所有其他答案都没有考虑到Span的性能，它比。net中字符串类型的Substring的性能更好

如果你还不知道有一个版本的系统。内存(为以下情况提供了Span, ReadOnlySpan, Memory和ReadOnlyMemory:

这样的简单实现可以如下所示:

public static string Truncate(this string value, int maxLength)
{
    if (!string.IsNullOrEmpty(value) && value.Length > maxLength)
    {
        return value.AsSpan(0, maxLength).ToString(); /* Note the use of AsSpan instead of Substring. */
    }

    return value;
}

该方法理论上可以返回Span<char>，以避免使用Span<T>的ToString()成员分配新字符串。

The BCL itself internally uses Span's, ReadOnlySpan's, Memory's, and ReadOnlyMemory's where possible to avoid issues and to help optimize the code, especially when you compile arrays that are known at compile time and so using an property that returns that new'd up array as an ReadOnlySpan<byte> actually optimizes the code at runtime as then the JIT would not call memcpy on the data and instead uses it since it just returns a Span and as such is a window to the data that is already allocated ahead of time resulting in:

更少的分配。 更少的分配时间。 使代码总体上更快地使用。

你可以用LINQ…它消除了检查字符串长度的需要。不可否认，这可能不是最有效的，但很有趣。

string result = string.Join("", value.Take(maxLength)); // .NET 4 Join

or

string result = new string(value.Take(maxLength).ToArray());

另一个解决方案:

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

或者，您可以使用Math.min代替三元操作符

public static class StringExt
{
    public static string Truncate( this string value, int maxLength )
    {
        if (string.IsNullOrEmpty(value)) { return value; }

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

截断字符串

public static string TruncateText(string strText, int intLength)
{
    if (!(string.IsNullOrEmpty(strText)))
    {                                
        // split the text.
        var words = strText.Split(' ');

        // calculate the number of words
        // based on the provided characters length 
        // use an average of 7.6 chars per word.
        int wordLength = Convert.ToInt32(Math.Ceiling(intLength / 7.6));

        // if the text is shorter than the length,
        // display the text without changing it.
        if (words.Length <= wordLength)
            return strText.Trim();                

        // put together a shorter text
        // based on the number of words
        return string.Join(" ", words.Take(wordLength)) + " ...".Trim();
    }
        else
        {
            return "";
        }            
    }

在。net 4.0中，你可以使用Take方法:

string.Concat(myString.Take(maxLength));

没有测试效率!