标题: C#格式化输出再讨论
作者: dnawo
日期: 2009-05-08 06:21 PM
分类: Web编程
内容: 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！
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