C# Code Control

	FirstName = = "Frank"			// equal to a string literal
	txtYears.Text = = " "				// equal to an empty string
	Message = = null		 		// equal to a null value
	DiscountPercent = = 2.3			// equal to a numeric literal
	IsValid = = false				// equal to the false value
	Code = = ProductCode			// equal to another variable
	
	LastName ! = "Jones"			// not equal to a string literal

	Years > 0						// greater than a numeric literal
	i < Months					// less than a variable

	Subtotal > = 500				// greater than or equal to a literal value
	Quantity < = ReorderPoint		// less than or equal to a variable
						

• You can use the relational operators to create a Boolean expression that compares two operands and returns a Boolean value.
• To compare two operands for equality, make sure you use two equals signs. If you use a single equals sign, the compiler will interpret it as an assignment statement, and your code won't compile.
• When comparing strings, you can only use the equality and inequality operators.
• If you compare two numeric operands with different data types, C# will cast the less precise operand to the type of the more precise operand.

Figure CS-54: C# Relational Operator Table

	Subtotal > = 250 && Subtotal < 500
	TimeInService < = 4 || TimeInService > = 12

	IsValid = = true & Counter ++ < Years
	IsValid = = true | Counter -- > Years

	Date > StartDate && Date < ExpirationDate || IsValid = = true
	( ( ThisYTD > LastYTD ) || EmployeeType = = " Part Time " ) && StartYear < CurrentYear )
	
	! ( Counter ++ > = Years )
						

• You can use the logical operators to create a Boolean expression that combines two or more Boolean expressions.
• Since the && and || operators only evaluate the second expression if necessary, they're sometimes referred to as short-circuit operators. These operators are slightly more efficient than the & and | operators.
• By default, Not operations are performed first, followed by And operations, and then Or operations. These operations are performed after arithmetic operations and relational operations.
• You can use parentheses to change the sequence in which the operations will be performed or to clarify the sequence of operations.

Figure CS-55: C# Logical Operators, Examples and Description

	if ( booleanExpression ) { statements }
	[else if ( booleanExpression ) { statements } ] ...
	[else { statements } ]
						

	if ( Subtotal > = 100 )
		DiscountPercent = .2;
						

	if ( Subtotal > = 100 )
	{
		DiscountPercent = .2;
		Status = "Bulk Rate";
	}
						

	if ( Subtotal > = 100 )
		DiscountPercent = .2;
	else
		DiscountPercent = .1;
						

	if ( Subtotal > = 100 && Subtotal < 200 )
		DiscountPercent = .2;
	else if ( Subtotal > = 200 && Subtotal < 300 )
		DiscountPercent = .3;
	else if ( Subtotal > = 300 )
		DiscountPercent = .4;
	else
		DiscountPercnet = .1;
						

	if ( CustomerType = = "R" )
	{								// begin nested if
		if ( Subtotal > = 100 )
			DiscountPercent = .2;
		else
			DiscountPercent = .1;
	}								// end nested if
	
	else								// customer type isn't "R"
	{
		DiscountPercent = .4;
	}
						

• An if-else statement, or just if statement, always contains an if clause. In addition, it can contain one or more else if clauses and a final else clause.
• If a clause requires just one statement, you don't have to enclose the statement in braces. You can just end the clause with a semicolon.
• If a clause requires more than one statement, you enclose the block of statements in braces.

Figure CS-56: C# if-else Construct Syntax, Examples and Description

	switch ( SwitchExpression )
	{
		case Constant Expression:
			Statements
			break;
		[case Constant Expression:
			Statements
			break;]. . .
		[default:
			Statements
			break;]
	}
						

	switch ( CustomerType )
	{
		case "R":
			DiscountPercent = .1;
			break;

		case "C"
			DiscountPercent = .2;
			break;

		default:
			DiscountPercent = 0;
			break;
	}
						

	switch ( CustomerType )
	{
		case "R":
		case "C":
			DiscountPercent = .2;
			break;

		case "T":
			DiscountPercent = .4;
			break;
	}
						

• A switch statement begins by evaluating its switch expression. This expression must evaluate to a string, char, long sbtye, shorth, ushort, int, uint, long, or ulong type.
• After evaluating the switch expression, the switch statement transfers control to the appropriate case label. If control isn't transferred to one of the case labels, the optional default label is executed.
• The break statement exits the switch statement. If a label contains one or more statements, the label must end with a break statement.
• If a label doesn't contain any statements, code execution will fall through to the next label. That means that the statements contained by the next label will be executed.

Figure CS-57: C# switch Construct Syntax, Examples and Description