12 345 45
2342 234 45 2 2 45345
234 546 34 3 45 65 765
12 23 434 34 56 76 5

{12,345,45}
{2342,234,45,2,45345}
{234,546,34,3,65,765}
{12,23,434,56,76,5}

解决方法@H_403_12@

添加一些LINQ来获取乐趣和利润(优化空间))：

System.IO.File.WriteAllLines(
    "outfilename.txt",System.IO.File.ReadAllLines("infilename.txt").Select(line =>
        "{" +
        string.Join(",",line.Split(new string[] { " " },StringSplitOptions.RemoveEmptyEntries)
        ) + "}"
    ).ToArray()
);