System.FormatException: '输入字符串的格式不正确。' 在 C# 黄皮书教科书中

Question

I am following an example in C# Programming Yellow Book by Rob Miles. I copied and pasted this example from the e-book.

• Here is the error:

   width = double.Parse(widthString); <- this gives the error of: System.FormatException: 'Input string was not in a correct format.' <- Why???

• Full code:

   double width, height, woodLength, glassArea; string widthString, heightString; widthString = Console.ReadLine(); width = double.Parse(widthString); <- this gives the error of: System.FormatException: 'Input string was not in a correct format.' <- Why??? heightString = Console.ReadLine(); height = double.Parse(heightString); woodLength = 2 * (width + height) * 3.25; glassArea = 2 * (width * height); Console.WriteLine("The length of the wood is " + woodLength + " feet"); Console.WriteLine("The area of the glass is " + glassArea + " square metres"); Console.ReadLine(); }

Answer 1

The code reads a string from the console. The string is expected to be a valid double (ie 1.2, 4.3, etc) but you seem to be entering something that is not a valid double.

I'd recommend adding some prompts to instruct the user what they're expected to enter. For example:

    double width, height, woodLength, glassArea; string widthString, heightString;

    Console.Write("Enter the width (as a decimal number): "); // <- Add this
    widthString = Console.ReadLine(); 
    width = double.Parse(widthString); <- this gives the error of: System.FormatException: 'Input string was not in a correct format.' <- Why??? 

    Console.Write("Enter the height (as a decimal number): "); // <- And this
    heightString = Console.ReadLine(); 
    height = double.Parse(heightString);

    woodLength = 2 * (width + height) * 3.25;

    glassArea = 2 * (width * height);

    Console.WriteLine("The length of the wood is " + woodLength + " feet"); 
    Console.WriteLine("The area of the glass is " + glassArea + " square metres");

    Console.ReadLine();
}

Cheers, Ian

Answer 2

If your input string is empty or non numeric(or too large for the scope of a double var) if will throw that error. In these situations I prefer to use TryParse.

double width, height, woodLength, glassArea; string widthString, heightString;

widthString = Console.ReadLine();
heightString = Console.ReadLine();

double.TryParse(widthString, out width);
double.TryParse(heightString, out height);

woodLength = 2 * (width + height) * 3.25;

glassArea = 2 * (width * height);

Console.WriteLine("The length of the wood is " + woodLength + " feet");
Console.WriteLine("The area of the glass is " + glassArea + " square metres");

Console.ReadLine();

this will output an updated value for height / width if the input is able to be parsed.. If not it will default to whatever it was set to before the parsing attempt.

As mentioned in the comments below (thanks for the feedback) you would also want to validate the input. You could do that in many ways. Below is an example on how to do so:

        double width, height, woodLength, glassArea; string widthString, heightString;

        width = GetValidDblInput("Please enter the width:");
        height = GetValidDblInput("Please enter the height:");
        woodLength = 2 * (width + height) * 3.25;

        glassArea = 2 * (width * height);

        Console.WriteLine("The length of the wood is " + woodLength + " feet");
        Console.WriteLine("The area of the glass is " + glassArea + " square metres");

        Console.ReadLine();

    }

    static double GetValidDblInput(string inputRequest)
    {
        bool IsValid = false;
        double val = 0;
        while(!IsValid)
        {
            Console.WriteLine(inputRequest);
            if(double.TryParse(Console.ReadLine(), out val))
            {
                IsValid = true;
            }
            else
            {
                Console.WriteLine("Invalid input. Please enter a valid value (e.g. 1, 3.25, 400)");
            }
        }
        return val;
    }

Answer 3

Make sure that you use the correct decimal separator when entering the value on the console. If you don't specify a specific format when using parsing methods, the framework usually falls back to the regional format of the operation system the program is running on.

If you want to use a specific format, you can do so by using one of the following overloads:

• Parse(String, NumberStyles)
• Parse(String, IFormatProvider)
• Parse(String, NumberStyles, IFormatProvider)

Check out the docs here .