-to-date value is present in the field at all times.
The volatile modifier is usually used for a field that is accessed by multiple threads without using the lock statement to serialize access.
The volatile keyword can only be applied to fields of a class or struct. Local variables cannot be declared volatile.
---------------------
The stackalloc keyword is used in an unsafe code context to allocate a block of memory on the stack.
The keyword is valid only in local variable initializers.
