format a number with commas and decimals in C# (asp.net MVC3)

int number = 1234567890;
Convert.ToDecimal(number).ToString("#,##0.00");

You will get the result 1,234,567,890.00.

Maybe you simply want the standard format string "N", as in

number.ToString("N")

It will use thousand separators, and a fixed number of fractional decimals. The symbol for thousands separators and the symbol for the decimal point depend on the format provider (typically CultureInfo) you use, as does the number of decimals (which will normally by 2, as you require).

If the format provider specifies a different number of decimals, and if you don't want to change the format provider, you can give the number of decimals after the N, as in .ToString("N2").

Edit: The sizes of the groups between the commas are governed by the

CultureInfo.CurrentCulture.NumberFormat.NumberGroupSizes

array, given that you don't specify a special format provider.

I had the same problem. I wanted to format numbers like the "General" format in spreadsheets, meaning show decimals if they're significant, but chop them off if not. In other words:

1234.56 => 1,234.56

1234 => 1,234

It needs to support a maximum number of places after the decimal, but don't put trailing zeros or dots if not required, and of course, it needs to be culture friendly. I never really figured out a clean way to do it using String.Format alone, but a combination of String.Format and Regex.Replace with some culture help from NumberFormatInfo.CurrentInfo did the job (LinqPad C# Program).

string FormatNumber<T>(T number, int maxDecimals = 4) {
    return Regex.Replace(String.Format("{0:n" + maxDecimals + "}", number),
                         @"[" + System.Globalization.NumberFormatInfo.CurrentInfo.NumberDecimalSeparator + "]?0+$", "");
}   

void Main(){
    foreach (var test in new[] { 123, 1234, 1234.56, 123456.789, 1234.56789123 } )
        Console.WriteLine(test + " = " + FormatNumber(test));
}

Produces:

123 = 123
1234 = 1,234
1234.56 = 1,234.56
123456.789 = 123,456.789
1234.56789123 = 1,234.5679