Specific(exact) characters from char array to string C#

Question

I just saw in one source code something like this:

char[] letters = { 'k', 'n', 'p', 's' };

    for (int d1 = 0; d1 < letters.Length; d1++)
    {
        for (int d2 = 0; d2 < letters.Length; d2++)
        {
            for (int d3 = 0; d3 < letters.Length; d3++)
            {
                for (int d4 = 0; d4 < letters.Length; d4++)
                {
                    string left = ""+ letters[d1] + letters[d2] + letters[d3] + letters[d4];
                }
           }
        }
    }

What I am realy wondering is how exactly is that string strName = "" +.... working. What are these "" for? I looked wherever I could, but didnt find answer. Sorry if answer is very easy, I am new in programming, so I will appreciate your understanding.

Community · Accepted Answer · 2020-06-20 09:12:55Z

5

Somewhere in the C# Language Specifications (7.8.4 I think) it is written:

String concatenation:

string operator +(string x, string y);

string operator +(string x, object y);

string operator +(object x, string y);

These overloads of the binary + operator perform string concatenation. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string argument is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.

So the + operator between a string and a char converts the char to a string (through the .ToString() method of the char) and concatenates the two.

So for example:

char ch = 'x';

string str = "" + ch;

is "converted" to

string str = "" + ch.ToString();

where ch.ToString() is "x" (with double quotes, so a string)

In the end "" + something is a (safe) way to call .ToString() for something, safe because it handles the case that something == null, by not calling .ToString() and returning an empty string.

In your specific case, you can see that line of code as something like:

string left = "" + letters[d1].ToString();
left = left + letters[d2].ToString();
left = left + letters[d3].ToString();
left = left + letters[d4].ToString();

(in truth it is a little more complex if I remember correctly, because the C# compiler optimizes multiple string concatenations through the use of a single call to string.Concat, but this is an implementation detail, see for example https://stackoverflow.com/a/288913/613130)

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Apr 19, 2015 at 12:52

xanatos

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2 Comments

Kadiyski Over a year ago

Great anwer, appreciate it!

xanatos Over a year ago

For the one that suggested the edit, no, it is wrong. There is an explicit addition between "" and the first letters[d1]. Then the compiler can clearly be clever and remove it, but it is present.

Zied R. · Accepted Answer · 2015-04-19 12:45:26Z

3

Simple : letters[i] is a char and to cast it to a string "" is added to the expression.

e.g. int x = 5;

  string str = ""+x;  // "5" as string.

Same goes for char to string :)

edited Apr 19, 2015 at 12:45

Zied R.

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answered Apr 19, 2015 at 12:42

Sachin Sharma

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4 Comments

Steve Over a year ago

You have got it, but perhaps you should try to expand a little explaining why you cannot build a string just concatenating characters

Sachin Sharma Over a year ago

Just joined SO. Will keep that in Mind. :)

Kadiyski Over a year ago

Yes, I made the math for that :D but it looks very.. strange. Isn't there more official way to say "Hey, you- character! Go in that string!" I found something like string someStr = String.Join("",someCh[0],someCh[1],someCh[2]);)

Sachin Sharma Over a year ago

or you can use String.Concat(letters[d1] + letters[d2] + letters[d3] + letters[d4]);

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