Copy string.Fromat

从.net Framework中copy出来的，修改了一下

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            MessageBox.Show(Format(null, "ss{0}ww{0}ee{1}", "AAA", "BBB"));

        }

        private static void FormatError()
        {
            throw new FormatException("参数格式错误"); 
        }

        public static string Format(IFormatProvider provider, String format, params Object[] args)
        {
            if (format == null || args == null)
            {
                throw new ArgumentNullException((format == null) ? "format" : "args");
            }
            StringBuilder sb = new StringBuilder();
            char[] chars = format.ToCharArray(0, format.Length);
            int pos = 0;
            int len = chars.Length;
            char ch = '\x0';

            ICustomFormatter cf = null;
            if (provider != null)
            {
                cf = (ICustomFormatter)provider.GetFormat(typeof(ICustomFormatter));
            }

            while (true)
            {
                int p = pos;
                int i = pos;
                while (pos < len)
                {
                    ch = chars[pos];

                    pos++;
                    if (ch == '}')
                    {
                        if (pos < len && chars[pos] == '}') // Treat as escape character for }} 
                            pos++;
                        else
                            FormatError();
                    }

                    if (ch == '{')
                    {
                        if (pos < len && chars[pos] == '{') // Treat as escape character for {{
                            pos++;
                        else
                        {
                            pos--;
                            break;
                        }
                    }

                    chars[i++] = ch;
                }
                if (i > p) sb.Append(chars, p, i - p);
                if (pos == len) break;
                pos++;
                if (pos == len || (ch = chars[pos]) < '0' || ch > '9') FormatError();
                int index = 0;
                do
                {
                    index = index * 10 + ch - '0';
                    pos++;
                    if (pos == len) FormatError();
                    ch = chars[pos];
                } while (ch >= '0' && ch <= '9' && index < 1000000);
                if (index >= args.Length) throw new FormatException("索引(从零开始)必须大于或等于零，且小于参数列表的大小。");
                while (pos < len && (ch = chars[pos]) == ' ') pos++;
                bool leftJustify = false;
                int width = 0;
                if (ch == ',')
                {
                    pos++;
                    while (pos < len && chars[pos] == ' ') pos++;

                    if (pos == len) FormatError();
                    ch = chars[pos];
                    if (ch == '-')
                    {
                        leftJustify = true;
                        pos++;
                        if (pos == len) FormatError();
                        ch = chars[pos];
                    }
                    if (ch < '0' || ch > '9') FormatError();
                    do
                    {
                        width = width * 10 + ch - '0';
                        pos++;
                        if (pos == len) FormatError();
                        ch = chars[pos];
                    } while (ch >= '0' && ch <= '9' && width < 1000000);
                }

                while (pos < len && (ch = chars[pos]) == ' ') pos++;
                Object arg = args[index];
                String fmt = null;
                if (ch == ':')
                {
                    pos++;
                    p = pos;
                    i = pos;
                    while (true)
                    {
                        if (pos == len) FormatError();
                        ch = chars[pos];
                        pos++;
                        if (ch == '{')
                        {
                            if (pos < len && chars[pos] == '{')  // Treat as escape character for {{
                                pos++;
                            else
                                FormatError();
                        }
                        else if (ch == '}')
                        {
                            if (pos < len && chars[pos] == '}')  // Treat as escape character for }} 
                                pos++;
                            else
                            {
                                pos--;
                                break;
                            }
                        }

                        chars[i++] = ch;
                    }
                    if (i > p) fmt = new String(chars, p, i - p);
                }
                if (ch != '}') FormatError();
                pos++;
                String s = null;
                if (cf != null)
                {
                    s = cf.Format(fmt, arg, provider);
                }

                if (s == null)
                {
                    if (arg is IFormattable)
                    {
                        s = ((IFormattable)arg).ToString(fmt, provider);
                    }
                    else if (arg != null)
                    {
                        s = arg.ToString();
                    }
                }

                if (s == null) s = String.Empty;
                int pad = width - s.Length;
                if (!leftJustify && pad > 0) sb.Append(' ', pad);
                sb.Append(s);
                if (leftJustify && pad > 0) sb.Append(' ', pad);
            }
            return sb.ToString();
        }

    }