C# 使用 TryParse 时遇到问题

Question

How would I add TryParse to this code, so if user enters a letter, it would tell him, "Not valid, please enter numbers only". I have tried a couple of ways, but it breaks my code. I have tried it with for and while loops. But, when I did get it to work, it only took 1 number, and then assigned that number to all my arrays.

{
    const int SIZE = 2;

    double[] array = new double[SIZE];
    Console.WriteLine("Please Sir Enter 2 numbers");
    for (int i = 0; i < SIZE; i++)
    {
         array[i] = Convert.ToDouble(Console.ReadLine());
    }
    Console.WriteLine("===============================================");
    Console.WriteLine("The Values you've entered are");
    Console.WriteLine("{0}{1,8}", "index", "value");
    for (int counter = 0; counter< SIZE; counter++)
    {
         Console.WriteLine("{0,5}{1,8}", counter, array[counter]);
    }
    Console.WriteLine("===============================================");            
    Console.ReadLine();
}

Answer 1

Use the while loop inside of the for loop like this:

for (int i = 0; i < SIZE; i++)
{
   string input = Console.ReadLine();
   double num = 0;
   while(!Double.TryParse(input, out num))
   {
       Console.WriteLine("Not valid, please enter numbers only");
       input = Console.ReadLine();
   }
    array[i] = num;
}

Answer 2

For getting validated input from the user (especially when the type is expected to be something other than a string), I find it really useful to use a helper method.

The method below takes in a string that is displayed to the user (the prompt for input), and then continues to ask the user for input until they enter a valid number ( TryParse is part of the while condition).

It also takes in an optional function parameter that can be used to validate the input, in case you want to place additional restrictions on it. The function is defined to take in a double (the user input) and return a bool ( true if the input is valid):

private static double GetDoubleFromUser(string prompt, Func<double, bool> validator = null)
{
    double result;

    do
    {
        Console.Write(prompt);
    } while (!double.TryParse(Console.ReadLine(), out result) ||
             (validator != null && !validator.Invoke(result)));

    return result;
}

To use this in your code, you would just do something like:

Console.WriteLine($"Please sir, enter {SIZE} numbers");
for (int i = 0; i < SIZE; i++)
{
    array[i] = GetDoubleFromUser($" Enter number #{i + 1}: ");
}

Output

---

But other times you may want to restrict the number even further, in which case you can pass a validation method to the function. One simple way to do this is to pass the function as a lambda expression. For example this expression: i => i > 10 means "return the result of the comparison if i is greater than 10 ", or in other words, "return true if i > 10 , otherwise return false ".

You could pass this to our GetDoubleFromUser method like this:

double greaterThan10 = GetDoubleFromUser("Enter a number greater than 10: ", i => i > 10);

and now the method will keep looping while the input is not a number or the input is not greater than 10!

Output