double d = 1234567;
string res = d.ToString("#,##0.00"); //in the formatting, the comma always represents the group separator and the dot the decimal separator. The format part is culture independant and is replaced with the culture dependant values in runtime.
如果显示必须与文化无关,则可以使用特定的 numberformatinfo:
var nfi = new NumberFormatInfo { NumberDecimalSeparator = ",", NumberGroupSeparator = "." };
double d = 1234567;
string res = d.ToString("#,##0.00", nfi); //result will always be 1.234.567,00