Number Formatting (edit)

NumberFormatInfo.CurrentInfo Property (System.Globalization) | Microsoft Docs

Standard numeric format strings | Microsoft Docs

Number Formatting - Globalization | Microsoft Docs

Namespace: System.Globalization

Assembly: System.Runtime.dll

Format a number with commas and decimals in C# (asp.net MVC3) - Stack Overflow

C# - How to change symbol for decimal point in double.ToString()? - Stack Overflow

How to round up or down in C#?

c# math round down Code Example (codegrepper.com)

decimal.Round(x, 2)

decimal.Round(x, 2, MidpointRounding.AwayFromZero);

Program.cs

using System;
using System.Globalization;

namespace ConsoleApp2
{
 internal class Program
 {
 private static void Main(string[] args)
 {
 //var x = Math.Floor(-1.23 * 10) / 10; //==> -1.3
 var x = Math.Floor(0m * 10) / 10; //==> 0
 //var x = Math.Floor(1.23 * 10) / 10; //==> 1.2
 //var x = Math.Floor(0.99 * 10) / 10; //==> 0.9
 //var x = Math.Floor(1.87 * 10) / 10; //==> 1.8
 Console.WriteLine(x);

 //Console.WriteLine( CultureInfo.CurrentUICulture.Name);

 NumberFormatInfo nfi = NumberFormatInfo.CurrentInfo;
 Console.WriteLine(nfi.NumberDecimalSeparator);
 Console.WriteLine(nfi.NumberGroupSeparator);

 //NumberFormatInfo nfi = new NumberFormatInfo();
 //nfi.NumberDecimalSeparator = ",";
 //nfi.NumberGroupSeparator = ".";

 double num = 4.36;
 string ret = num.ToString(nfi); // 4,3
 Console.WriteLine(ret);

 //string s0 = string.Format(System.Globalization.CultureInfo.GetCultureInfo("en-US"), "{0:0.0}", 4.35);
 //Console.WriteLine(s0);
 //string s1 = string.Format(System.Globalization.CultureInfo.GetCultureInfo("de-DE"), "{0:0.0}", 4.35);
 //Console.WriteLine(s1);
 //string s2 = string.Format(System.Globalization.CultureInfo.GetCultureInfo("ja-JP"), "{0:0.0}", 4.35);
 //Console.WriteLine(s2);
 //string s3 = string.Format(CultureInfo.CurrentUICulture, "{0:0.0}", 14524.35);
 //Console.WriteLine(s3);
 }

 /// <summary>
 /// 
 /// </summary>
 /// <param name="tax"></param>
 /// <returns></returns>
 private static decimal RoundTax(decimal tax)
 {
 if (tax <= 0) return 0;
 return Math.Floor(0m * 10) / 10;
 }
 }
}

Làm tròn xuống

return Math.Floor(1m * 10) / 10;