用 C# 写个人住房贷款计算器
现在，很多人都有个人住房贷款，或者将要有个人住房贷款。那么，就让我们用 C# 写一个计算器，用于计算个人住房贷款的还款计划表。这个计算器能够根据你给出的贷款金额、贷款期数、贷款日期、还款方式、贷款种类，计算出相应的还款计划表，如上图所示。
这样，就很容易知道每月要还多少钱，到现在为止剩余多少贷款未还，最终要付出多少贷款利息，等等。
贷款利率是由贷款种类决定的，存放在 LoanCalculator.xml 文件中：
<?xml version="1.0" encoding="utf-8" ?>
<LoanCalculator>
<option balance="13.8" months="180" date="2004-07-23" method="等本息" item="公积金" />
<items>
<item title="公积金">
<rate date="1001-01-01" low="3.6"high="4.05" />
<rate date="2005-01-01" low="3.78" high="4.23" />
<rate date="2006-01-01" low="3.96" high="4.41" />
<rate date="2007-01-01" low="4.14" high="4.59" />
<rate date="2008-01-01" low="5.04" high="5.22" />
<rate date="2009-01-01" low="3.33" high="3.87" />
</item>
<item title="商业性基准">
<rate date="1001-01-01" low="5.51"high="5.75" />
<rate date="2007-01-01" low="5.51"high="5.81" />
<rate date="2008-01-01" low="6.579" high="6.65" />
<rate date="2009-01-01" low="5.76"high="5.94" />
</item>
<item title="商业性优惠">
<rate date="1001-01-01" low="5.51"high="5.75" />
<rate date="2007-01-01" low="5.51"high="5.81" />
<rate date="2008-01-01" low="6.579" high="6.65" />
<rate date="2009-01-01" low="4.03"high="4.16" />
</item>
</items>
</LoanCalculator>你可以自行修改这个文件，以适应不同银行的贷款利率。
这个文件由 Config.cs 文件中的 Config 类读取：
using System;
using System.Xml;
using System.Drawing;
using System.Collections.Generic;
namespace Skyiv.Ben.LoanCalculator
{
sealed class Config
{
static readonly string ElmOption = "option";
static readonly string ElmItems = "items";
static readonly string AttrBalance = "balance";
static readonly string AttrMonths = "months";
static readonly string AttrDate = "date";
static readonly string AttrMethod = "method";
static readonly string AttrItem = "item";
static readonly string AttrTitle = "title";
static readonly string AttrLow = "low";
static readonly string AttrHigh = "high";
public decimal Balance { get; private set; } // 贷款金额(万元)
public int Months { get; private set; } // 贷款期数(月)
public DateTime Date { get; private set; } // 贷款日期
public bool IsEq { get; private set; } // 还款方式: true:等本息false: 等本金
public string Item { get; private set; } // 贷款种类
public string[] Items { get; private set; }// 贷款种类列表
public KeyValuePair<DateTime, PointF>[] Rates { get; private set; } // 贷款利率
KeyValuePair<DateTime, PointF>[][] ratesArray; // 各种类的“贷款利率”列表
public Config(string fileName)
{
try
{
var doc = new XmlDocument();
doc.Load(fileName);
var elm = doc.DocumentElement;
if (elm == null) throw new Exception("未能找到 <" + ElmOption + "> 元素");
Balance = GetDecimal(elm, AttrBalance);
Months = GetInt32(elm, AttrMonths);
Date = GetDateTime(elm, AttrDate);
IsEq = GetBooleanFromMethod(elm, AttrMethod);
Item = GetString(elm, AttrItem);
Items = GetItemsAndLoadRatesArray(doc);
SetRates(Item);
}
catch (Exception ex)
{
throw new Exception("读配置文件(" + fileName + ")", ex);
}
}
// 根据贷款种类设置贷款利率
public void SetRates(string key)
{
var idx = Array.IndexOf(Items, key);
if (idx < 0) throw new Exception("无此贷款种类: " + key);
Rates = ratesArray;
}
string[] GetItemsAndLoadRatesArray(XmlDocument doc)
{
var elm = doc.DocumentElement;
if (elm == null) throw new Exception("未能找到 <" + ElmItems + "> 元素");
var elms = elm.ChildNodes;
var items = new string;
ratesArray = new KeyValuePair<DateTime, PointF>[];
for (var i = 0; i < elms.Count; i++)
{
items = GetString(elms, AttrTitle);
ratesArray = GetRates(elms);
}
return items;
}
KeyValuePair<DateTime, PointF>[] GetRates(XmlNode elm)
{
var elms = elm.ChildNodes;
var rates = new KeyValuePair<DateTime, PointF>;
for (var i = 0; i < elms.Count; i++)
rates = new KeyValuePair<DateTime, PointF>(GetDateTime(elms, AttrDate),
new PointF(GetFloat(elms, AttrLow), GetFloat(elms, AttrHigh)));
return rates;
}
string GetString(XmlNode elm, string key)
{
if (elm.Attributes == null) throw new Exception("未能找到 <" + elm.Name + "> 元素的 " + key + " 属性");
return elm.Attributes.Value;
}
float GetFloat(XmlNode elm, string key)
{
float value;
if (!float.TryParse(GetString(elm, key), out value))
throw new Exception("<" + elm.Name + "> 元素的 " + key + " 属性的值必须为浮点数");
return value;
}
decimal GetDecimal(XmlNode elm, string key)
{
decimal value;
if (!decimal.TryParse(GetString(elm, key), out value))
throw new Exception("<" + elm.Name + "> 元素的 " + key + " 属性的值必须为实数");
return value;
}
int GetInt32(XmlNode elm, string key)
{
int value;
if (!int.TryParse(GetString(elm, key), out value))
throw new Exception("<" + elm.Name + "> 元素的 " + key + " 属性的值必须为整数");
return value;
}
DateTime GetDateTime(XmlNode elm, string key)
{
DateTime value;
if (!DateTime.TryParseExact(GetString(elm, key), "yyyy-MM-dd", null, System.Globalization.DateTimeStyles.None, out value))
throw new Exception("<" + elm.Name + "> 元素的 " + key + " 属性的值必须为日期值");
return value;
}
bool GetBooleanFromMethod(XmlNode elm, string key)
{
var value = GetString(elm, key);
if (value == "等本息") return true;
if (value == "等本金") return false;
throw new Exception("<" + elm.Name + "> 元素的 " + key + " 属性的值必须为“等本息”或者“等本金”");
}
}
}
而 Pub.cs 文件中的 Pub 静态类提供的 GetMessage 方法用于显示错误信息：
using System;
using System.Text;
namespace Skyiv.Ben.LoanCalculator
{
static class Pub
{
public static string GetMessage(Exception ex)
{
var sb = new StringBuilder("错误: ");
for (var e = ex; e != null; e = e.InnerException) sb.Append(e.Message + ": ");
sb.Length -= 2;
return sb.ToString();
}
}
}
接着，就是 LoanBase.cs 文件中的抽象基类 LoanBase 了：
using System;
using System.Data;
using System.Drawing;
using System.Collections.Generic;
namespace Skyiv.Ben.LoanCalculator
{
abstract class LoanBase
{
public DataTable Table { get; private set; }
public LoanBase(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
{
Table = GetTable();
Calculate(balance, months, date, rates);
}
protected virtual void Calculate(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
{
}
protected decimal GetMonthRate(DateTime date, int months, KeyValuePair<DateTime, PointF>[] rates)
{
int i;
for (i = rates.Length - 1; i >= 0; i--) if (date >= rates.Key) break;
return (decimal)(months <= 60 ? rates.Value.X : rates.Value.Y) / 100 / 12;
}
protected decimal Round(decimal dec)
{
return decimal.Round(dec, 2);
}
protected DateTime NextMonth(DateTime date, int day)
{
date = date.AddMonths(1);
return (date.Day >= day) ? date : new DateTime(date.Year, date.Month,
Math.Min(day, DateTime.DaysInMonth(date.Year, date.Month)));
}
DataTable GetTable()
{
var dt = new DataTable();
dt.Columns.Add("期数", typeof(int));
dt.Columns.Add("还款日期", typeof(DateTime));
dt.Columns.Add("本金", typeof(decimal));
dt.Columns.Add("利息", typeof(decimal));
dt.Columns.Add("还款", typeof(decimal));
dt.Columns.Add("贷款余额", typeof(decimal));
dt.Columns.Add("累计还款", typeof(decimal));
dt.Columns.Add("累计利息", typeof(decimal));
return dt;
}
}
}该类中的 Round 方法用于决定在计算时如何进行舍入，如有需要，可以修改该方法。
在该类的 GetMonthRate 方法中，根据贷款期数(months)来判断是该笔贷款是短期贷款还是中长期贷款，从而决定应该使用什么利率。
表示等本息法的 LoanEq 类是从 LoanBase 类中派生的：
using System;
using System.Drawing;
using System.Collections.Generic;
namespace Skyiv.Ben.LoanCalculator
{
// 等本息法
sealed class LoanEq : LoanBase
{
public LoanEq(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
: base(balance, months, date, rates)
{
}
protected override void Calculate(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
{
decimal baseAmount = 0, totalAmount = 0, totalInterest = 0;
decimal monthRate0 = decimal.MinValue, monthAmount = decimal.MinValue;
for (int day = date.Day, month = months; month >= 1; month--, date = NextMonth(date, day))
{
var monthRate = GetMonthRate(date, months, rates);
var interest = Round(balance * monthRate);
if (monthRate0 != monthRate) monthAmount = GetMonthAmount(balance, monthRate0 = monthRate, month);
baseAmount = monthAmount - interest;
balance -= baseAmount;
if (month == 1 && balance != 0)
{
baseAmount += balance;
interest -= balance;
balance = 0;
}
totalAmount += monthAmount;
totalInterest += interest;
Table.Rows.Add(new object[] { months - month + 1, date, baseAmount, interest, monthAmount, balance, totalAmount, totalInterest });
}
}
decimal GetMonthAmount(decimal balance, decimal monthRate, int months)
{
double tmp = Math.Pow(1 + (double)monthRate, months);
return Round((decimal)((double)balance * (double)monthRate * tmp / (tmp - 1)));
}
}
}在该类中覆盖了基类的 Calculate 虚方法，在主循环中逐月计算还款计划表。
等本息法在利率不变的情况下，每月的还款额是固定的，所以也称为“等额法”，计算公式如下：
月还款额 =贷款金额 x 月利率 x (1 + 月利率)期数(1 + 月利率)期数 - 1
这个公式在 GetMonthAmount 方法中计算。
而月还利息等于上月剩余贷款余额乘以月利率，月还本金等于月还款额减去月还利息。
然后，本月剩余贷款余额等于上月剩余贷款余额减去月还本金。
最后，由于计算时需要进行舍入处理，到最后一期还款后可能剩余的贷款余额不为零，这就需要在保持月还款额不变的情况下调整月还本金和月还利息。
表示等本金法的 LoanDesc 类也是从 LoanBase 类中派生的：
using System;
using System.Drawing;
using System.Collections.Generic;
namespace Skyiv.Ben.LoanCalculator
{
// 等本金法
class LoanDesc : LoanBase
{
public LoanDesc(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
: base(balance, months, date, rates)
{
}
protected override void Calculate(decimal balance, int months, DateTime date, KeyValuePair<DateTime, PointF>[] rates)
{
decimal baseAmount = Round(balance / months), totalAmount = 0, totalInterest = 0;
for (int day = date.Day, month = months; month >= 1; month--, date = NextMonth(date, day))
{
var monthRate = GetMonthRate(date, months, rates);
var interest = Round(balance * monthRate);
var monthAmount = baseAmount + interest;
balance -= baseAmount;
if (month == 1 && balance != 0)
{
baseAmount += balance;
monthAmount += balance;
balance = 0;
}
totalAmount += monthAmount;
totalInterest += interest;
Table.Rows.Add(new object[] { months - month + 1, date, baseAmount, interest, monthAmount, balance, totalAmount, totalInterest });
}
}
}
}在该类中同样也覆盖了基类的 Calculate 虚方法，在主循环中逐月计算还款计划表。
等本金法的月还本金是固定的，并且在调整贷款利率时也不变，等于贷款金额除以总期数。
但是，在贷款利率不变的情况下，每月还款额却是递减的，所以也称为“递减法”。
月还利息等于上月剩余贷款余额乘以月利率，月还款额等于月还本金加上月还利息。
然后，本月剩余贷款余额等于上月剩余贷款余额减去月还本金。
最后，由于计算时需要进行舍入处理，到最后一期还款后可能剩余的贷款余额不为零，这就需要在保持月还利息不变的情况下调整月还本金和月还款额。
最后，MainForm.cs 文件中的 MainForm 类如下：
using System;using System.Data;using System.Windows.Forms;namespace Skyiv.Ben.LoanCalculator{public sealed partial class MainForm : Form{ Config cfg; public MainForm() { InitializeComponent(); } private void MainForm_Load(object sender, EventArgs e) { btnCalculte.Enabled = false; try { cfg = new Config("LoanCalculator.xml"); tbxBalance.Text = cfg.Balance.ToString(); tbxMonths.Text = cfg.Months.ToString(); dtpBegin.Value = cfg.Date; rbnDesc.Checked = !(rbnEq.Checked = cfg.IsEq); lbxType.DataSource = cfg.Items; lbxType.SelectedIndex = lbxType.FindStringExact(cfg.Item); btnCalculate.Enabled = true; } catch (Exception ex) { tbxOut.Text = Pub.GetMessage(ex); } } private void lbxType_SelectedIndexChanged(object sender, EventArgs e) { cfg.SetRates(lbxType.SelectedValue.ToString()); dgvRate.Rows.Clear(); foreach (var kvp in cfg.Rates) dgvRate.Rows.Add(new object[] { kvp.Key.ToString("yyyy-MM-dd"), kvp.Value.X, kvp.Value.Y }); } private void btnCalculate_Click(object sender, EventArgs e) { btnCalculate.Enabled = false; try { tbxOut.Text = ""; var isEq = rbnEq.Checked; var date = dtpBegin.Value; int months; decimal balance; if (!int.TryParse(tbxMonths.Text, out months) || months
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