provider.NumberDecimalDigits =intDecLength;     //要设定的小数位数
double strCashAmt=Convert.ToDouble(this.txtCashAmt.Text); //先把控件內的值转成double

this.txtCashAmt.Text = strCashAmt.ToString("N",provider); //再利用ToString函数格式化小数位数

2.保留N位,四舍五入 .      

decimal d= decimal.Round(decimal.Parse("0.55555"),2);

3.保留N位四舍五入

Math.Round(0.55555,2)

4,保留N位四舍五入
        double dbdata = 0.55555;
        string str1 = dbdata.ToString("f2");//fN 保留N位，四舍五入

5.保留N位四舍五入

        string result = String.Format("{0:N2}", 0.55555);//2位

string result = String.Format("{0:N3}", 0.55555);//3位

6.     保留N位四舍五入   (不错)

double s=0.55555;
      result=s.ToString("#0.00");//点后面几个0就保留几位

C#下如果显示保留小数位数，及百分号的解决方法:

1、用NumberFormatInfo类来解决:
        System.Globalization.NumberFormatInfo provider = new System.Globalization.NumberFormatInfo();

        provider.PercentDecimalDigits = 2;//小数点保留几位数.
        provider.PercentPositivePattern = 2;//百分号出现在何处.
        double result = (double)1 / 3;//一定要用double类型.
        Response.Write(result.ToString("P", provider));

2、用toString方法.:
public string getRate(double hcount, double task)
        {
            string rValue;
            string temp = "";

            if (task == 0)
            {
                task = 1;
            }

            double db = (hcount / task) * 100;

            if (hcount >= task)
            {
                rValue = "100%";
            }
            else
            {
                rValue = db.ToString("#0.#0") + "%";
            }
            return rValue;
        }

string str1 = String.Format("{0:N1}",56789); //result: 56,789.0
string str2 = String.Format("{0:N2}",56789); //result: 56,789.00
string str3 = String.Format("{0:N3}",56789); //result: 56,789.000
string str8 = String.Format("{0:F1}",56789); //result: 56789.0
string str9 = String.Format("{0:F2}",56789); //result: 56789.00
string str11 =(56789 / 100.0).ToString("#.##"); //result: 567.89
string str12 =(56789 / 100).ToString("#.##"); //result: 567