double a = 123456, b = 123456.1, c = 123456.12, d = 123456.123, e = 123456.126;
            Console.WriteLine(a.ToString("N"));  //123,456.00
            Console.WriteLine(b.ToString("N"));  //123,456.10
            Console.WriteLine(c.ToString("N"));  //123,456.12
            Console.WriteLine(d.ToString("
 
N"));  //123,456.12
            Console.WriteLine(e.ToString("N"));  //123,456.13
            Console.WriteLine();

            Console.WriteLine(a.ToString("N2"));  //123,456.00
            Console.WriteLine(b.ToString("N2"));  //123,456.10
            Console.WriteLine(c.ToString("N2"));  //123,456.12
            Console.WriteLine(d.ToString("
 
N2"));  //123,456.12
            Console.WriteLine(e.ToString("N2"));  //123,456.13
            Console.WriteLine();

            Console.WriteLine(a.ToString("F2"));  //123456.00
            Console.WriteLine(b.ToString("F2"));  //123456.10
            Console.WriteLine(c.ToString("F2"));  //123456.12
            Console.WriteLine(d.ToString("
 
F2"));  //123456.12
            Console.WriteLine(e.ToString("F2"));  //123456.13
            Console.WriteLine();

            Console.WriteLine(a.ToString("0.00"));  //123456.00
            Console.WriteLine(b.ToString("0.00"));  //123456.10
            Console.WriteLine(c.ToString("0.00"));  //123456.12
            Console.WriteLine(d.ToString("0.00"));  //123456.12
            Console.WriteLine(e.ToString("0.00"));  //123456.13
            Console.WriteLine();

            Console.WriteLine(a.ToString("G"));  //123456
            Console.WriteLine(b.ToString("G"));  //123456.1
            Console.WriteLine(c.ToString("G"));  //123456.12
            Console.WriteLine(d.ToString("G"));  //123456.123
            Console.WriteLine(e.ToString("G"));  //123456.126
            Console.WriteLine();

            //本地貨幣
            Console.WriteLine(a.ToString("C"));  //NT$123,456
            Console.WriteLine(b.ToString("C"));  //NT$123,456.1
            Console.WriteLine(c.ToString("C"));  //NT$123,456.12
            Console.WriteLine(d.ToString("C"));  //NT$123,456.123
            Console.WriteLine(e.ToString("C"));  //NT$123,456.126
            Console.WriteLine();
            Console.Read();

c# 數字轉字符串保留兩位小數