IFormatProvider scientific conversion from double to string - number of digits

问题

I have a problem with the conversion from double to string.

I want to convert:

double value: 0.0772486324655191

string value: 0.0772486324655191

and if the length is bigger than 16 digits after the decimal point I want it like this:

double value: 0.00063500244832493823

string value: 6.3500244832493823e-004

I have tried to convert it with IFormatProvider Pattern:

0.0000000000000000e000

But the result in the first case is

7.7248632465519100e-002

How can I get the number of digits in my double vector? Or better: how should I use the format provider correctly?

String specifier;
CultureInfo culture;
specifier = "0.0000000000000000e000";
culture = CultureInfo.CreateSpecificCulture("en-US");
Console.WriteLine(DoubleMirrored[0].ToString(specifier, CultureInfo.InvariantCulture));

回答1:

To do this, you definitely need to create a custom formatter.

To create a custom formatter, here's what you should know:
string.Format has the following overload: string.Format(IFormatProvider, string, object[]), so you must create a IFormatProvider that will "provide" a ICustomFormatter, which will handle your custom formatting. The same class can be easily used for both interfaces.

Here's some code that does exactly what you describe:

public class DoubleFormatter : IFormatProvider, ICustomFormatter 
{
    // Implementation of IFormatProvider:
    public object GetFormat(Type t) {
        if (t == typeof(ICustomFormatter)) {
            return this;
        }
        return null;
    }
    // Implementation of ICustomFormatter:
    public string Format(string format, object arg, IFormatProvider provider) {
        // Search for the custom "EE" format specifier:
        if (format == null || !format.StartsWith("EE")) return null;
        format = format.Substring(2); // Trim "EE"
        // Determine how many digits before we cutoff:
        int digits;
        if (!int.TryParse(format, out digits)) {
            throw new FormatException("Format must contain digits");
        }

        // Get the value: (note, this will work for any numeric type)
        var value = Convert.ToDouble(arg);
        // Convert to string without using Exponential format:
        var output = value.ToString("0."+(new string('#',digits)), provider);
        // Determine how many digits are showing: (this part isn't culture-compatible)
        var length = output.Length - output.IndexOf(".");
        if (length <= digits) {
            return output;
        } else {
            return value.ToString("E"+format, provider);
        }
    }
}

And here's an example of how to use this code:

var tests = new[]{
    0.0000055555,
    0.00000555555555555555555,
};

var formatter = new DoubleFormatter();
foreach (var t in tests){ 
    var result = string.Format(formatter, "{0:EE15}", t);
    Console.WriteLine(result);
}

来源：https://stackoverflow.com/questions/8414873/iformatprovider-scientific-conversion-from-double-to-string-number-of-digits

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double