int array to string - c#

In C#, I have an array of ints, containing digits only. I want to convert this array to string.
Array example:
int[] arr = {0,1,2,3,0,1};
How can I convert this to a string formatted as: "012301"?


at.net 3.5 use:
String.Join("", new List<int>(array).ConvertAll(i => i.ToString()).ToArray());
at.net 4.0 or above use: (see #Jan Remunda's answer)
string result = string.Join("", array);


You can simply use String.Join function, and as separator use string.Empty because it uses StringBuilder internally.
string result = string.Join(string.Empty, new []{0,1,2,3,0,1});
E.g.: If you use semicolon as separator, the result would be 0;1;2;3;0;1.
It actually works with null separator, and second parameter can be enumerable of any objects, like:
string result = string.Join(null, new object[]{0,1,2,3,0,"A",DateTime.Now});


I realize my opinion is probably not the popular one, but I guess I have a hard time jumping on the Linq-y band wagon. It's nifty. It's condensed. I get that and I'm not opposed to using it where it's appropriate. Maybe it's just me, but I feel like people have stopped thinking about creating utility functions to accomplish what they want and instead prefer to litter their code with (sometimes) excessively long lines of Linq code for the sake of creating a dense 1-liner.
I'm not saying that any of the Linq answers that people have provided here are bad, but I guess I feel like there is the potential that these single lines of code can start to grow longer and more obscure as you need to handle various situations. What if your array is null? What if you want a delimited string instead of just purely concatenated? What if some of the integers in your array are double-digit and you want to pad each value with leading zeros so that the string for each element is the same length as the rest?
Taking one of the provided answers as an example:
result = arr.Aggregate(string.Empty, (s, i) => s + i.ToString());
If I need to worry about the array being null, now it becomes this:
result = (arr == null) ? null : arr.Aggregate(string.Empty, (s, i) => s + i.ToString());
If I want a comma-delimited string, now it becomes this:
result = (arr == null) ? null : arr.Skip(1).Aggregate(arr[0].ToString(), (s, i) => s + "," + i.ToString());
This is still not too bad, but I think it's not obvious at a glance what this line of code is doing.
Of course, there's nothing stopping you from throwing this line of code into your own utility function so that you don't have that long mess mixed in with your application logic, especially if you're doing it in multiple places:
public static string ToStringLinqy<T>(this T[] array, string delimiter)
{
// edit: let's replace this with a "better" version using a StringBuilder
//return (array == null) ? null : (array.Length == 0) ? string.Empty : array.Skip(1).Aggregate(array[0].ToString(), (s, i) => s + "," + i.ToString());
return (array == null) ? null : (array.Length == 0) ? string.Empty : array.Skip(1).Aggregate(new StringBuilder(array[0].ToString()), (s, i) => s.Append(delimiter).Append(i), s => s.ToString());
}
But if you're going to put it into a utility function anyway, do you really need it to be condensed down into a 1-liner? In that case why not throw in a few extra lines for clarity and take advantage of a StringBuilder so that you're not doing repeated concatenation operations:
public static string ToStringNonLinqy<T>(this T[] array, string delimiter)
{
if (array != null)
{
// edit: replaced my previous implementation to use StringBuilder
if (array.Length > 0)
{
StringBuilder builder = new StringBuilder();
builder.Append(array[0]);
for (int i = 1; i < array.Length; i++)
{
builder.Append(delimiter);
builder.Append(array[i]);
}
return builder.ToString()
}
else
{
return string.Empty;
}
}
else
{
return null;
}
}
And if you're really so concerned about performance, you could even turn it into a hybrid function that decides whether to do string.Join or to use a StringBuilder depending on how many elements are in the array (this is a micro-optimization, not worth doing in my opinion and possibly more harmful than beneficial, but I'm using it as an example for this problem):
public static string ToString<T>(this T[] array, string delimiter)
{
if (array != null)
{
// determine if the length of the array is greater than the performance threshold for using a stringbuilder
// 10 is just an arbitrary threshold value I've chosen
if (array.Length < 10)
{
// assumption is that for arrays of less than 10 elements
// this code would be more efficient than a StringBuilder.
// Note: this is a crazy/pointless micro-optimization. Don't do this.
string[] values = new string[array.Length];
for (int i = 0; i < values.Length; i++)
values[i] = array[i].ToString();
return string.Join(delimiter, values);
}
else
{
// for arrays of length 10 or longer, use a StringBuilder
StringBuilder sb = new StringBuilder();
sb.Append(array[0]);
for (int i = 1; i < array.Length; i++)
{
sb.Append(delimiter);
sb.Append(array[i]);
}
return sb.ToString();
}
}
else
{
return null;
}
}
For this example, the performance impact is probably not worth caring about, but the point is that if you are in a situation where you actually do need to be concerned with the performance of your operations, whatever they are, then it will most likely be easier and more readable to handle that within a utility function than using a complex Linq expression.
That utility function still looks kind of clunky. Now let's ditch the hybrid stuff and do this:
// convert an enumeration of one type into an enumeration of another type
public static IEnumerable<TOut> Convert<TIn, TOut>(this IEnumerable<TIn> input, Func<TIn, TOut> conversion)
{
foreach (TIn value in input)
{
yield return conversion(value);
}
}
// concatenate the strings in an enumeration separated by the specified delimiter
public static string Delimit<T>(this IEnumerable<T> input, string delimiter)
{
IEnumerator<T> enumerator = input.GetEnumerator();
if (enumerator.MoveNext())
{
StringBuilder builder = new StringBuilder();
// start off with the first element
builder.Append(enumerator.Current);
// append the remaining elements separated by the delimiter
while (enumerator.MoveNext())
{
builder.Append(delimiter);
builder.Append(enumerator.Current);
}
return builder.ToString();
}
else
{
return string.Empty;
}
}
// concatenate all elements
public static string ToString<T>(this IEnumerable<T> input)
{
return ToString(input, string.Empty);
}
// concatenate all elements separated by a delimiter
public static string ToString<T>(this IEnumerable<T> input, string delimiter)
{
return input.Delimit(delimiter);
}
// concatenate all elements, each one left-padded to a minimum length
public static string ToString<T>(this IEnumerable<T> input, int minLength, char paddingChar)
{
return input.Convert(i => i.ToString().PadLeft(minLength, paddingChar)).Delimit(string.Empty);
}
Now we have separate and fairly compact utility functions, each of which are arguable useful on their own.
Ultimately, my point is not that you shouldn't use Linq, but rather just to say don't forget about the benefits of creating your own utility functions, even if they are small and perhaps only contain a single line that returns the result from a line of Linq code. If nothing else, you'll be able to keep your application code even more condensed than you could achieve with a line of Linq code, and if you are using it in multiple places, then using a utility function makes it easier to adjust your output in case you need to change it later.
For this problem, I'd rather just write something like this in my application code:
int[] arr = { 0, 1, 2, 3, 0, 1 };
// 012301
result = arr.ToString<int>();
// comma-separated values
// 0,1,2,3,0,1
result = arr.ToString(",");
// left-padded to 2 digits
// 000102030001
result = arr.ToString(2, '0');


To avoid the creation of an extra array you could do the following.
var builder = new StringBuilder();
Array.ForEach(arr, x => builder.Append(x));
var res = builder.ToString();


string result = arr.Aggregate("", (s, i) => s + i.ToString());
(Disclaimer: If you have a lot of digits (hundreds, at least) and you care about performance, I suggest eschewing this method and using a StringBuilder, as in JaredPar's answer.)


You can do:
int[] arr = {0,1,2,3,0,1};
string results = string.Join("",arr.Select(i => i.ToString()).ToArray());
That gives you your results.


I like using StringBuilder with Aggregate(). The "trick" is that Append() returns the StringBuilder instance itself:
var sb = arr.Aggregate( new StringBuilder(), ( s, i ) => s.Append( i ) );
var result = sb.ToString();


string.Join("", (from i in arr select i.ToString()).ToArray())
In the .NET 4.0 the string.Join can use an IEnumerable<string> directly:
string.Join("", from i in arr select i.ToString())


I've left this here for posterity but don't recommend its use as it's not terribly readable. This is especially true now that I've come back to see if after a period of some time and have wondered what I was thinking when I wrote it (I was probably thinking 'crap, must get this written before someone else posts an answer'.)
string s = string.Concat(arr.Cast<object>().ToArray());


The most efficient way is not to convert each int into a string, but rather create one string out of an array of chars. Then the garbage collector only has one new temp object to worry about.
int[] arr = {0,1,2,3,0,1};
string result = new string(Array.ConvertAll<int,char>(arr, x => Convert.ToChar(x + '0')));


This is a roundabout way to go about it its not much code and easy for beginners to understand
int[] arr = {0,1,2,3,0,1};
string joined = "";
foreach(int i in arr){
joined += i.ToString();
}
int number = int.Parse(joined);


If this is long array you could use
var sb = arr.Aggregate(new StringBuilder(), ( s, i ) => s.Append( i ), s.ToString());


// This is the original array
int[] nums = {1, 2, 3};
// This is an empty string we will end up with
string numbers = "";
// iterate on every char in the array
foreach (var item in nums)
{
// add the char to the empty string
numbers += Convert.ToString(item);
}
// Write the string in the console
Console.WriteLine(numbers);

Related

how to censor the first 10 char in a string using c#

so I want to create a credit card encoder (if this even a word) that takes a string and put the first 10 digits of the string as '*'
this is the code I came up with:
public static string[] ToCencoredString(this string str)
{
char[] array = Enumerable.Repeat('*', str.Length-1).ToArray();
array = array.Select((cha, index) =>
{
if (index < 10)
array[index] = str[index];
});
}
(ignoringe the fact the function returns string[] there is another part of the code which is not relevant)
I don't know why, but I keep getting ArgumentNullException which is odd because there is not a single value in array witch is null.
what am I doing wrong?

What about changing it to something a bit more simple:
var result = string.Concat(Enumerable.Repeat("*", 10)) + str.Substring(10);

I'd use this more efficient version using String.Substring and the string constructor:
public static string ToCencoredString(this string str, int length = 10)
{
if (String.IsNullOrEmpty(str)) return str;
string censored = new string('*', length);
if (str.Length <= length) return censored;
return censored + str.Substring(length);
}

I suggest you use the original array for iterating so that you can make use of its index to create the mask. A String.Join() may help you to produce the masked output. The code would be something like this:
string maskedInput = String.Join("", str.Select((c, index) => index < 10? '*' : c));
Here is a working example for your reference

Your code doesn't compile. So I dont know how you managed to reach ArgumentNullException.
And that's not how you use LINQ. The correct way (although not a good way, since the answers above are apparently way better) to implement what's in your mind
array = array.Select((cha, index) =>
{
if (index < 10)
return array[index];
else
return str[index];
}).ToArray();

$"{string.Concat(Enumerable.Repeat("*", 10))}{FIELD.Substring(10)}";

Generate all combinations with unknown number of slots

I have a text file full of strings, one on each line. Some of these strings will contain an unknown number of "#" characters. Each "#" can represent the numbers 1, 2, 3, or 4. I want to generate all possible combinations (permutations?) of strings for each of those "#"s. If there were a set number of "#"s per string, I'd just use nested for loops (quick and dirty). I need help finding a more elegant way to do it with an unknown number of "#"s.
Example 1: Input string is a#bc
Output strings would be:
a1bc
a2bc
a3bc
a4bc
Example 2: Input string is a#bc#d
Output strings would be:
a1bc1d
a1bc2d
a1bc3d
a1bc4d
a2bc1d
a2bc2d
a2bc3d
...
a4bc3d
a4bc4d
Can anyone help with this one? I'm using C#.

This is actually a fairly good place for a recursive function. I don't write C#, but I would create a function List<String> expand(String str) which accepts a string and returns an array containing the expanded strings.
expand can then search the string to find the first # and create a list containing the first part of the string + expansion. Then, it can call expand on the last part of the string and add each element in it's expansion to each element in the last part's expansion.
Example implementation using Java ArrayLists:
ArrayList<String> expand(String str) {
/* Find the first "#" */
int i = str.indexOf("#");
ArrayList<String> expansion = new ArrayList<String>(4);
/* If the string doesn't have any "#" */
if(i < 0) {
expansion.add(str);
return expansion;
}
/* New list to hold the result */
ArrayList<String> result = new ArrayList<String>();
/* Expand the "#" */
for(int j = 1; j <= 4; j++)
expansion.add(str.substring(0,i-1) + j);
/* Combine every expansion with every suffix expansion */
for(String a : expand(str.substring(i+1)))
for(String b : expansion)
result.add(b + a);
return result;
}

I offer you here a minimalist approach for the problem at hand.
Yes, like other have said recursion is the way to go here.
Recursion is a perfect fit here, since we can solve this problem by providing the solution for a short part of the input and start over again with the other part until we are done and merge the results.
Every recursion must have a stop condition - meaning no more recursion needed.
Here my stop condition is that there are no more "#" in the string.
I'm using string as my set of values (1234) since it is an IEnumerable<char>.
All other solutions here are great, Just wanted to show you a short approach.
internal static IEnumerable<string> GetStrings(string input)
{
var values = "1234";
var permutations = new List<string>();
var index = input.IndexOf('#');
if (index == -1) return new []{ input };
for (int i = 0; i < values.Length; i++)
{
var newInput = input.Substring(0, index) + values[i] + input.Substring(index + 1);
permutations.AddRange(GetStrings(newInput));
}
return permutations;
}
An even shorter and cleaner approach with LINQ:
internal static IEnumerable<string> GetStrings(string input)
{
var values = "1234";
var index = input.IndexOf('#');
if (index == -1) return new []{ input };
return
values
.Select(ReplaceFirstWildCardWithValue)
.SelectMany(GetStrings);
string ReplaceFirstWildCardWithValue(char value) => input.Substring(0, index) + value + input.Substring(index + 1);
}

This is shouting out loud for a recursive solution.
First, lets make a method that generates all combinations of a certain length from a given set of values. Because we are only interested in generating strings, lets take advantage of the fact that string is immutable (see P.D.2); this makes recursive functions so much easier to implement and reason about:
static IEnumerable<string> GetAllCombinations<T>(
ISet<T> set, int length)
{
IEnumerable<string> getCombinations(string current)
{
if (current.Length == length)
{
yield return current;
}
else
{
foreach (var s in set)
{
foreach (var c in getCombinations(current + s))
{
yield return c;
}
}
}
}
return getCombinations(string.Empty);
}
Study carefully how this methods works. Work it out by hand for small examples to understand it.
Now, once we know how to generate all possible combinations, building the strings is easy:
Figure out the number of wildcards in the specified string: this will be our combination length.
For every combination, insert in order each character into the string where we encounter a wildcard.
Ok, lets do just that:
public static IEnumerable<string> GenerateCombinations<T>(
this string s,
IEnumerable<T> set,
char wildcard)
{
var length = s.Count(c => c == wildcard);
var combinations = GetAllCombinations(set, length);
var builder = new StringBuilder();
foreach (var combination in combinations)
{
var index = 0;
foreach (var c in s)
{
if (c == wildcard)
{
builder.Append(combination[index]);
index += 1;
}
else
{
builder.Append(c);
}
}
yield return builder.ToString();
builder.Clear();
}
}
And we're done. Usage would be:
var set = new HashSet<int>(new[] { 1, 2, 3, 4 });
Console.WriteLine(
string.Join("; ", "a#bc#d".GenerateCombinations(set, '#')));
And sure enough, the output is:
a1bc1d; a1bc2d; a1bc3d; a1bc4d; a2bc1d; a2bc2d; a2bc3d;
a2bc4d; a3bc1d; a3bc2d; a3bc3d; a3bc4d; a4bc1d; a4bc2d;
a4bc3d; a4bc4d
Is this the most performant or efficient implementation? Probably not but its readable and maintainable. Unless you have a specific performance goal you are not meeting, write code that works and is easy to understand.
P.D. I’ve omitted all error handling and argument validation.
P.D.2: if the length of the combinations is big, concatenting strings inside GetAllCombinations might not be a good idea. In that case I’d have GetAllCombinations return an IEnumerable<IEnumerable<T>>, implement a trivial ImmutableStack<T>, and use that as the combination buffer instead of string.

Shove a delimited string into a List<int>

If I have for example the following string:
"123;3344;4334;12"
and I want these numbers in a generic List<int>, I guess I don't know of a good way here other than to split in a loop and do a conversion then add to a List<int> through each iteration. Does anyone have other ways to go about this?
Updated. Here's what I came up with. I want to do this the old fashion way, not with LINQ because I'm trying to get better with just strings, arrays, lists and manipulating and converting in general.
public List<int> StringToList(string stringToSplit, char splitDelimiter)
{
List<int> list = new List<int>();
if (string.IsNullOrEmpty(stringToSplit))
return list;
string[] values = stringToSplit.Split(splitDelimiter);
if (values.Length <= 1)
return list;
foreach (string s in values)
{
int i;
if (Int32.TryParse(s, out i))
list.Add(i);
}
return list;
}
This is a new string utility method I plan on using whenever I need to convert a delimited string list to List
So I'm returning an empty list back to the caller if something fails. Good/Bad? is it pretty common to do this?
Yes, there are more "elegant" ways to do this with LINQ but I want to do it manually..the old way for now just for my own understanding.
Also, what bothers me about this:
list.AddRange(str.Split(';').Select(Int32.Parse));
is that I have no idea:
How to shove in a TryParse there instead.
What if the str.Split(';').Select(Int32.Parse) just fails for whatever reason...then the method that this AddRange resides in is going to blow up and unless I add a try/catch around this whole thing, I'm screwed if I don't handle it properly.

static int? ToInt32OrNull(string s)
{
int value;
return (Int32.TryParse(s, out value)) ? value : default(int?);
}
// ...
var str = "123;3344;4334;12";
var list = new List<int>();
list.AddRange(str.Split(';')
.Select(ToInt32OrNull)
.Where(i => i != null)
.Cast<int>());
Questioner notes:
I don't know of a good way here other than to split in a loop and do a conversion then add to a List
In general, this is a major reason why LINQ was brought into C# - to remove the need to work with sequences of values by implementing loops, and instead just declare your intention to transform the sequence. If you ever find yourself thinking "I don't know how to do this except with a loop" - it's time to look into a LINQ construct which will do the work for you.
Performance Update:
Performance of LINQ has been quesioned below. While in the comments the idea of LINQ being slower is defended since we gain the benefits of readability, maintainability and composibility, there is another aspect which gives LINQ an easy performance advantage: parallelism. Here is an example where adding just one extension method call, AsParallel() doubles the performance. This is a great example of where scale-out beats micro-optimization without even needing to measure very carefully. Note I'm not claiming that micro-optimizations are not ever needed, but with the tools we have available at this level of absraction, the need becomes vanishingly small.
class Program
{
private const int ElementCount = 10000000;
static void Main(string[] args)
{
var str = generateString();
var stopwatch = new Stopwatch();
var list1 = new List<int>(ElementCount);
var list2 = new List<int>(ElementCount);
var split = str.Split(';');
stopwatch.Start();
list1.AddRange(split
.Select(ToInt32OrNull)
.Where(i => i != null)
.Cast<int>());
stopwatch.Stop();
TimeSpan nonParallel = stopwatch.Elapsed;
stopwatch.Restart();
list2.AddRange(split
.AsParallel()
.Select(ToInt32OrNull)
.Where(i => i != null)
.Cast<int>());
stopwatch.Stop();
TimeSpan parallel = stopwatch.Elapsed;
Debug.WriteLine("Non-parallel: {0}", nonParallel);
Debug.WriteLine("Parallel: {0}", parallel);
}
private static String generateString()
{
var builder = new StringBuilder(1048576);
var rnd = new Random();
for (int i = 0; i < ElementCount; i++)
{
builder.Append(rnd.Next(99999));
builder.Append(';');
}
builder.Length--;
return builder.ToString();
}
static int? ToInt32OrNull(string s)
{
int value;
return (Int32.TryParse(s, out value)) ? value : default(int?);
}
}
Non-parallel: 00:00:07.0719911
Parallel: 00:00:04.5933906

string str = "123;3344;4334;12";
List<int> list = new List<int>();
foreach (string s in str.Split(';'))
{
list.Add( Int32.Parse(s));
}

List<int> list = (from numString in "123;3344;4334;12".Split(';')
select int.Parse(numString)).ToList();

string myString = "123;3344;4334;12";
var ints = new List<int>();
(from s in myString.Split(';')
select int.Parse()).ToList().ForEach(i=>ints.Add(i));
I've heard .Net 4.0 may have added ForEach to Enumerable<T>, so the ToList might be unnecessary there (can't test).

I think this is simplest
var str = "123;3344;4334;12";
var list = str.Split(';').ToList().Cast<int>();

Can I rewrite this more elegantly using LINQ?

I have a double[][] that I want to convert to a CSV string format (i.e. each row in a line, and row elements separated by commas). I wrote it like this:
public static string ToCSV(double[][] array)
{
return String.Join(Environment.NewLine,
Array.ConvertAll(array,
row => String.Join(",",
Array.ConvertAll(row, x => x.ToString())));
}
Is there a more elegant way to write this using LINQ?
(I know, one could use temporary variables to make this look better, but this code format better conveys what I am looking for.)

You can, but I wouldn't personally do all the lines at once - I'd use an iterator block:
public static IEnumerable<string> ToCSV(IEnumerable<double[]> source)
{
return source.Select(row => string.Join(",",
Array.ConvertAll(row, x=>x.ToString())));
}
This returns each line (the caller can then WriteLine etc efficiently, without buffering everything). It is also now callable from any source of double[] rows (including but not limited to a jagged array).
Also - with a local variable you could use StringBuilder to make each line slightly cheaper.
To return the entire string at once, I'd optimize it to use a single StringBuilder for all the string work; a bit more long-winded, but much more efficient (far fewer intermediate strings):
public static string ToCSV(IEnumerable<double[]> source) {
StringBuilder sb = new StringBuilder();
foreach(var row in source) {
if (row.Length > 0) {
sb.Append(row[0]);
for (int i = 1; i < row.Length; i++) {
sb.Append(',').Append(row[i]);
}
}
}
return sb.ToString();
}

You could also use Aggregate
public static string ToCSV(double[][] array)
{
return array.Aggregate(string.Empty, (multiLineStr, arrayDouble) =>
multiLineStr + System.Environment.NewLine +
arrayDouble.Aggregate(string.Empty, (str, dbl) => str + "," + dbl.ToString()));
}

This is compatible with any nested sequences of double. It also defers the ToString implementation to the caller, allowing formatting while avoiding messy IFormatProvider overloads:
public static string Join(this IEnumerable<string> source, string separator)
{
return String.Join(separator, source.ToArray());
}
public static string ToCsv<TRow>(this IEnumerable<TRow> rows, Func<double, string> valueToString)
where TRow : IEnumerable<double>
{
return rows
.Select(row => row.Select(valueToString).Join(", "))
.Join(Environment.NewLine);
}

You can do it with LINQ, but I'm not sure if you like this one better than yours. I'm afraid you don't. :)
var q = String.Join(Environment.NewLine, (from a in d
select String.Join(", ", (from b in a
select b.ToString()).ToArray())).ToArray());
Cheers,
Matthias

Most efficient way to concatenate strings?

What's the most efficient way to concatenate strings?

Rico Mariani, the .NET Performance guru, had an article on this very subject. It's not as simple as one might suspect. The basic advice is this:
If your pattern looks like:
x = f1(...) + f2(...) + f3(...) + f4(...)
that's one concat and it's zippy, StringBuilder probably won't help.
If your pattern looks like:
if (...) x += f1(...)
if (...) x += f2(...)
if (...) x += f3(...)
if (...) x += f4(...)
then you probably want StringBuilder.
Yet another article to support this claim comes from Eric Lippert where he describes the optimizations performed on one line + concatenations in a detailed manner.

The StringBuilder.Append() method is much better than using the + operator. But I've found that, when executing 1000 concatenations or less, String.Join() is even more efficient than StringBuilder.
StringBuilder sb = new StringBuilder();
sb.Append(someString);
The only problem with String.Join is that you have to concatenate the strings with a common delimiter.
Edit: as #ryanversaw pointed out, you can make the delimiter string.Empty.
string key = String.Join("_", new String[]
{ "Customers_Contacts", customerID, database, SessionID });

There are 6 types of string concatenations:
Using the plus (+) symbol.
Using string.Concat().
Using string.Join().
Using string.Format().
Using string.Append().
Using StringBuilder.
In an experiment, it has been proved that string.Concat() is the best way to approach if the words are less than 1000(approximately) and if the words are more than 1000 then StringBuilder should be used.
For more information, check this site.
string.Join() vs string.Concat()
The string.Concat method here is equivalent to the string.Join method invocation with an empty separator. Appending an empty string is fast, but not doing so is even faster, so the string.Concat method would be superior here.

From Chinh Do - StringBuilder is not always faster:
Rules of Thumb
When concatenating three dynamic string values or less, use traditional string concatenation.
When concatenating more than three dynamic string values, use StringBuilder.
When building a big string from several string literals, use either the # string literal or the inline + operator.
Most of the time StringBuilder is your best bet, but there are cases as shown in that post that you should at least think about each situation.

If you're operating in a loop, StringBuilder is probably the way to go; it saves you the overhead of creating new strings regularly. In code that'll only run once, though, String.Concat is probably fine.
However, Rico Mariani (.NET optimization guru) made up a quiz in which he stated at the end that, in most cases, he recommends String.Format.

Here is the fastest method I've evolved over a decade for my large-scale NLP app. I have variations for IEnumerable<T> and other input types, with and without separators of different types (Char, String), but here I show the simple case of concatenating all strings in an array into a single string, with no separator. Latest version here is developed and unit-tested on C# 7 and .NET 4.7.
There are two keys to higher performance; the first is to pre-compute the exact total size required. This step is trivial when the input is an array as shown here. For handling IEnumerable<T> instead, it is worth first gathering the strings into a temporary array for computing that total (The array is required to avoid calling ToString() more than once per element since technically, given the possibility of side-effects, doing so could change the expected semantics of a 'string join' operation).
Next, given the total allocation size of the final string, the biggest boost in performance is gained by building the result string in-place. Doing this requires the (perhaps controversial) technique of temporarily suspending the immutability of a new String which is initially allocated full of zeros. Any such controversy aside, however...
...note that this is the only bulk-concatenation solution on this page which entirely avoids an extra round of allocation and copying by the String constructor.
Complete code:
/// <summary>
/// Concatenate the strings in 'rg', none of which may be null, into a single String.
/// </summary>
public static unsafe String StringJoin(this String[] rg)
{
int i;
if (rg == null || (i = rg.Length) == 0)
return String.Empty;
if (i == 1)
return rg[0];
String s, t;
int cch = 0;
do
cch += rg[--i].Length;
while (i > 0);
if (cch == 0)
return String.Empty;
i = rg.Length;
fixed (Char* _p = (s = new String(default(Char), cch)))
{
Char* pDst = _p + cch;
do
if ((t = rg[--i]).Length > 0)
fixed (Char* pSrc = t)
memcpy(pDst -= t.Length, pSrc, (UIntPtr)(t.Length << 1));
while (pDst > _p);
}
return s;
}
[DllImport("MSVCR120_CLR0400", CallingConvention = CallingConvention.Cdecl)]
static extern unsafe void* memcpy(void* dest, void* src, UIntPtr cb);
I should mention that this code has a slight modification from what I use myself. In the original, I call the cpblk IL instruction from C# to do the actual copying. For simplicity and portability in the code here, I replaced that with P/Invoke memcpy instead, as you can see. For highest performance on x64 (but maybe not x86) you may want to use the cpblk method instead.

From this MSDN article:
There is some overhead associated with
creating a StringBuilder object, both
in time and memory. On a machine with
fast memory, a StringBuilder becomes
worthwhile if you're doing about five
operations. As a rule of thumb, I
would say 10 or more string operations
is a justification for the overhead on
any machine, even a slower one.
So if you trust MSDN go with StringBuilder if you have to do more than 10 strings operations/concatenations - otherwise simple string concat with '+' is fine.

Try this 2 pieces of code and you will find the solution.
static void Main(string[] args)
{
StringBuilder s = new StringBuilder();
for (int i = 0; i < 10000000; i++)
{
s.Append( i.ToString());
}
Console.Write("End");
Console.Read();
}
Vs
static void Main(string[] args)
{
string s = "";
for (int i = 0; i < 10000000; i++)
{
s += i.ToString();
}
Console.Write("End");
Console.Read();
}
You will find that 1st code will end really quick and the memory will be in a good amount.
The second code maybe the memory will be ok, but it will take longer... much longer.
So if you have an application for a lot of users and you need speed, use the 1st. If you have an app for a short term one user app, maybe you can use both or the 2nd will be more "natural" for developers.
Cheers.

It's also important to point it out that you should use the + operator if you are concatenating string literals.
When you concatenate string literals or string constants by using the + operator, the compiler creates a single string. No run time concatenation occurs.
How to: Concatenate Multiple Strings (C# Programming Guide)

Adding to the other answers, please keep in mind that StringBuilder can be told an initial amount of memory to allocate.
The capacity parameter defines the maximum number of characters that can be stored in the memory allocated by the current instance. Its value is assigned to the Capacity property. If the number of characters to be stored in the current instance exceeds this capacity value, the StringBuilder object allocates additional memory to store them.
If capacity is zero, the implementation-specific default capacity is used.
Repeatedly appending to a StringBuilder that hasn't been pre-allocated can result in a lot of unnecessary allocations just like repeatedly concatenating regular strings.
If you know how long the final string will be, can trivially calculate it, or can make an educated guess about the common case (allocating too much isn't necessarily a bad thing), you should be providing this information to the constructor or the Capacity property. Especially when running performance tests to compare StringBuilder with other methods like String.Concat, which do the same thing internally. Any test you see online which doesn't include StringBuilder pre-allocation in its comparisons is wrong.
If you can't make any kind of guess about the size, you're probably writing a utility function which should have its own optional argument for controlling pre-allocation.

Following may be one more alternate solution to concatenate multiple strings.
String str1 = "sometext";
string str2 = "some other text";
string afterConcate = $"{str1}{str2}";
string interpolation

Another solution:
inside the loop, use List instead of string.
List<string> lst= new List<string>();
for(int i=0; i<100000; i++){
...........
lst.Add(...);
}
return String.Join("", lst.ToArray());;
it is very very fast.

The most efficient is to use StringBuilder, like so:
StringBuilder sb = new StringBuilder();
sb.Append("string1");
sb.Append("string2");
...etc...
String strResult = sb.ToString();
#jonezy: String.Concat is fine if you have a couple of small things. But if you're concatenating megabytes of data, your program will likely tank.

System.String is immutable. When we modify the value of a string variable then a new memory is allocated to the new value and the previous memory allocation released. System.StringBuilder was designed to have concept of a mutable string where a variety of operations can be performed without allocation separate memory location for the modified string.

I've tested all the methods in this page and at the end I've developed my solution that is the fastest and less memory expensive.
Note: tested in Framework 4.8
[MemoryDiagnoser]
public class StringConcatSimple
{
private string
title = "Mr.", firstName = "David", middleName = "Patrick", lastName = "Callan";
[Benchmark]
public string FastConcat()
{
return FastConcat(
title, " ",
firstName, " ",
middleName, " ",
lastName);
}
[Benchmark]
public string StringBuilder()
{
var stringBuilder =
new StringBuilder();
return stringBuilder
.Append(title).Append(' ')
.Append(firstName).Append(' ')
.Append(middleName).Append(' ')
.Append(lastName).ToString();
}
[Benchmark]
public string StringBuilderExact24()
{
var stringBuilder =
new StringBuilder(24);
return stringBuilder
.Append(title).Append(' ')
.Append(firstName).Append(' ')
.Append(middleName).Append(' ')
.Append(lastName).ToString();
}
[Benchmark]
public string StringBuilderEstimate100()
{
var stringBuilder =
new StringBuilder(100);
return stringBuilder
.Append(title).Append(' ')
.Append(firstName).Append(' ')
.Append(middleName).Append(' ')
.Append(lastName).ToString();
}
[Benchmark]
public string StringPlus()
{
return title + ' ' + firstName + ' ' +
middleName + ' ' + lastName;
}
[Benchmark]
public string StringFormat()
{
return string.Format("{0} {1} {2} {3}",
title, firstName, middleName, lastName);
}
[Benchmark]
public string StringInterpolation()
{
return
$"{title} {firstName} {middleName} {lastName}";
}
[Benchmark]
public string StringJoin()
{
return string.Join(" ", title, firstName,
middleName, lastName);
}
[Benchmark]
public string StringConcat()
{
return string.
Concat(new String[]
{ title, " ", firstName, " ",
middleName, " ", lastName });
}
}
Yes, it use unsafe
public static unsafe string FastConcat(string str1, string str2, string str3, string str4, string str5, string str6, string str7)
{
var capacity = 0;
var str1Length = 0;
var str2Length = 0;
var str3Length = 0;
var str4Length = 0;
var str5Length = 0;
var str6Length = 0;
var str7Length = 0;
if (str1 != null)
{
str1Length = str1.Length;
capacity = str1Length;
}
if (str2 != null)
{
str2Length = str2.Length;
capacity += str2Length;
}
if (str3 != null)
{
str3Length = str3.Length;
capacity += str3Length;
}
if (str4 != null)
{
str4Length = str4.Length;
capacity += str4Length;
}
if (str5 != null)
{
str5Length = str5.Length;
capacity += str5Length;
}
if (str6 != null)
{
str6Length = str6.Length;
capacity += str6Length;
}
if (str7 != null)
{
str7Length = str7.Length;
capacity += str7Length;
}
string result = new string(' ', capacity);
fixed (char* dest = result)
{
var x = dest;
if (str1Length > 0)
{
fixed (char* src = str1)
{
Unsafe.CopyBlock(x, src, (uint)str1Length * 2);
x += str1Length;
}
}
if (str2Length > 0)
{
fixed (char* src = str2)
{
Unsafe.CopyBlock(x, src, (uint)str2Length * 2);
x += str2Length;
}
}
if (str3Length > 0)
{
fixed (char* src = str3)
{
Unsafe.CopyBlock(x, src, (uint)str3Length * 2);
x += str3Length;
}
}
if (str4Length > 0)
{
fixed (char* src = str4)
{
Unsafe.CopyBlock(x, src, (uint)str4Length * 2);
x += str4Length;
}
}
if (str5Length > 0)
{
fixed (char* src = str5)
{
Unsafe.CopyBlock(x, src, (uint)str5Length * 2);
x += str5Length;
}
}
if (str6Length > 0)
{
fixed (char* src = str6)
{
Unsafe.CopyBlock(x, src, (uint)str6Length * 2);
x += str6Length;
}
}
if (str7Length > 0)
{
fixed (char* src = str7)
{
Unsafe.CopyBlock(x, src, (uint)str7Length * 2);
}
}
}
return result;
}
You can edit the method and adapt it to your case. For example you can make it something like
public static unsafe string FastConcat(string str1, string str2, string str3 = null, string str4 = null, string str5 = null, string str6 = null, string str7 = null)

For just two strings, you definitely do not want to use StringBuilder. There is some threshold above which the StringBuilder overhead is less than the overhead of allocating multiple strings.
So, for more that 2-3 strings, use DannySmurf's code. Otherwise, just use the + operator.

It really depends on your usage pattern.
A detailed benchmark between string.Join, string,Concat and string.Format can be found here: String.Format Isn't Suitable for Intensive Logging
(This is actually the same answer I gave to this question)

It would depend on the code.
StringBuilder is more efficient generally, but if you're only concatenating a few strings and doing it all in one line, code optimizations will likely take care of it for you. It's important to think about how the code looks too: for larger sets StringBuilder will make it easier to read, for small ones StringBuilder will just add needless clutter.

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