问题
I would like to truncate a string such that its length is not longer than a given value. I am writing to a database table and want to ensure that the values I write meet the constraint of the column\'s datatype.
For instance, it would be nice if I could write the following:
string NormalizeLength(string value, int maxLength)
{
return value.Substring(0, maxLength);
}
Unfortunately, this raises an exception because maxLength generally exceeds the boundaries of the string value. Of course, I could write a function like the following, but I was hoping that something like this already exists.
string NormalizeLength(string value, int maxLength)
{
return value.Length <= maxLength ? value : value.Substring(0, maxLength);
}
Where is the elusive API that performs this task? Is there one?
回答1:
There isn't a Truncate() method on string, unfortunately. You have to write this kind of logic yourself. What you can do, however, is wrap this in an extension method so you don't have to duplicate it everywhere:
public static class StringExt
{
public static string Truncate(this string value, int maxLength)
{
if (string.IsNullOrEmpty(value)) return value;
return value.Length <= maxLength ? value : value.Substring(0, maxLength);
}
}
Now we can write:
var someString = "...";
someString = someString.Truncate(2);
回答2:
Or instead of the ternary operator, you could use Math.min
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));
}
}
回答3:
I figured I would throw in my implementation since I believe it covers all of the cases that have been touched on by the others and does so in a concise way that is still readable.
public static string Truncate(this string value, int maxLength)
{
if (!string.IsNullOrEmpty(value) && value.Length > maxLength)
{
return value.Substring(0, maxLength);
}
return value;
}
This solution mainly builds upon the Ray's solution and opens up the method for use as an extension method by using the this keyword just as LBushkin does in his solution.
回答4:
In .NET 4.0 you can use the Take method:
string.Concat(myString.Take(maxLength));
Not tested for efficiency!
回答5:
Because performance testing is fun: (using linqpad extension methods)
var val = string.Concat(Enumerable.Range(0, 50).Select(i => i % 10));
foreach(var limit in new[] { 10, 25, 44, 64 })
new Perf<string> {
{ "newstring" + limit, n => new string(val.Take(limit).ToArray()) },
{ "concat" + limit, n => string.Concat(val.Take(limit)) },
{ "truncate" + limit, n => val.Substring(0, Math.Min(val.Length, limit)) },
{ "smart-trunc" + limit, n => val.Length <= limit ? val : val.Substring(0, limit) },
{ "stringbuilder" + limit, n => new StringBuilder(val, 0, Math.Min(val.Length, limit), limit).ToString() },
}.Vs();
The truncate method was "significantly" faster. #microoptimization
Early
- truncate10 5788 ticks elapsed (0.5788 ms) [in 10K reps, 5.788E-05 ms per]
- smart-trunc10 8206 ticks elapsed (0.8206 ms) [in 10K reps, 8.206E-05 ms per]
- stringbuilder10 10557 ticks elapsed (1.0557 ms) [in 10K reps, 0.00010557 ms per]
- concat10 45495 ticks elapsed (4.5495 ms) [in 10K reps, 0.00045495 ms per]
- newstring10 72535 ticks elapsed (7.2535 ms) [in 10K reps, 0.00072535 ms per]
Late
- truncate44 8835 ticks elapsed (0.8835 ms) [in 10K reps, 8.835E-05 ms per]
- stringbuilder44 13106 ticks elapsed (1.3106 ms) [in 10K reps, 0.00013106 ms per]
- smart-trunc44 14821 ticks elapsed (1.4821 ms) [in 10K reps, 0.00014821 ms per]
- newstring44 144324 ticks elapsed (14.4324 ms) [in 10K reps, 0.00144324 ms per]
- concat44 174610 ticks elapsed (17.461 ms) [in 10K reps, 0.0017461 ms per]
Too Long
- smart-trunc64 6944 ticks elapsed (0.6944 ms) [in 10K reps, 6.944E-05 ms per]
- truncate64 7686 ticks elapsed (0.7686 ms) [in 10K reps, 7.686E-05 ms per]
- stringbuilder64 13314 ticks elapsed (1.3314 ms) [in 10K reps, 0.00013314 ms per]
- newstring64 177481 ticks elapsed (17.7481 ms) [in 10K reps, 0.00177481 ms per]
- concat64 241601 ticks elapsed (24.1601 ms) [in 10K reps, 0.00241601 ms per]
回答6:
You could use LINQ... it eliminates the need to check string length. Admittedly maybe not the most efficient, but it's fun.
string result = string.Join("", value.Take(maxLength)); // .NET 4 Join
or
string result = new string(value.Take(maxLength).ToArray());
回答7:
Seems no one has posted this yet:
public static class StringExt
{
public static string Truncate(this string s, int maxLength)
{
return s != null && s.Length > maxLength ? s.Substring(0, maxLength) : s;
}
}
Using the && operator makes it marginally better than the accepted answer.
回答8:
I did mine in one line sort of like this
value = value.Length > 1000 ? value.Substring(0, 1000) : value;
回答9:
The .NET Framework has an API to truncate a string like this:
Microsoft.VisualBasic.Strings.Left(string, int);
But in a C# app you'll probably prefer to roll your own than taking a dependency on Microsoft.VisualBasic.dll, whose main raison d'etre is backwards compatibility.
回答10:
Another solution:
return input.Substring(0, Math.Min(input.Length, maxLength));
回答11:
I know this is an old question, but here is a nice solution:
public static string Truncate(this string text, int maxLength, string suffix = "...")
{
string str = text;
if (maxLength > 0)
{
int length = maxLength - suffix.Length;
if (length <= 0)
{
return str;
}
if ((text != null) && (text.Length > maxLength))
{
return (text.Substring(0, length).TrimEnd(new char[0]) + suffix);
}
}
return str;
}
var myString = "hello world"
var myTruncatedString = myString.Truncate(4);
Returns: hello...
回答12:
A similar variant with C# 6's Null propagation operator
public static string Truncate(this string value, int maxLength)
{
return value?.Length <= maxLength ? value : value?.Substring(0, maxLength);
}
Please note, we are essentially checking if value is null twice here.
回答13:
Still no Truncate method in 2016 for C# strings. But - Using C# 6.0 Syntax:
public static class StringExtension
{
public static string Truncate(this string s, int max)
{
return s?.Length > max ? s.Substring(0, max) : s ?? throw new ArgumentNullException(s);
}
}
It works like a charm:
"Truncate me".Truncate(8);
Result: "Truncate"
回答14:
Taking @CaffGeek and simplifying it:
public static string Truncate(this string value, int maxLength)
{
return string.IsNullOrEmpty(value) ? value : value.Substring(0, Math.Min(value.Length, maxLength));
}
回答15:
Kndly note that truncating a string not merely means justing cutting a string at a specified length alone but have to take care not to split the word.
eg string : this is a test string.
I want to cut it at 11 . If we use any of the method given above the result will be
this is a te
This is not the thing we want
The method i am using may also not so perfect but it can handle most of the situation
public string CutString(string source, int length)
{
if (source== null || source.Length < length)
{
return source;
}
int nextSpace = source.LastIndexOf(" ", length);
return string.Format("{0}...", input.Substring(0, (nextSpace > 0) ? nextSpace : length).Trim());
}
回答16:
Why not:
string NormalizeLength(string value, int maxLength)
{
//check String.IsNullOrEmpty(value) and act on it.
return value.PadRight(maxLength).Substring(0, maxLength);
}
i.e. in the event value.Length < maxLength pad spaces to the end or truncate the excess.
回答17:
Just in case there's not enough answers here, here's mine :)
public static string Truncate(this string str,
int totalLength,
string truncationIndicator = "")
{
if (string.IsNullOrEmpty(str) || str.Length < totalLength)
return str;
return str.Substring(0, totalLength - truncationIndicator.Length)
+ truncationIndicator;
}
to use:
"I use it like this".Truncate(5,"~")
回答18:
For the sake of (over)complexity I'll add my overloaded version which replaces the last 3 characters with an ellipsis in respect with the maxLength parameter.
public static string Truncate(this string value, int maxLength, bool replaceTruncatedCharWithEllipsis = false)
{
if (replaceTruncatedCharWithEllipsis && maxLength <= 3)
throw new ArgumentOutOfRangeException("maxLength",
"maxLength should be greater than three when replacing with an ellipsis.");
if (String.IsNullOrWhiteSpace(value))
return String.Empty;
if (replaceTruncatedCharWithEllipsis &&
value.Length > maxLength)
{
return value.Substring(0, maxLength - 3) + "...";
}
return value.Substring(0, Math.Min(value.Length, maxLength));
}
回答19:
I prefer jpierson's answer, but none of the examples here that I can see are handling an invalid maxLength parameter, such as when maxLength < 0.
Choices would be either handle the error in a try/catch, clamp the maxLength parameter min to 0, or if maxLength is less than 0 return an empty string.
Not optimized code:
public string Truncate(this string value, int maximumLength)
{
if (string.IsNullOrEmpty(value) == true) { return value; }
if (maximumLen < 0) { return String.Empty; }
if (value.Length > maximumLength) { return value.Substring(0, maximumLength); }
return value;
}
回答20:
Here is a vb.net solution, mark that the if (although ugly) statement improves performance because we do not need the substring statement when string is already smaller than maxlength... By making it an extention to string it is easy to use...
<System.Runtime.CompilerServices.Extension()> _
Public Function Truncate(String__1 As String, maxlength As Integer) As String
If Not String.IsNullOrEmpty(String__1) AndAlso String__1.Length > maxlength Then
Return String__1.Substring(0, maxlength)
Else
Return String__1
End If
End Function
回答21:
I know there are a ton of answers already but my need was to keep the beginning and end of the string intact but shorten it to under the max length.
public static string TruncateMiddle(string source)
{
if (String.IsNullOrWhiteSpace(source) || source.Length < 260)
return source;
return string.Format("{0}...{1}",
source.Substring(0, 235),
source.Substring(source.Length - 20));
}
This is for creating SharePoint URLs that have a max length of 260 characters.
I didn't make length a parameter since it is a constant 260. I also didn't make the first substring length a parameter because I want it to break at a specific point. Finally, the second substring is the length of the source - 20 since I know the folder structure.
This could easily be adapted to your specific needs.
回答22:
I know there are a ton of answers here already, but this is the one I have gone with, which handles both null strings and the situation where the length passed in is negative:
public static string Truncate(this string s, int length)
{
return string.IsNullOrEmpty(s) || s.Length <= length ? s
: length <= 0 ? string.Empty
: s.Substring(0, length);
}
回答23:
My two cents with example length of 30 :
var truncatedInput = string.IsNullOrEmpty(input) ?
string.Empty :
input.Substring(0, Math.Min(input.Length, 30));
回答24:
TruncateString
public static string _TruncateString(string input, int charaterlimit)
{
int characterLimit = charaterlimit;
string output = input;
// Check if the string is longer than the allowed amount
// otherwise do nothing
if (output.Length > characterLimit && characterLimit > 0)
{
// cut the string down to the maximum number of characters
output = output.Substring(0, characterLimit);
// Check if the character right after the truncate point was a space
// if not, we are in the middle of a word and need to remove the rest of it
if (input.Substring(output.Length, 1) != " ")
{
int LastSpace = output.LastIndexOf(" ");
// if we found a space then, cut back to that space
if (LastSpace != -1)
{
output = output.Substring(0, LastSpace);
}
}
// Finally, add the "..."
output += "...";
}
return output;
}
回答25:
As an addition to the possibilities discussed above I'd like to share my solution. It's an extension method that allows null (returns string.Empty) also there is a second .Truncate() for using it with an ellipsis. Beware, it's not performance optimized.
public static string Truncate(this string value, int maxLength) =>
(value ?? string.Empty).Substring(0, (value?.Length ?? 0) <= (maxLength < 0 ? 0 : maxLength) ? (value?.Length ?? 0) : (maxLength < 0 ? 0 : maxLength));
public static string Truncate(this string value, int maxLength, string ellipsis) =>
string.Concat(value.Truncate(maxLength - (((value?.Length ?? 0) > maxLength ? ellipsis : null)?.Length ?? 0)), ((value?.Length ?? 0) > maxLength ? ellipsis : null)).Truncate(maxLength);
回答26:
There is nothing in .net for this that I am aware of - here is my version which adds "...":
public static string truncateString(string originalString, int length) {
if (string.IsNullOrEmpty(originalString)) {
return originalString;
}
if (originalString.Length > length) {
return originalString.Substring(0, length) + "...";
}
else {
return originalString;
}
}
回答27:
public static string Truncate( this string value, int maxLength )
{
if (string.IsNullOrEmpty(value)) { return value; }
return new string(value.Take(maxLength).ToArray());// use LINQ and be happy
}
回答28:
Truncate String
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 "";
}
}
回答29:
I'd Recommend using the substring method for the same effective functionality.
// Gets first n characters.
string subString = inputString.Substring(0, n);
This has the benefit of letting you splice your string from either side or even somewhere in the middle without writing additional methods. Hope that helps :)
For additional reference: https://www.dotnetperls.com/substring
回答30:
This is the code I usually use:
string getSubString(string value, int index, int length)
{
if (string.IsNullOrEmpty(value) || value.Length <= length)
{
return value;
}
System.Text.StringBuilder sb = new System.Text.StringBuilder();
for (int i = index; i < length; i++)
{
sb.AppendLine(value[i].ToString());
}
return sb.ToString();
}
来源：https://stackoverflow.com/questions/2776673/how-do-i-truncate-a-net-string