Web编程 订阅所有【Web编程】的日志

C#格式化输出再讨论

编辑:dnawo 日期:2009-05-08

字体大小: 小 中 大
先来定义一个类MyFormat，它实现了ICustomFormatter和IFormatProvider接口，作用是将字符串转化为大写：

复制内容到剪贴板程序代码 程序代码
public class MyFormat : ICustomFormatter, IFormatProvider
{
    /// <summary>
    /// 实现IFormatProvider的GetFormat成员
    /// </summary>
    /// <param name="formatType"></param>
    /// <returns></returns>
    public object GetFormat(Type formatType)
    {
        if (typeof(ICustomFormatter) == formatType)
        {
            return this;
        }
        return null;
    }

    /// <summary>
    /// 实现ICustomFormatter的Format成员
    /// </summary>
    /// <param name="format"></param>
    /// <param name="arg"></param>
    /// <param name="formatProvider"></param>
    /// <returns></returns>
    public string Format(string format, object arg, IFormatProvider formatProvider)
    {
        if (arg == null)
        {
            throw new ArgumentException("The object should not be null");
        }

        return arg.ToString().ToUpper();
    }
}

使用下看：

复制内容到剪贴板程序代码 程序代码
Console.WriteLine(string.Format(new MyFormat(), "{0}", "abcd")); //ABCD
Console.WriteLine("abcd".ToString(new MyFormat())); //abcd

为什么下边一句没有发生作用？我们用Reflector来查看下String类的ToString(IFormatProvider provider)和Format(IFormatProvider provider, string format, params object[] args)两个方法代码：

String.ToString(IFormatProvider provider)代码：
复制内容到剪贴板程序代码 程序代码
public string ToString(IFormatProvider provider)
{
    return this;
}

String.Format(IFormatProvider provider, string format, params object[] args)代码：
复制内容到剪贴板程序代码 程序代码
public static string Format(IFormatProvider provider, string format, params object[] args)
{
    if ((format == null) || (args == null))
    {
        throw new ArgumentNullException((format == null) ? "format" : "args");
    }
    StringBuilder builder = new StringBuilder(format.Length + (args.Length * 8));
    builder.AppendFormat(provider, format, args);
    return builder.ToString();
}

StringBuilder.AppendFormat(IFormatProvider provider, string format, params object[] args)代码：
复制内容到剪贴板程序代码 程序代码
public StringBuilder AppendFormat(IFormatProvider provider, string format, params object[] args)
{
    int num3;
    if ((format == null) || (args == null))
    {
        throw new ArgumentNullException((format == null) ? "format" : "args");
    }
    char[] chArray = format.ToCharArray(0, format.Length);
    int index = 0;
    int length = chArray.Length;
    char ch = '\0';
    ICustomFormatter formatter = null;
    if (provider != null)
    {
        formatter = (ICustomFormatter) provider.GetFormat(typeof(ICustomFormatter));
    }
Label_004E:
    num3 = index;
    int num4 = index;
    while (index < length)
    {
        ch = chArray[index];
        index++;
        if (ch == '}')
        {
            if ((index < length) && (chArray[index] == '}'))
            {
                index++;
            }
            else
            {
                FormatError();
            }
        }
        if (ch == '{')
        {
            if ((index < length) && (chArray[index] == '{'))
            {
                index++;
            }
            else
            {
                index--;
                break;
            }
        }
        chArray[num4++] = ch;
    }
    if (num4 > num3)
    {
        this.Append(chArray, num3, num4 - num3);
    }
    if (index == length)
    {
        return this;
    }
    index++;
    if (((index == length) || ((ch = chArray[index]) < '0')) || (ch > '9'))
    {
        FormatError();
    }
    int num5 = 0;
    do
    {
        num5 = ((num5 * 10) + ch) - 0x30;
        index++;
        if (index == length)
        {
            FormatError();
        }
        ch = chArray[index];
    }
    while (((ch >= '0') && (ch <= '9')) && (num5 < 0xf4240));
    if (num5 >= args.Length)
    {
        throw new FormatException(Environment.GetResourceString("Format_IndexOutOfRange"));
    }
    while ((index < length) && ((ch = chArray[index]) == ' '))
    {
        index++;
    }
    bool flag = false;
    int num6 = 0;
    if (ch == ',')
    {
        index++;
        while ((index < length) && (chArray[index] == ' '))
        {
            index++;
        }
        if (index == length)
        {
            FormatError();
        }
        ch = chArray[index];
        if (ch == '-')
        {
            flag = true;
            index++;
            if (index == length)
            {
                FormatError();
            }
            ch = chArray[index];
        }
        if ((ch < '0') || (ch > '9'))
        {
            FormatError();
        }
        do
        {
            num6 = ((num6 * 10) + ch) - 0x30;
            index++;
            if (index == length)
            {
                FormatError();
            }
            ch = chArray[index];
        }
        while (((ch >= '0') && (ch <= '9')) && (num6 < 0xf4240));
    }
    while ((index < length) && ((ch = chArray[index]) == ' '))
    {
        index++;
    }
    object arg = args[num5];
    string str = null;
    if (ch == ':')
    {
        index++;
        num3 = index;
        num4 = index;
        while (true)
        {
            if (index == length)
            {
                FormatError();
            }
            ch = chArray[index];
            index++;
            switch (ch)
            {
                case '{':
                    if ((index < length) && (chArray[index] == '{'))
                    {
                        index++;
                    }
                    else
                    {
                        FormatError();
                    }
                    break;

                case '}':
                    if ((index < length) && (chArray[index] == '}'))
                    {
                        index++;
                    }
                    else
                    {
                        index--;
                        if (num4 > num3)
                        {
                            str = new string(chArray, num3, num4 - num3);
                        }
                        goto Label_0253;
                    }
                    break;
            }
            chArray[num4++] = ch;
        }
    }
Label_0253:
    if (ch != '}')
    {
        FormatError();
    }
    index++;
    string str2 = null;
    if (formatter != null)
    {
        str2 = formatter.Format(str, arg, provider);
    }
    if (str2 == null)
    {
        if (arg is IFormattable)
        {
            str2 = ((IFormattable) arg).ToString(str, provider);
        }
        else if (arg != null)
        {
            str2 = arg.ToString();
        }
    }
    if (str2 == null)
    {
        str2 = string.Empty;
    }
    int repeatCount = num6 - str2.Length;
    if (!flag && (repeatCount > 0))
    {
        this.Append(' ', repeatCount);
    }
    this.Append(str2);
    if (flag && (repeatCount > 0))
    {
        this.Append(' ', repeatCount);
    }
    goto Label_004E;
}

注意红色部分，正是它保证了我们的自定义类MyFormat可以正常使用！总结一下，当且仅当一个类的格式化方法中有IFormatProvider类型的参数，并且存在上边红色部分代码，我们的自定义格式化类才能发挥作用！

上一篇: C#格式化输出
下一篇: 关于接口多态性一点说明
文章来自: 本站原创
引用通告: 查看所有引用 | 我要引用此文章
Tags:
最新日志:
评论: 0 | 引用: 0 | 查看次数: 4304
发表评论
请登录后再发表评论！