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[ElmOption];
        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[idx];
    }

    string[] GetItemsAndLoadRatesArray(XmlDocument doc)
    {
      var elm = doc.DocumentElement[ElmItems];
      if (elm == null) throw new Exception("未能找到 <" + ElmItems + "> 元素");
      var elms = elm.ChildNodes;
      var items = new string[elms.Count];
      ratesArray = new KeyValuePair<DateTime, PointF>[elms.Count][];
      for (var i = 0; i < elms.Count; i++)
      {
        items[i] = GetString(elms[i], AttrTitle);
        ratesArray[i] = GetRates(elms[i]);
      }
      return items;
    }

    KeyValuePair<DateTime, PointF>[] GetRates(XmlNode elm)
    {
      var elms = elm.ChildNodes;
      var rates = new KeyValuePair<DateTime, PointF>[elms.Count];
      for (var i = 0; i < elms.Count; i++)
        rates[i] = new KeyValuePair<DateTime, PointF>(GetDateTime(elms[i], AttrDate),
          new PointF(GetFloat(elms[i], AttrLow), GetFloat(elms[i], AttrHigh)));
      return rates;
    }

    string GetString(XmlNode elm, string key)
    {
      if (elm.Attributes[key] == null) throw new Exception("未能找到 <" + elm.Name + "> 元素的 " + key + " 属性");
      return elm.Attributes[key].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[i].Key) break;
      return (decimal)(months <= 60 ? rates[i].Value.X : rates[i].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 <= 0) throw new Exception("贷款期数必须是正整数");
        if (!decimal.TryParse(tbxBalance.Text, out balance) || balance <= 0) throw new Exception("贷款金额必须是正数");
        balance *= 10000; // 贷款金额单位是万元
        var loan = isEq ? (new LoanEq(balance, months, date, cfg.Rates) as LoanBase) :
          (new LoanDesc(balance, months, date, cfg.Rates) as LoanBase);
        dgvOut.Rows.Clear();
        foreach (DataRow row in loan.Table.Rows) dgvOut.Rows.Add(row.ItemArray);
      }
      catch (Exception ex)
      {
        tbxOut.Text = Pub.GetMessage(ex);
      }
      btnCalculate.Enabled = true;
    }
  }
}

当用户改变贷款种类时，调用该类的 lbxType_SelectedIndexChanged 方法来相应改变贷款利率。

当用户点击“计算”按钮时，就调用该类的 btnCalculate_Click 方法来计算还款计划表。

 

个人住房贷款计算器以及全部的源程序，可以在这里下载。