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;
    }

public static string TruncateLongString(this string str, int maxLength)
{
    if (string.IsNullOrEmpty(str)) return str;

    return str.Substring(0, Math.Min(str.Length, maxLength));
}

在c# 8或更高版本中，也可以使用Range使其更简洁:

public static string TruncateLongString(this string str, int maxLength)
{
    return str?[0..Math.Min(str.Length, maxLength)];
}

可以使用表达式体进一步简化:

public static string TruncateLongString(this string str, int maxLength) =>
    str?[0..Math.Min(str.Length, maxLength)];

注意空条件操作符(?)用于处理str为空的情况。这取代了显式空检查的需要。

简单:

public static String Truncate(String input,int maxLength)
{
   if(input.Length > maxLength)
      return input.Substring(0,maxLength);
   return input;
}

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

substring(int startpos, int lenght);

你可以使用LINQ str.Take(n)或str.SubString(0, n)，后者将为n > str.Length抛出ArgumentOutOfRangeException异常。

注意，LINQ版本返回一个IEnumerable<char>，所以你必须将IEnumerable<char>转换为string: new string(s.t take (n). toarray())。