[SOLVED] How come for loops in C# are so slow when concatenating strings?

Issue

I wrote a program that runs a simple for loop in both C++ and C#, yet the same thing takes dramatically longer in C#, why is that? Did I fail to account for something in my test?

C# (13.95s)

static double timeStamp() {
    return (double)(DateTime.UtcNow.Subtract(new DateTime(1970, 1, 1))).TotalSeconds;
}

static void Main(string[] args) {
    double timeStart = timeStamp();

    string f = "";
    for(int i=0; i<100000; i++) {
        f += "Sample";
    }

    double timeEnd = timeStamp();
    double timeDelta = timeEnd - timeStart;
    Console.WriteLine(timeDelta.ToString());
    Console.Read();
}

C++ (0.20s)

long int timeStampMS() {
    milliseconds ms = duration_cast<milliseconds> (system_clock::now().time_since_epoch());
    return ms.count();
}

int main() {
    long int timeBegin = timeStampMS();

    string test = "";

    for (int i = 0; i < 100000; i++) {
        test += "Sample";
    }


    long int timeEnd = timeStampMS();
    long double delta = timeEnd - timeBegin;

    cout << to_string(delta) << endl;
    cin.get();
}

Solution

On my PC, changing the code to use StringBuilder and converting to a String at the end, the execution time went from 26.15 seconds to 0.0012 seconds, or over 20,000 times faster.

var fb = new StringBuilder();
for (int i = 0; i < 100000; ++i) {
    fb.Append("Sample");
}
var f = fb.ToString();

As explained in the .Net documentation, the StringBuilder class is a mutable string object that is useful for when you are making many changes to a string, as opposed to the String class, which is an immutable object that requires a new object creation every time you e.g. concatenate two Strings together. Because the implementation of StringBuilder is a linked list of character arrays, and new blocks are added up to 8000 characters at a time, StringBuilder.Append is much faster.

Answered By – NetMage

Answer Checked By – Timothy Miller (BugsFixing Admin)