如果用户在没有输入数字的情况下按下回车键，我该如何编写错误消息。（例如输入字母或什么都不输入）

Question

 'the variables
        Dim watertemp As Decimal 'changed variable to decimal so that decimal numbers can be input.
        Dim fahren As Decimal
        Dim newtemp As String


        'this code asks the intial question (the temp of the water)
        Console.WriteLine("Enter the tempurature of the water, between -273.15 and 100 degrees celcius.")
        watertemp = Console.ReadLine





        'this code is to work out what the water should be called and the colour of the text it will be displayed in.
        If watertemp <= 0 Then
            Console.ForegroundColor = ConsoleColor.Blue
            Console.WriteLine("The water is freezing")
        End If
        If watertemp >= 1 And watertemp <= 25 Then
            Console.ForegroundColor = ConsoleColor.Cyan
            Console.WriteLine("The water is warm")
        End If
        If watertemp >= 26 And watertemp <= 60 Then
            Console.WriteLine("The water is hot")
        End If
        If watertemp >= 61 And watertemp <= 99 Then
            Console.ForegroundColor = ConsoleColor.Magenta
            Console.WriteLine("The water is very hot")
        End If
        If watertemp >= 100 Then
            Console.ForegroundColor = ConsoleColor.Red
            Console.WriteLine("The water is boiling")
        End If

        'this code converts the tempurature to fahrenheit
        fahren = (9 / 5 * watertemp) + 32
        Console.WriteLine("The water is " & (Format(fahren, "0.00")) & " °F")

        'this code changes the text back to white
        Console.ForegroundColor = ConsoleColor.White
        'this code asks the user if they would like to convert another input
        Console.WriteLine("Would you like to convert another tempurature?")
        newtemp = Console.ReadLine

        'this code determines whether to carry on with converting another input or end the program
        If newtemp = "no" Then
            Console.WriteLine("This is the end of the program")
        End If
        If newtemp = "yes" Then
            Console.WriteLine("What is the tempurature of the water?")
            watertemp = Console.ReadLine
        End If
        fahren = (9 / 5 * watertemp) + 32
        Console.WriteLine("The water is " & (Format(fahren, "0.00")) & " °F")

        Console.ReadLine()

I need to have an error message if the user inputs something that the program cannot run, such as letters or nothing at all.

I tried a few things but I dont know if I did any of them right as I am very new to coding.

Answer 1

Dim watertemp As Decimal 'changed variable to decimal so that decimal numbers can be input.
Dim fahren As Decimal
Dim newtemp As String

reenter:
        'this code asks the intial question (the temp of the water)
        Console.WriteLine("Enter the tempurature of the water, between -273.15 and 100 degrees celcius.")
        watertemp = Console.ReadLine
        If String.IsNullOrEmpty(watertemp) OrElse Not IsNumeric(watertemp) Then
            MsgBox("Wrong input.")
            GoTo reenter
        End If