Programming in general often requires a decision or a branch within the code to account for how the code operates under different inputs or conditions. Within the C# programming language (and most programming languages for this matter), the simplest and sometimes the most useful way of creating a branch within your program is through an If-Else statement.

Lets assume we have method (a.k.a. a function) which takes an int parameter which will represent a score up to 100, and the method will print out a message saying whether we pass or fail.

static void PrintPassOrFail(int score)
{
    if (score >= 50) // If score is greater or equal to 50
    {
        Console.WriteLine("Pass!");
    }
    else // If score is not greater or equal to 50
    {
        Console.WriteLine("Fail!");
    }
}

When looking at this method, you may notice this line of code (score >= 50) inside the If statement. This can be seen as a boolean condition, where if the condition is evaluated to equal true, then the code that is in between the if { } is ran.

For example, if this method was called like this: PrintPassOrFail(60);, the output of the method would be a Console Print saying Pass! since the parameter value of 60 is greater or equal to 50.

However, if the method was called like: PrintPassOrFail(30);, the output of the method would print out saying Fail!. This is because the value 30 is not greater or equal to 50, thus the code in between the else { } is ran instead of the If statement.

In this example, we've said that score should go up to 100, which hasn't been accounted for at all. To account for score not going past 100 or possibly dropping below 0, see the If-Else If-Else Statement example.

Following on from the If-Else Statement example, it is now time to introduce the Else If statement. The Else If statement follows directly after the If statement in the If-Else If-Else structure, but intrinsically has has a similar syntax as the If statement. It is used to add more branches to the code than what a simple If-Else statement can.

In the example from If-Else Statement, the example specified that the score goes up to 100; however there were never any checks against this. To fix this, lets modify the method from If-Else Statement to look like this:

static void PrintPassOrFail(int score)
{
    if (score > 100) // If score is greater than 100
    {
        Console.WriteLine("Error: score is greater than 100!");
    }
    else if (score < 0) // Else If score is less than 0
    {
        Console.WriteLine("Error: score is less than 0!");
    }
    else if (score >= 50) // Else if score is greater or equal to 50
    {
        Console.WriteLine("Pass!");
    }
    else // If none above, then score must be between 0 and 49
    {
        Console.WriteLine("Fail!");
    }
}

All these statements will run in order from the top all the way to the bottom until a condition has been met. In this new update of the method, we've added two new branches to now accommodate for the score going out of bounds.

For example, if we now called the method in our code as PrintPassOFail(110);, the output would be a Console Print saying Error: score is greater than 100!; and if we called the method in our code like PrintPassOrFail(-20);, the output would say Error: score is less than 0!.

The following statement

if (conditionA && conditionB && conditionC) //...

is exactly equivalent to

bool conditions = conditionA && conditionB && conditionC;
if (conditions) // ...

in other words, the conditions inside the "if" statement just form an ordinary Boolean expression.

A common mistake when writing conditional statements is to explicitly compare to true and false:

if (conditionA == true && conditionB == false && conditionC == true) // ...

This can be rewritten as

if (conditionA && !conditionB && conditionC)

A switch statement allows a variable to be tested for equality against a list of values. Each value is called a case, and the variable being switched on is checked for each switch case.

A switch statement is often more concise and understandable than if...else if... else.. statements when testing multiple possible values for a single variable.

Syntax is as follows

switch(expression) {
   case constant-expression:
      statement(s);
      break;
   case constant-expression:
      statement(s);
      break;
  
   // you can have any number of case statements
   default : // Optional
      statement(s);
      break;
}

there are sevaral things that have to consider while using the switch statement

• The expression used in a switch statement must have an integral or enumerated type, or be of a class type in which the class has a single conversion function to an integral or enumerated type.
• You can have any number of case statements within a switch. Each case is followed by the value to be compared to and a colon. The values to compare to have to be unique within each switch statement.
• A switch statement can have an optional default case. The default case can be used for performing a task when none of the cases is true.
• Each case has to end with a break statement unless it is an empty statement. In that case execution will continue at the case below it. The break statement can also be omitted when a return, throw or goto case statement is used.

Example can be given with the grades wise

char grade = 'B';

switch (grade)
{
    case 'A':
        Console.WriteLine("Excellent!");
        break;
    case 'B':
    case 'C':
        Console.WriteLine("Well done");
        break;
    case 'D':
        Console.WriteLine("You passed");
        break;
    case 'F':
        Console.WriteLine("Better try again");
        break;
    default:
        Console.WriteLine("Invalid grade");
        break;
}