I need some help about how I can validate my if , else if statements.

No. Whatever your teacher said, its right. I would suggest you to go ahead with your original post..

When you are having it in sequence, like this.

if (annual_salary >= 0 && annual_salary <= 16500)
            {
                tax_rate = 11.32F;
                pay_rate = 8.68F;
            }
            else if (annual_salary <= 19500)
            {
                tax_rate = 15.14F;
                pay_rate = 10.26F;
            }

its same as

if (annual_salary >= 0 && annual_salary <= 16500)
{
    tax_rate = 11.32F;
    pay_rate = 8.68F;
}
else if (annual_salary >= 16500 && annual_salary <= 19500 )
{
    tax_rate = 15.14F;
    pay_rate = 10.26F;
}

For eg, when i pass 17000, definitely it will take tax rate as 15.14F, since it will fail first if loop and enters else if.

If you want to test the complete code,

use the following values and run once and test your code.

Input:Annual_salary Expected:Tax_rate
14000 11.32F
17000 15.14F
21000 22.65F
32000 27.1F
34000 30.92F
45000 35.72F
78000 40.72F
91000 50.52F

Hi,

You need to add some exception handling at many places.

Example 1: User should not allowed to enter annual_salary below 0.
So your code would be like,

If(annual_salary<=0)
{
throw new ArgumentOutOfRangeException(); //// Here you can even prompt some message. if you are developing API then you should throw Exception to handled from client.
}

Example 2: User should not enter Empty name.
the code would be like,

if (string.IsNullOrEmpty(full_name))
{
throw new ArgumentNullException(); //// Here you can even prompt to enter correct name.
}

This way so many validation can be added in your application.

Best of luck

Thanks
-Amit Gajjar.

A similar code snippet was posted in the Horrors recently. I think the correct answer here is the same as it was there: a list of bands and a simple lookup into them:

public class Band {
 public float UpperBound, TaxRate, PayRate;
 public Band(float upperBound, float taxRate, float payRate) { 
  UpperBound = upperBound; TaxRate = taxRate; PayRate = payRate;
 }
}

class MainClass {
 private static readonly Band[] bands = {
  new Band(16500, 11.32F, 8.68F),
  new Band(19500, 15.14F, 10.26F),
  // etc
  new Band(89500, 40.72F, 47.12F),
  new Band(float.Infinity, 50.52F, 55.67F) // Infinity matches any finite salary
 );

 void YourMethod(){
  // All the code that gets stuff off the console, etc
//Annual Salary data
  Console.Write("Enter your annual salary >");
  annual_salary = int.Parse(Console.ReadLine());
              
  Band salaryBand = bands.First(b => b.UpperBound >= annual_salary);
  tax_rate = salaryBand.TaxRate; pay_rate = salaryBand.PayRate;

  // Calculate gross pay
  gross_pay = hours_worked * pay_rate;
  // ... etc
 }
}

And yes you should probably guard against a salary less than zero, otherwise you'll get the first one, which might not be what you want.