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Why does this code not work when I try to convert it

This is only a small piece of the code I'm trying to make work. The original code I wrote works but I want to make sure when the user inputs a number it is in fact a number.
Console.WriteLine("Give the number of A");
A =Convert.ToDouble( Console.ReadLine());
if (char.IsNumber(Convert.ToDouble(A)) == correct)
{
Console.WriteLine(Convert.ToDouble( A * A));
}
else
{
Console.WriteLine("Incorrecrt input");
}
The Console.WriteLine(Convert.ToDouble(A*A)); I only wrote to see if that will work and it doesn't. After the user inputs only a number I must use it in another equation for a final answer. The user must input 3 numbers.

For me, you should check is the input is a number that you can convert to double before converting it.
Console.WriteLine("Give the number of A");
var a = Console.ReadLine();
double number;
if (double.TryParse(a, out number))//double.TryParse takes a parameter and tries to convert it to double. If the convertion is successfull, sets the out parameter as result and returns true. If not, returns false. And you can use with other types like int.Tryparse(param, out outParam);
{
A = number;
}
else
{
//Not a number, show a message etc...
{

If you break this down:
A =Convert.ToDouble( Console.ReadLine());
if (char.IsNumber(Convert.ToDouble(A)) == correct)
{
}
What you're basically doing is:
Double A = Convert.ToDouble(Console.ReadLine());
Double dbl = Convert.ToDouble(A);
Boolean bl = char.IsNumber(dbl);
if (bl== correct)
{
}
So there's multiple things wrong here.
Firstly, you're trying to convert the user input without any sort of guarantee of success, so if someone typed "A" instead of a number it will throw an exception.
You should use TryParse and then if it's a valid conversion proceed.
Secondly, you're checking if a Double is a char that can be converted to a number. Obviously not because it's already a Double.
Thirdly, you're checking if a Boolean is equal to some variable called correct (which you haven't provided the definition of) so it's not clear if this is a valid comparsion.
EDIT:
This is what I would do:
bool validInput = false;
do
{
Console.WriteLine("Give the number of A");
string userInput = Console.ReadLine();
if (double.TryParse(userInput, out double result))
{
validInput = true;
Console.WriteLine(result * result);
Console.ReadLine();
}
else
{
Console.WriteLine("Invalid input. Please type a number.");
}
} while (!validInput);

How to print an error message if input is not a number?

I wonder how to print an error message if user's input is not a number.
Console.WriteLine("Water amount in ml:");
int waterAmount = Convert.ToInt32(Console.ReadLine());
Most answers from other posts don't work, because waterAmount is a Int32, not a string.
Also, sorry if my English is weak, it's not my native language

I see you did not accept the other answers. Maybe you want to make the user try again after he did not input a number. You can do that with a while loop, or easier, using the goto keyword. You simply put a tag before everything is happening (in my case, waterAmountInput), and if the input was not a number, you write your error message and go to the beginning again. Here is the code:
int waterAmount;
waterAmountInput:
Console.Write("Water amount in ml: ");
try
{
waterAmount = Convert.ToInt32(Console.ReadLine());
}
catch
{
Console.WriteLine("The water amount needs to be a number!");
goto waterAmountInput;
}

You can try using C#'s TryParse() functions.
These attempt to convert values, but if they fail they return false rather than erroring.
I would suggest trying this code:
Console.WriteLine("Water amount in ml:");
string input = Console.ReadLine();
if (Int32.TryParse(input, out var value))
// Do something here. The converted int is stored in "value".
else
Console.WriteLine("Please enter a number");

You should use TryParse function:
Console.WriteLine("Water amount in ml:");
int waterAmount = 0;
if (int.TryParse(Console.ReadLine(), waterAmount)) {
} else {
Console.WriteLine("Error Message");
}

You will need to use TryParse before presuming it to be a number.
After Console.WriteLine you can capture and parse the input and if TryParse returns false, you can display an error message.
Note: if the conversion succeeds the numeric value is available in the waterAmt local variable.
Console.WriteLine("Water amount in ml:");
var input = Console.ReadLine();
if(!int.TryParse(input, out int waterAmt))
Console.WriteLine("Input is not a number");

is there a way to instead of making double not freeze project and instead say invalid number?

i am new in stack overflow, but i've ran into a problem.
what i want to do exactly is an advanced calculator, but can't find out a way to give an error message (then i will add a retry option) instead of making the console freeze.
Console.Write("Type A Number: ");
double num1 = Convert.ToDouble(Console.ReadLine());
//detects numbers
if (num1 != "1")
Console.WriteLine("invalid number!");
//detects numbers
Console.Write("Type An Operator: ");
string op = Console.ReadLine();
Console.Write("Type Another Number: ");
double num2 = Convert.ToDouble(Console.ReadLine());
//the operator logic system!!!
the operator logic system detects if the operator (+ - x/* / etc) is valid, but I can't find out for number. after that, I am going to make a game, but I can't find out for this one now.

Try this:
Console.Write("Type A Number: ");
if (!double.TryParse (Console.ReadLine(), out double num1))
Console.WriteLine("invalid number!");
TryParse returns false if the entered text is not a valid double. If it is valid it returns true and puts the number into num1.

This code is going to cause you issues if the user enters an unexpected value. E.g. They enter a string instead of a number. So you need to also look for and account for that.
Best is to create different methods which will check the input and only return a valid result, otherwise to prompt the user. So you have something like this;
The main method calls submethods, so is relatively simple.
You call GetValidNumber to return the number inputs for bother number 1 and number 2
GetValidOperator to return the valid operator, which itself calls IsValidOperator to check the input is valid (as there is more to check).
Double.TryParse(...) is the main check if the double number entered is valid or not. But using TryParese it will not through an exception if the user enters an non-valid number, but rather just return false. So it is a safe way of checking user input in this case.
The code might need refining and error checking improved, but it should put you on the right course.
static void Main(string[] args)
{
double number1 = GetValidNumber("Enter first number");
string op = GetValidOperator();
double number2 = GetValidNumber("Enter second number");
Console.WriteLine($"Operation is {number1} {op} {number2}");
//Add code to perform your operation
}
private static double GetValidNumber(string message)
{
Console.WriteLine(message);
string input = Console.ReadLine();
double output;
while (!Double.TryParse(input, out output) )
{
Console.WriteLine("Invalid number! Please try again");
input = Console.ReadLine();
}
return output;
}
private static string GetValidOperator()
{
Console.WriteLine("Enter an operator + - * / ");
string input = Console.ReadLine();
while (!IsValidOperator(input))
{
Console.WriteLine("Invalid Operator! Please try again");
input = Console.ReadLine();
}
return input;
}
private static bool IsValidOperator(string input)
{
bool result = false;
if (String.IsNullOrWhiteSpace(input))
return false;
if (input.Length != 1)
return false;
if (input[0] == '+' || input[0] == '-' || input[0] == '/' || input[0] == '*')
return true;
else
return result;
}

Well, you can use Double.Parse instead of Convert.ToDouble. It will throw exception on failed conversion and you will be able to handle that.

C# read value from user console.read [duplicate]

What I am looking for is how to read an integer that was given by the user from the command line (console project). I primarily know C++ and have started down the C# path. I know that Console.ReadLine(); only takes a char/string. So in short I am looking for the integer version of this.
Just to give you an idea of what I'm doing exactly:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
Console.ReadLine(); // Needs to take in int rather than string or char.
I have been looking for quite a while for this. I have found a lot on C but not C#. I did find however a thread, on another site, that suggested to convert from char to int. I'm sure there has to be a more direct way than converting.

You can convert the string to integer using Convert.ToInt32() function
int intTemp = Convert.ToInt32(Console.ReadLine());

I would suggest you use TryParse:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
string input = Console.ReadLine();
int number;
Int32.TryParse(input, out number);
This way, your application does not throw an exception, if you try to parse something like "1q" or "23e", because somebody made a faulty input.
Int32.TryParse returns a boolean value, so you can use it in an if statement, to see whether or not you need to branch of your code:
int number;
if(!Int32.TryParse(input, out number))
{
//no, not able to parse, repeat, throw exception, use fallback value?
}
To your question: You will not find a solution to read an integer because ReadLine() reads the whole command line, threfor returns a string. What you can do is, try to convert this input into and int16/32/64 variable.
There are several methods for this:
Int.Parse()
Convert.ToInt()
Int.TryParse()
If you are in doubt about the input, which is to be converted, always go for the TryParse methods, no matter if you try to parse strings, int variable or what not.
Update
In C# 7.0 out variables can be declared directly where they are passed in as an argument, so the above code could be condensed into this:
if(Int32.TryParse(input, out int number))
{
/* Yes input could be parsed and we can now use number in this code block
scope */
}
else
{
/* No, input could not be parsed to an integer */
}
A complete example would look like this:
class Program
{
static void Main(string[] args)
{
Console.WriteLine("Hello World!");
var foo = Console.ReadLine();
if (int.TryParse(foo, out int number1)) {
Console.WriteLine($"{number1} is a number");
}
else
{
Console.WriteLine($"{foo} is not a number");
}
Console.WriteLine($"The value of the variable {nameof(number1)} is {number1}");
Console.ReadLine();
}
}
Here you can see, that the variable number1 does get initialized even if the input is not a number and has the value 0 regardless, so it is valid even outside the declaring if block

You need to typecast the input. try using the following
int input = Convert.ToInt32(Console.ReadLine());
It will throw exception if the value is non-numeric.
Edit
I understand that the above is a quick one. I would like to improve my answer:
String input = Console.ReadLine();
int selectedOption;
if(int.TryParse(input, out selectedOption))
{
switch(selectedOption)
{
case 1:
//your code here.
break;
case 2:
//another one.
break;
//. and so on, default..
}
}
else
{
//print error indicating non-numeric input is unsupported or something more meaningful.
}

int op = 0;
string in = string.Empty;
do
{
Console.WriteLine("enter choice");
in = Console.ReadLine();
} while (!int.TryParse(in, out op));

Use this simple line:
int x = int.Parse(Console.ReadLine());

I didn't see a good and complete answer to your question, so I will show a more complete example. There are some methods posted showing how to get integer input from the user, but whenever you do this you usually also need to
validate the input
display an error message if invalid input
is given, and
loop through until a valid input is given.
This example shows how to get an integer value from the user that is equal to or greater than 1. If invalid input is given, it will catch the error, display an error message, and request the user to try again for a correct input.
static void Main(string[] args)
{
int intUserInput = 0;
bool validUserInput = false;
while (validUserInput == false)
{
try
{
Console.Write("Please enter an integer value greater than or equal to 1: ");
intUserInput = int.Parse(Console.ReadLine()); //try to parse the user input to an int variable
}
catch (Exception e) //catch exception for invalid input, such as a letter
{
Console.WriteLine(e.Message);
}
if (intUserInput >= 1) { validUserInput = true; }
else { Console.WriteLine(intUserInput + " is not a valid input, please enter an integer greater than 0."); }
} //end while
Console.WriteLine("You entered " + intUserInput);
Console.WriteLine("Press any key to exit ");
Console.ReadKey();
} //end main
In your question it looks like you wanted to use this for menu options. So if you wanted to get int input for choosing a menu option you could change the if statement to
if ( (intUserInput >= 1) && (intUserInput <= 4) )
This would work if you needed the user to pick an option of 1, 2, 3, or 4.

I used int intTemp = Convert.ToInt32(Console.ReadLine()); and it worked well, here's my example:
int balance = 10000;
int retrieve = 0;
Console.Write("Hello, write the amount you want to retrieve: ");
retrieve = Convert.ToInt32(Console.ReadLine());

Better way is to use TryParse:
Int32 _userInput;
if(Int32.TryParse (Console.Readline(), out _userInput) {// do the stuff on userInput}

Try this it will not throw exception and user can try again:
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice = 0;
while (!Int32.TryParse(Console.ReadLine(), out choice))
{
Console.WriteLine("Wrong input! Enter choice number again:");
}

static void Main(string[] args)
{
Console.WriteLine("Please enter a number from 1 to 10");
int counter = Convert.ToInt32(Console.ReadLine());
//Here is your variable
Console.WriteLine("The numbers start from");
do
{
counter++;
Console.Write(counter + ", ");
} while (counter < 100);
Console.ReadKey();
}

You could create your own ReadInt function, that only allows numbers
(this function is probably not the best way to go about this, but does the job)
public static int ReadInt()
{
string allowedChars = "0123456789";
ConsoleKeyInfo read = new ConsoleKeyInfo();
List<char> outInt = new List<char>();
while(!(read.Key == ConsoleKey.Enter && outInt.Count > 0))
{
read = Console.ReadKey(true);
if (allowedChars.Contains(read.KeyChar.ToString()))
{
outInt.Add(read.KeyChar);
Console.Write(read.KeyChar.ToString());
}
if(read.Key == ConsoleKey.Backspace)
{
if(outInt.Count > 0)
{
outInt.RemoveAt(outInt.Count - 1);
Console.CursorLeft--;
Console.Write(" ");
Console.CursorLeft--;
}
}
}
Console.SetCursorPosition(0, Console.CursorTop + 1);
return int.Parse(new string(outInt.ToArray()));
}

Declare a variable that will contain the value of the user input :
Ex :
int userInput = Convert.ToInt32(Console.ReadLine());

I know this question is old, but with some newer C# features like lambda expressions, here's what I actually implemented for my project today:
private static async Task Main()
{
// -- More of my code here
Console.WriteLine("1. Add account.");
Console.WriteLine("2. View accounts.");
int choice = ReadInt("Please enter your choice: ");
// -- Code that uses the choice variable
}
// I have this as a public function in a utility class,
// but you could use it directly in Program.cs
private static int ReadInt(string prompt)
{
string? text;
do
{
Console.Write(prompt);
text = Console.ReadLine();
} while (text == null || !text.Where(c => char.IsNumber(c)).Any());
return int.Parse(new string(text.Where(c => char.IsNumber(c)).ToArray()));
}
The difference here is that if you accidentally type a number and any other text along with that number, only the number is parsed.

You could just go ahead and try :
Console.WriteLine("1. Add account.");
Console.WriteLine("Enter choice: ");
int choice=int.Parse(Console.ReadLine());
That should work for the case statement.
It works with the switch statement and doesn't throw an exception.

C# How to loop user input until the datatype of the input is correct?

How to make this piece of code loop asking for input from the user until int.TryParse()
is successful?
//setX
public void setX()
{
//take the input from the user
string temp;
int temp2;
System.Console.WriteLine("Enter a value for X:");
temp = System.Console.ReadLine();
if (int.TryParse(temp, out temp2))
x = temp2;
else
System.Console.WriteLine("You must enter an integer type value"); 'need to make it ask user for another input if first one was of invalid type'
}
Version of the code after the helpful answer:
//setX
public void setX()
{
//take the input from the user
string temp;
int temp2;
System.Console.WriteLine("Enter a value for X:");
temp = System.Console.ReadLine();
if (int.TryParse(temp, out temp2))
x = temp2;
else
{
Console.WriteLine("The value must be of integer type");
while (!int.TryParse(Console.ReadLine(), out temp2))
Console.WriteLine("The value must be of integer type");
x = temp2;
}
}

while (!int.TryParse(Console.ReadLine(), out mynum))
Console.WriteLine("Try again");
edit:
public void setX() {
Console.Write("Enter a value for X (int): ");
while (!int.TryParse(Console.ReadLine(), out x))
Console.Write("The value must be of integer type, try again: ");
}
Try this. I personally prefer to use while, but do .. while is also valid solution. The thing is that I don't really want to print error message before any input. However while has also problem with more complicated input that can't be pushed into one line. It really depends on what exactly you need. In some cases I'd even recommend to use goto even tho some people would probably track me down and slap me with a fish because of it.

Even though the question has been already marked as answered, do-while loops are much better for validating user input.
Notice your code:
Console.WriteLine("The value must be of integer type");
while (!int.TryParse(Console.ReadLine(), out temp2))
Console.WriteLine("The value must be of integer type");
You have the same code at top and bottom. This can be changed:
do {
Console.WriteLine("The value must be of integer type");
} while (!int.TryParse(Console.ReadLine(), out temp2));

This can help too
public int fun()
{
int Choice=0;
try
{
Choice = int.Parse(Console.ReadLine());
return choice;
}
catch (Exception)
{
return fun();
}
}

I've been wondering quite a lot, but I just figured it out!
int number;
bool check;
do
{
Console.WriteLine("Enter an integer:");
check = int.TryParse(Console.ReadLine(), out num1);
}
while (!check);
This code will loop until the user has entered an integer number. This way, the program doesn't simply report an error, but instead immediately allows the user to input again another, correct value.