Develop Reference

A problem with getting/printing answers in a C# calculator

I'm trying to make simple projects for learning C# and have tried to make a simple console calculator. I have only found this current error when getting to the getting/printing the answer bit when test-running my program, so I have no idea if there are any other errors/things that will or may not work or run properly/as intended. So if there are any of those, please let me know and if you want to you can fix them yourself. It only recognized the error when it reached that specific line of code, and otherwise will run the program until it reaches the error.
Code:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Calculator
{
class Program
{
static void Main(string[] args)
{
string num1;
string num2;
string condition;
string answer;
Console.WriteLine("Calculator");
Console.WriteLine("For division, use /. For multiplication, use *.\n");
while (true)
{
Console.WriteLine("Enter a number: "); // gets first number to add in problem
num1 = Console.ReadLine();
Console.WriteLine("Enter a condition: "); // gets condition to add in problem
condition = Console.ReadLine();
Console.WriteLine("Enter your second number: "); // gets second number to add in problem
num2 = Console.ReadLine();
Console.WriteLine("Calculating..");
// converting strings to int and working out answer
Convert.ToInt32(num1);
Convert.ToInt32(num2);
// error is from here on (not sure if the Convert.ToInt32() code above causes errors)
answer = num1 + condition + num2;
Convert.ToInt32(answer);
Console.WriteLine(answer);
// sets values to null after getting & printing answer (probably unnessessary)
answer = null;
num1 = null;
num2 = null;
condition = null;
}
}
}
}

When facing problems like this one - the routine is too complex to be tested:
if there are any other errors/things that will or may not work"
split routine into smaller ones: start extracting methods.
// Get integer value from user
public static int ReadInteger(string title) {
while (true) {
if (!string.IsNullOrEmpty(title))
Console.WriteLine(title);
if (int.TryParse(Console.ReadLine(), out int result))
return result;
Console.WriteLine("Sorry, not a valid integer value, please, try again.");
}
}
// Get character operator ('+', '-' etc.) from user
public static char ReadOperator(string title, string operators) {
while (true) {
if (!string.IsNullOrEmpty(title))
Console.WriteLine(title);
string input = Console.ReadLine().Trim();
if (input.Length == 1 && operators.Contains(input[0]))
return input[0];
Console.WriteLine("Sorry, not a valid operator, please, try again.");
}
}
Now we are ready to implement Main method:
static void Main(string[] args) {
while (true) {
int num1 = ReadInteger("Enter a number: ");
char op = ReadOperator("Enter a condition: ", "+-*/");
int num2 = ReadInteger("Enter your second number: ");
//TODO: I've skipped error handling (zero division, overflow)
int answer =
op == '+' ? num1 + num2 :
op == '-' ? num1 - num2 :
op == '*' ? num1 * num2 :
op == '/' ? num1 / num2 : 0;
Console.WriteLine($"{num1} {op} {num2} = {answer}");
//TODO: it's a right place here to ask user if (s)he wants to continue
Console.WriteLine();
}
}

You receive input (condition) as a string
cant do this : answer = num1 + condition + num2 because those variables are
string
you have to check that with switch , for example :
int num1 = 0, num2 = 0, answer = 0;
string condition;
Console.WriteLine("Calculator");
Console.WriteLine("For division, use /. For multiplication, use *.\n");
while (true)
{
Console.WriteLine("Enter a number: "); // gets first number to add in problem
num1 = int.Parse(Console.ReadLine());
Console.WriteLine("Enter a condition: "); // gets condition to add in problem
condition = Console.ReadLine();
Console.WriteLine("Enter your second number: "); // gets second number to add in problem
num2 = int.Parse(Console.ReadLine());
Console.WriteLine("Calculating..");
// converting strings to int and working out answer
Convert.ToInt32(num1);
Convert.ToInt32(num2);
// error is from here on (not sure if the Convert.ToInt32() code above causes errors)
switch (condition)
{
case "/":
answer = num1 / num2;
break;
case "*":
answer = num1 * num2;
break;
case "+":
answer = num1 + num2;
break;
case "-":
answer = num1 - num2;
break;
default:
Console.WriteLine("error : unknown operator");
break;
}
Console.WriteLine(answer);
// sets values to null after getting & printing answer (probably unnessessary)
}

You can't do this line because all the variables are strings and not the real values (e.g: num1 & num2 should ints/doubles... and condition should be a real operator).
answer = num1 + condition + num2;
Also, you can't do it because let's assume the user inputs the multiplication sign, and then the string of "*" is not equal to the sign of *.
Instead, you can do some switch-case statements to check the value of the condition variable. Also, you need to make sure num1 & num2 are numbers (you can parse/try-parse them to a number (int/double...)). You will also have to parse them to another variables as they are strings.
switch (condition)
{
case '*':
answer = numX * numY;
break;
case "/":
// Validate that numY is not 0 to avoid [DivideByZeroException][1]
answer = numX / numY;
break;
...
}
Note, it is just one way to do it and I just gave you a small example that might help you to continue.
I would also offer you to make a default case (in case that the input of the user is not something you expect, in this case, you may ask him again to write an input because the current input is invalid).

Allowing user to choose to display entire number in calculator or only 2 decimals

I'm making a basic calculator and I'm wondering how I would go on to make the user choose if he/she wants to display the full result or only the result with 2 decimals. For example, if user puts in first number 4.213 and second number 4.6321, method "+" then the console asks "Do you wish to display the entire result or round it to 2 decimals" at which the user can type in "Yes" or "No".
I'm not looking for the program to round up the decimals, just display 2 decimals or the entire number.
I'm guessing if and else statement would be the way to go here.
Code:
using System.Collections.Generic;
namespace ConsoleApp1
{
class Program
{
private static int runda;
static void Main(string[] args)
{
List<double> Numbers = new List<double>();
string Method = "";
while (true)
{
loop:
try
{
Numbers.Add(ConvStr(TakeUserInput("First Number:")));
}
catch
{
Console.Clear();
Console.WriteLine("Error, try again.");
Console.ReadLine();
goto loop;
}
Console.Clear();
looop:
try
{
Numbers.Add(ConvStr(TakeUserInput("Second Number:")));
}
catch
{
Console.Clear();
Console.WriteLine("Error, try again.");
Console.ReadLine();
goto looop;
}
Console.Clear();
do
{
Method = TakeUserInput("Choose method: ");
Console.Clear();
Console.WriteLine("Error (Endast Addition (+) Subtraktion (-) Multiplikation (*) samt division (/)");
}
while (!CheckMethod(Method));
Console.Clear();
**HERE IS WHERE I WOULD LIKE USER TO CHOOSE IF DISPLAY ENTIRE NUMBER OR ONLY 2 DECIMALS**
Console.WriteLine("Result:");
Console.WriteLine(Calc(Numbers, Method));
Console.WriteLine("If you wish to keep using this calculator press Enter.");
Console.ReadLine();
Numbers.Clear();
}
}
private static string TakeUserInput(string DisplayText)
{
Console.Write(DisplayText);
return Console.ReadLine();
}
private static bool CheckMethod(string method)
{
switch(method)
{
case "+":
break;
case "-":
break;
case "*":
break;
case "/":
break;
default:
return false;
}
return true;
}
private static double Calc(List<double> input, string method)
{
double Answer = 0;
switch (method)
{
case "+":
Answer = input[0] + input[1];
break;
case "-":
Answer = input[0] - input[1];
break;
case "*":
Answer = input[0] * input[1];
break;
case "/":
Answer = input[0] / input[1];
break;
}
return Answer;
}
private static double ConvStr(string input)
{
return Convert.ToDouble(input = input.Replace(".", ","));
}
}
}

Might be wrong but couldn’t you
1). Turn the integer(result) into a string
2). Use the .indexOf(“”) function on the string to get the location of the period
3). check the index+3 to be in range of the result in order to not give any errors (3 to skip the period as well)
4). Use the .remove(index) function to remove all other numbers at the new index
5). Use Convert.toInt32(string) to get its value back
Also there is a “Data.” something function which enables you to calculate the result of a string representation of the expression if that is of any help

Pseudocode for switch statement in c#

How do I write the pseudocode for a switch (case) statement in C#?
switch (option)
{
case 1:
Console.Write("Enter First Number: ");
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Second Number: ");
num2 = Convert.ToDouble(Console.ReadLine());
result = num1 + num2;
Console.WriteLine(result);
break;
case 2:
Console.Write("Enter First Number: ");
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Second Number: ");
num2 = Convert.ToDouble(Console.ReadLine());
result = num1 - num2;
Console.WriteLine(result);
break;
case 3:
Console.Write("Enter First Number: ");
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Second Number: ");
num2 = Convert.ToDouble(Console.ReadLine());
result = num1 * num2;
Console.WriteLine(result);
break;
default:
Console.WriteLine("\n Next time follow instructions. You can only choose numbers 1 - 4");
break;
}

So, if I was going to write this, I'd start with an enumerated type for the operations:
public enum ArithmeticOperation
{
Add,
Subtract,
Multiply,
Divide,
}
I'd write a little helper function:
private static string ShowEnumOptions<T>() where T : struct
{
if (!typeof(T).IsEnum)
{
throw new ArgumentException($"Type: {typeof(T).ToString()} must be an enumerated type");
}
var options = Enum.GetNames(typeof(T));
return string.Join("/", options);
}
(the newest version of C# (which I don't use yet) allows a System.Enum constraint on a generic type parameter which would simplify this)
Then I'd write my main program to look like this:
static void Main(string[] args)
{
while (true)
{
ArithmeticOperation operation = default(ArithmeticOperation);
var goodOperation = false;
while (!goodOperation)
{
Console.Write(
$"Enter operation (one of [{ShowEnumOptions<ArithmeticOperation>()}] or \"Quit\"): ");
var response = Console.ReadLine();
if (string.Equals(response, "Quit", StringComparison.InvariantCultureIgnoreCase))
{
return; //quit the app
}
if (Enum.TryParse<ArithmeticOperation>(response, true, out operation))
{
goodOperation = true;
}
}
double value1 = 0.0;
double value2 = 0.0; //initialize them to keep the compiler happy
var goodDouble = false;
while (!goodDouble)
{
Console.Write("Enter the first number: ");
var response = Console.ReadLine();
if (double.TryParse(response, out value1))
{
goodDouble = true;
}
}
goodDouble = false;
while (!goodDouble)
{
Console.Write("Enter the second number: ");
var response = Console.ReadLine();
if (double.TryParse(response, out value2))
{
goodDouble = true;
}
}
//ok, got an operation and two numbers
double result = 0.0;
switch (operation)
{
case ArithmeticOperation.Add:
result = value1 + value2;
break;
case ArithmeticOperation.Subtract:
result = value1 - value2;
break;
case ArithmeticOperation.Multiply:
result = value1 * value2;
break;
case ArithmeticOperation.Divide:
if (value2 == 0.0)
{
Console.WriteLine("Division by zero is invalid");
result = double.NaN; //NaN means "not a number"
break;
}
result = value1 / value2;
break;
}
Console.WriteLine($"Result is {result}");
}
}
Note that I check all input for validity. Always assume your users will enter bad data. Also note that I check my double for equality with zero. Checking for floating point equality is usually a bad idea, but it's the right thing to do here.
Then, as pseudo code, all I'd write would be:
// Get the operation (one of add/subtract/multiply or divide) - or allow the user to quit
// Get each of value1 and value2 as doubles
// Based on the operation, calculate the result (pay attention to division by zero)
// Show the result
// Loop back and let the user try again (or quit)

Pseudocode is basically writing what you're trying to do in comments. What your professor was probably trying to teach you is to make a bunch of comments to plan out the structure of your code, and then write your code. What you have above is already functional code. At the risk of answering your homework question, I'd say it goes something like this:
switch(option)
{
case 1:
//do something
break;
case 2:
//do something else
break;
default:
//what to do if none of the cases are met
break;
}

I don't know what you mean with pseudocode but this code is less repetitive:
double result = 0;
Console.Write("Enter First Number: ");
double num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Enter Second Number: ");
double num2 = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("Enter a number from 1 to 3");
string input = Console.ReadLine();
switch (input) {
case "1" :
result = num1 + num2;
break;
case "2":
result = num1 - num2;
break;
case "3":
result = num1 * num2;
break;
default:
Console.WriteLine("\n Next time follow instructions. You can only choose numbers 1 - 4");
break;
}
Console.WriteLine("Result = " + result);

Print message when invalid data entered in console application using switch C#

Im trying to write a basic calculator with a main menu and sub menu.
The code is working fine when a valid entry is inputted, but I want an error message to be displayed and then return the user to the main menu when invalid data is entered.
This is what I have done so far. Can someone tell me what am I doing wrong?
static void Main(string[] args)
{
// Main method (only to be used for mainmenu() execution)
MainMenu();
}
// Main Menu method
static void MainMenu()
{
// Declaring variables
// Selection variable, used in user's input to get to the desired operation
int sel;
char letter;
// Main menu styling
Console.WriteLine("Calculator");
Console.WriteLine("********************");
Console.WriteLine("1- Calculator");
Console.WriteLine("2- Exit Calculator");
Console.Write("Please enter your option here: ");
// Converting user's input to sel's type (byte)
sel = int.Parse(Console.ReadLine());
// Processing sel
switch (sel)
{
case 1:
// Execute Addition()
SecondMenu();
break;
case 2:
Console.ReadLine();
break;
default:
Console.WriteLine("Sorry that is not correct format! Please restart!"); //Catch
break;
}
}
static void SecondMenu()
{
char sel2; // Selection variable, used in user's input to get to the desired operation
// Display Menu Options
Console.WriteLine("");
Console.WriteLine("********************");
Console.WriteLine("A. Addition");
Console.WriteLine("S. Substraction");
Console.WriteLine("D. Division");
Console.WriteLine("********************");
Console.Write("Please enter your option here: ");
// Converting user's input to sel's type (byte)
sel2 = Convert.ToChar(Console.ReadLine());
// Processing sel
switch (sel2)
{
case 'a':
// Execute Addition()
Addition();
break;
case 's':
// Execute Substraction()
Substraction();
break;
case 'd':
// Execute Division()
Division();
break;
}
}
// Addition Method
static void Addition()
{
// Declaring variables
double num1, num2, res;
Console.Write("Please enter the first number: ");
// Getting user's input and converting it
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Please enter the second number: ");
// Getting user's input and converting it
num2 = Convert.ToDouble(Console.ReadLine());
// Processing numbers into one variable
res = num1 + num2;
// Printing out the result
Console.WriteLine("RESULT: " +res);
Console.WriteLine("");
Console.ReadKey(true);
MainMenu();
}
// Substraction Method
static void Substraction()
{
// Declaring variables
double num1, num2, res;
Console.Write("Please enter the first number: ");
// Getting user's input and converting it
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Please enter the second number: ");
// Getting user's input and converting it
num2 = Convert.ToDouble(Console.ReadLine());
// Processing numbers into one variable
res = num1 - num2;
// Printing out the result
Console.WriteLine("RESULT: " + res);
Console.ReadKey(true);
MainMenu();
}
// Division
static void Division()
{
// Declaring variables
double num1, num2, res;
Console.Write("Please enter the first number: ");
// Getting user's input and converting it
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Please enter the second number: ");
// Getting user's input and converting it
num2 = Convert.ToDouble(Console.ReadLine());
// Processing numbers into one variable
res = num1 / num2;
// Printing out the result
Console.WriteLine("RESULT: " + res);
Console.WriteLine("");
Console.ReadKey(true);
MainMenu();
}

You can call the MainMenu() method in the default part of the switch in it.
switch (sel)
{
case 1:
// Execute Addition()
SecondMenu();
break;
case 2:
Console.ReadLine();
break;
default:
Console.WriteLine("Sorry that is not correct format!");
MainMenu();
//Catch
break;
}
It is also recommended to have a condition to exit the program (like a maximum number of invalid inputs).

Something like this works, but you'll have to adjust accordingly to fit your requirements:
int sel;
char letter;
bool valid = false;
do
{
Console.WriteLine("Calculator");
Console.WriteLine("********************");
Console.WriteLine("1- Calculator");
Console.WriteLine("2- Exit Calculator");
Console.Write("Please enter your option here: ");
sel = int.Parse(Console.ReadLine());
switch (sel)
{
case 1:
SecondMenu();
break;
case 2:
Environment.Exit(0);
break;
default:
Console.WriteLine("Sorry that is not correct format! Please restart!");
break;
}
}
while (valid != true);

You could just do it like this
static void SecondMenu()
{
char sel2; // Selection variable, used in user's input to get to the desired operation
// Display Menu Options
Console.WriteLine("");
Console.WriteLine("********************");
Console.WriteLine("A. Addition");
Console.WriteLine("S. Substraction");
Console.WriteLine("D. Division");
Console.WriteLine("********************");
Console.Write("Please enter your option here: ");
sel2 = Convert.ToChar(Console.ReadLine());
switch (sel2)
{
case 'a':
Calc(1);
break;
case 's':
Calc(2);
break;
case 'd':
Calc(3);
break;
default:
Console.WriteLine("Wrong entry! Try again");
MainMenu();
return;
}
}
static void Calc(int f)
{
double num1, num2, res;
try
{
Console.Write("Please enter the first number: ");
num1 = Convert.ToDouble(Console.ReadLine());
Console.Write("Please enter the second number: ");
num2 = Convert.ToDouble(Console.ReadLine());
switch (f)
{
case 1:
res = num1 + num2;
break;
case 2:
res = num1 - num2;
break;
case 3:
res = num1 / num2;
break;
default:
Console.WriteLine("Wrong entry! Try again");
MainMenu();
return;
}
Console.WriteLine("RESULT: " + res);
Console.WriteLine("");
Console.ReadKey(true);
MainMenu();
}
catch
{
Console.WriteLine("Wrong entry! Try again");
MainMenu();
}
}
EDIT: To make sure the first menu wont crash, sorround it with a try catch block
try
{
sel = int.Parse(Console.ReadLine());
switch (sel)
{
case 1:
SecondMenu();
break;
case 2:
Console.ReadLine();
break;
default:
Console.WriteLine("Sorry that is not correct format! Please restart!");
MainMenu();
break;
}
}
catch
{
Console.WriteLine("Sorry that is not correct format! Please restart!");
MainMenu();
}
Your problem was, that it tried to parse a char into an int, what is not possible.

You can either write the switch case in another method and call the main method at the default case after printing the error message,
OR
Use a do-while loop,
Inside do, write the switch cases and put a condition at while to exit the calculator.
For example: 'Enter -1 to exit the calculator' and condition in while x!=-1

Try having a play with this:
static void Main(string[] args)
{
MainMenu();
}
static void MainMenu()
{
int sel = -1;
while (sel != 2)
{
Console.WriteLine("Calculator");
Console.WriteLine("********************");
Console.WriteLine("1- Calculator");
Console.WriteLine("2- Exit Calculator");
Console.Write("Please enter your option here: ");
sel = int.Parse(Console.ReadLine());
if (sel == 1)
{
SecondMenu();
}
else if (sel != 2)
{
Console.WriteLine("Sorry that is not correct format! Please restart!"); //Catch
}
}
}
static void SecondMenu()
{
var options = new Dictionary<char, Func<double, double, double>>()
{
{ 'a', (x, y) => x + y },
{ 's', (x, y) => x - y },
{ 'd', (x, y) => x / y },
};
while (true)
{
Console.WriteLine("");
Console.WriteLine("********************");
Console.WriteLine("A. Addition");
Console.WriteLine("S. Substraction");
Console.WriteLine("D. Division");
Console.WriteLine("********************");
Console.Write("Please enter your option here: ");
char sel = Convert.ToChar(Console.ReadLine());
if (options.ContainsKey(sel))
{
Calculate(options[sel]);
break;
}
}
}
static void Calculate(Func<double, double, double> operation)
{
Console.WriteLine("Please enter the first number: ");
double num1 = Convert.ToDouble(Console.ReadLine());
Console.WriteLine("Please enter the second number: ");
double num2 = Convert.ToDouble(Console.ReadLine());
double res = operation(num1, num2);
Console.WriteLine("RESULT: " + res);
Console.WriteLine("");
Console.ReadLine();
}

Float/Double type input validation in C#

This is literally my first program I've ever written (started learning this past Monday); I am a total newbie.
My question is, how can I prevent exceptions from being thrown when a user enters an invalid character when the program prompts the user for fahreinheit or celsius entry (expecting a number)??? So for example, when a user enters "asfasd", the program throws an exception.
I did a lot of searching on the site before posting this, and I was successfully able to find other input validation questions, however, they were all concerning C and C++ and since I am such a newbie, I have a hard time with understanding those languages and how they relate to C#. Thank you. Please see code:
using System;
namespace Converter
{
class Program
{
static void Main()
{
float? FahrenheitInput = null;
double? CelsiusInput = null;
float? KilogramInput = null;
float? PoundsInput = null;
int UserChoice = 0;
do
{
Console.WriteLine("What would you like to convert? Enter the corresponding number.\n1. Fahrenheit to Celsius");
Console.WriteLine("2. Celsius to Fahrenheit\n3. Pounds to Kilograms\n4. Kilograms to pounds\n5. Exit program");
UserChoice = int.Parse(Console.ReadLine());
switch (UserChoice)
{
case 1:
Console.WriteLine("Enter the temperature in Fahreinheit, number only:");
FahrenheitInput = float.Parse(Console.ReadLine());
Console.Clear();
Console.WriteLine(FahrenheitInput + " degrees fahrenheit in Celsius is " + Program.FahrenheitToCelsius(FahrenheitInput) + "\n\n");
break;
case 2:
Console.WriteLine("Enter the temperature in Celsius, number only:");
CelsiusInput = double.Parse(Console.ReadLine());
Console.Clear();
Console.WriteLine(CelsiusInput + " degrees Celius in fahrenheit is " + Program.CelsiusToFahrenheit(CelsiusInput) + "\n\n");
break;
case 5:
break;
default:
Console.WriteLine("This is not a valid entry. Please enter 1, 2, 3, 4, or 5.");
break;
}
} while (UserChoice != 5);
}
public static float? FahrenheitToCelsius(float? INPUT)
{
return (INPUT - 32) * 5 / 9;
}
public static double? CelsiusToFahrenheit(double? INPUT)
{
return INPUT * 1.8 + 32;
}
}
}

You can either put it in Try-Catch block or use a while loop to validate the user input.
below is your code with a while loop which validates users input.
class Program
{
static void Main(string[] args)
{
double FahrenheitInput = 0;
double CelsiusInput = 0;
double KilogramInput = 0;
double PoundsInput = 0;
int UserChoice = 0;
do
{
Console.WriteLine("What would you like to convert? Enter the corresponding number.\n1. Fahrenheit to Celsius");
Console.WriteLine("2. Celsius to Fahrenheit\n3. Pounds to Kilograms\n4. Kilograms to pounds\n5. Exit program");
UserChoice = int.Parse(Console.ReadLine());
switch (UserChoice)
{
case 1:
Console.WriteLine("Enter the temperature in Fahreinheit, number only:");
while (!double.TryParse(Console.ReadLine(), out FahrenheitInput))
{
Console.WriteLine("Invalid format, please input again!");
};
Console.Clear();
Console.WriteLine(FahrenheitInput + " degrees fahrenheit in Celsius is " + Program.FahrenheitToCelsius(FahrenheitInput) + "\n\n");
break;
case 2:
Console.WriteLine("Enter the temperature in Celsius, number only:");
while (!double.TryParse(Console.ReadLine(), out CelsiusInput))
{
Console.WriteLine("Invalid format, please input again!");
};
Console.Clear();
Console.WriteLine(CelsiusInput + " degrees Celius in fahrenheit is " + Program.CelsiusToFahrenheit(CelsiusInput) + "\n\n");
break;
case 5:
break;
default:
Console.WriteLine("This is not a valid entry. Please enter 1, 2, 3, 4, or 5.");
break;
}
} while (UserChoice != 5);
}
public static double FahrenheitToCelsius(double INPUT)
{
return (INPUT - 32) * 5 / 9;
}
public static double CelsiusToFahrenheit(double INPUT)
{
return INPUT * 1.8 + 32;
}
}

TryParse is your good friend here. In most scenarios, you should favor using TryParse than Parse. In your example, you can do something like:
int validInt;
int.TryParse(Console.ReadLine(), out validInt);
float validFloat;
float.TryParse(Console.ReadLine(), out validFloat);
Parse vs. TryParse

The easiest way, IMHO, to change the routine is to rewrite Parse into corresponding TryParse:
// UserChoice = int.Parse(Console.ReadLine());
UserChoice = int.TryParse(Console.ReadLine(), out UserChoice) ? UserChoice : -1;
...
A bit more complex (you have to convert float into float?)
// FahrenheitInput = float.Parse(Console.ReadLine());
float v;
FahrenheitInput = float.TryParse(Console.ReadLine(), out v) ? (float?) v : null;
The same scheme for CelsiusInput
// CelsiusInput = double.Parse(Console.ReadLine());
double d;
CelsiusInput = double.TryParse(Console.ReadLine(), out v) d (double?) d : null;
The underlying mechanic of the code is
We try to parse user input TryParse(Console.ReadLine()...
If parse succeeds (and thus TryParse returns true) we just return the out (parsed value).
If parse fails (and thus TryParse returns false) we return some special a value (-1 for UserChoice or null in case of FahrenheitInput or CelsiusInput)
P.S. in the first switch you have just case 1, case 2 and case 5; however, you've put "This is not a valid entry. Please enter 1, 2, 3, 4, or 5." in the error message. It seems, that you have to either implement case 3 and case 4 in the switch or edit the error message.

Use int.TryParse instead of int.Parse and float.tryParse instead of float.Parse

While all the answers provided seem to work, the question you asked was
how can I prevent exceptions from being thrown [..]
and I just want to point out that you do this by putting the part which throws the exception in an try-catch-block. What this does is it executes the code within try until an exception is beeing thrown and then passes this exceptions as a parameter to the catch-part where you can handle it:
EXAMPLE
do
{
try
{
// the code you already have
}
catch (Exception ex)
{
Console.WriteLine("This is no valid input (" + ex.Message + ")! Try again...");
}
} while (UserChoice != 5);
Of course preventing exceptions from beeing thrown at all in the first place is the better way (as all the other answers do suggest), but this approach works as well and ist more generic in case you run into a similar problem in the future. Using switch-case-statements for error-handling is quite common practice...