string text = "1234567";
var x = text.Substring(0, text.Length>5?6:text.Length);

substring(int startpos, int lenght);

string truncatedToNLength = new string(s.Take(n).ToArray());  

这个解决方案有一个小小的好处，如果n大于s。length，它仍然做正确的事情。

如果我们还在讨论验证，为什么我们没有检查空字符串条目。有什么具体原因吗?

我认为下面的方式帮助，因为IsNullOrEmpty是一个系统定义的方法，三元操作符的圈复杂度= 1，而if() {} else{}的值为2。

    public static string Truncate(string input, int truncLength)
    {
        return (!String.IsNullOrEmpty(input) && input.Length >= truncLength)
                   ? input.Substring(0, truncLength)
                   : input;
    }

string.Substring (0, n);// 0 -起始索引，n -字符数

The .NET Substring method is fraught with peril. I developed extension methods that handle a wide variety of scenarios. The nice thing is it preserves the original behavior, but when you add an additional "true" parameter, it then resorts to the extension method to handle the exception, and returns the most logical values, based on the index and length. For example, if length is negative, and counts backward. You can look at the test results with wide variety of values on the fiddle at: https://dotnetfiddle.net/m1mSH9. This will give you a clear idea on how it resolves substrings.

我总是将这些方法添加到我所有的项目中，并且从来不必担心代码破坏，因为某些东西发生了变化，索引无效。下面是代码。

    public static String Substring(this String val, int startIndex, bool handleIndexException)
    {
        if (!handleIndexException)
        { //handleIndexException is false so call the base method
            return val.Substring(startIndex);
        }
        if (string.IsNullOrEmpty(val))
        {
            return val;
        }
        return val.Substring(startIndex < 0 ? 0 : startIndex > (val.Length - 1) ? val.Length : startIndex);
    }

    public static String Substring(this String val, int startIndex, int length, bool handleIndexException)
    {
        if (!handleIndexException)
        { //handleIndexException is false so call the base method
            return val.Substring(startIndex, length);
        }
        if (string.IsNullOrEmpty(val))
        {
            return val;
        }
        int newfrom, newlth, instrlength = val.Length;
        if (length < 0) //length is negative
        {
            newfrom = startIndex + length;
            newlth = -1 * length;
        }
        else //length is positive
        {
            newfrom = startIndex;
            newlth = length;
        }
        if (newfrom + newlth < 0 || newfrom > instrlength - 1)
        {
            return string.Empty;
        }
        if (newfrom < 0)
        {
            newlth = newfrom + newlth;
            newfrom = 0;
        }
        return val.Substring(newfrom, Math.Min(newlth, instrlength - newfrom));
    }

早在2010年5月，我就在http://jagdale.blogspot.com/2010/05/substring-extension-method-that-does.html上写了一篇博文