What Is the Meaning of "This" in C#

What does ?? operator means in C#?

It's the null-coalescing operator.

It returns the first argument unless it is null, in which case it returns the second.

x ?? y is roughly equivalent to this (except that the first argument is only evaluated once):

if (x == null)
{
     result = y;
}
else
{
     result = x;
}

Or alternatively:

(x == null) ? y : x

It is useful for providing a default value for when a value can be null:

Color color = user.FavouriteColor ?? defaultColor;

COALESCE

When used in a LINQ to SQL query the ?? operator can be translated to a call to COALESCE. For example this LINQ query:

var query = dataContext.Table1.Select(x => x.Col1 ?? "default");

can result in this SQL query:

SELECT COALESCE([t0].[col1],@p0) AS [value]
FROM [dbo].[table1] AS [t0]

What does @ mean in C#

It means interpret the following string as literal. Meaning, the \ in the string will actually be a "\" in the output, rather than having to put "\\" to mean the literal character

What is the meaning of ~ in c#

~UnitOfWork() is the declaration of the destructor of the class public class UnitOfWork.

Here's an (IMO) an interesting part to note about the destructors:

The programmer has no control over when the destructor is called because this is determined by the garbage collector. The garbage collector checks for objects that are no longer being used by the application. If it considers an object eligible for destruction, it calls the destructor (if any) and reclaims the memory used to store the object. Destructors are also called when the program exits.

What does question mark and dot operator ?. mean in C# 6.0?

It's the null conditional operator. It basically means:

"Evaluate the first operand; if that's null, stop, with a result of null. Otherwise, evaluate the second operand (as a member access of the first operand)."

In your example, the point is that if a is null, then a?.PropertyOfA will evaluate to null rather than throwing an exception - it will then compare that null reference with foo (using string's == overload), find they're not equal and execution will go into the body of the if statement.

In other words, it's like this:

string bar = (a == null ? null : a.PropertyOfA);
if (bar != foo)
{
    ...
}

... except that a is only evaluated once.

Note that this can change the type of the expression, too. For example, consider FileInfo.Length. That's a property of type long, but if you use it with the null conditional operator, you end up with an expression of type long?:

FileInfo fi = ...; // fi could be null
long? length = fi?.Length; // If fi is null, length will be null

c# meaning of ! before a statement

! is the NOT (negation) logical operator.

It transform a false to true and true to false.

Boolean logical operators (C# reference)

Meaning of operator ? after an object in C#

It is a shorthand for Nullable<T>, i.e. Point? is the same as Nullable<Point>. This allows you to assign a null value for value types.

See the MSDN reference on Using Nullable Types.

NOTE, in this context it is not an operator, it is a shorthand syntax.

What does {} mean in C#'

It's something called DBNull.Value as stated before you can use it:

if (myDataGridView.Rows[0].Cells[1].Value == DBNull.Value) 
{
    //some code
}