Mohamed Abdeen

using as keyword in c# in conversion from type to another type (CASTING) , use it like “expression as type”  type and expression are references type but what different when as fail to cast the expression to certain type it won’t through any expression but it will fill the expression with null

when you create new form windows you want to show it so you use Show() or ShowDialog(), so what is the difference? ShowDialog is the same as Show, it just shows the form as a modal window. Modal meaning the form cannot lose focus until it is closed. (The user can’t click on other windows within the same application.)

in other meaning, ShowDialog the form is shown in a modal window. A modal form generally is used to show a dialog that must be closed until it takes back to the caller…for instance, the messagebox.show shows the message as a dialog, so the user should close it before the code continues to the next line. In the other hand, the show method just shows the form and continues executing the next line of the caller, when you open a form from a menu you usually shows it as no modal.

Nullable value…why are we introducing …

in studying of object oriented concepts, we know that we have two data type Value type & Reference type the famous difference between the two is the.

• value type contain the actual value of variable but,
• reference type contain the address (reference) of the object

but there is another difference which may be some people don’t know it

• value type can’t contain null value, if it hasn’t value, it will be default value (for example, int data type if it doesn’t have any value it will be Zero).
• reference type allow null value.

now you get the difference, let ask you a question, Do we need null in value type ? …thinking… the answer is yes, we need it. let’s imagine that we have database contain employee which consist some columns may be allow null like Salary and so on, how you can map this @code, if you use default value … so you didn’t get the actual meaning of your business @your code. null value doesn’t means zero or empty string, it means that you don’t have enough information  . so there are difference … the solution is Nullable Value. Nullable value is new generic type which allow nullability of value type, Nullable<T>.

Nullable @C#

• ? is new modifier means Nullable so when you need to define Nullable value type  int? i;
• ?? operator can convert from Nullable type to non-Nullable type with default value. So “ int x;     int? y;     x = y ?? 0“ will assign the value of y to x if it is not null, and will assign the value 0 to x otherwise.
• if you try to add two Nullable values and one of two is null value so the result of final addition will be NULL
• For comparison operators, comparing two nullable types is always going to result in a bool value, not a bool? value.

int? nullableInt;
int? nullValue = null;
int? notNull = 123;
bool? answer1 = true;
bool? answer2 = false;
bool? answer3 = null;

int? nullableInt;
int? copy = nullableInt;    // Invalid as nullableInt is not yet assigned

int standardInteger = 123;
int? nullableInteger;
decimal standardDecimal = 12.34M;
// Implicit conversion from int to int?
nullableInteger = standardInteger;
// Explicit conversion from int? to int
standardInteger = (int)nullableInteger;
// Explicit cast from decimal to int?
nullableInteger = (int?)standardDecimal;
int? a = 55;
int? n = null;
int? result;
result = a * 2;         // result = 110
result = a * n;         // result = null

bool? result;
result = true & null;       // result = null
result = false & null;      // result = false
result = true | null;       // result = true
result = false | null;      // result = null
result = true ^ null;       // result = null
result = false ^ null;      // result = null

My Source: http://www.blackwasp.co.uk/CSharpNullableDataTypes.aspx