c# - stringbuilder versus string concat

Question

In my project I am looping across a dataview result.

 string html =string.empty;
 DataView dV = data.DefaultView;
 for(int i=0;i< dV.Count;i++)
 {
     DataRowView rv = dV[i];
     html += rv.Row["X"].Tostring();
 }

Number of rows in dV will alway be 3 or 4.

Is it better to use the string concat += opearator or StringBuilder for this case and why?

Answer 1

I would use StringBuilder here, just because it describes what you're doing.

For a simple concatenation of 3 or 4 strings, it probably won't make any significant difference, and string concatenation may even be slightly faster - but if you're wrong and there are lots of rows, StringBuilder will start getting much more efficient, and it's always more descriptive of what you're doing.

Alternatively, use something like:

string html = string.Join("", dv.Cast<DataRowView>()
                                .Select(rv => rv.Row["X"]));

Note that you don't have any sort of separator between the strings at the moment. Are you sure that's what you want? (Also note that your code doesn't make a lot of sense at the moment - you're not using i in the loop. Why?)

I have an article about string concatenation which goes into more detail about why it's worth using StringBuilder and when.

EDIT: For those who doubt that string concatenation can be faster, here's a test - with deliberately "nasty" data, but just to prove it's possible:

using System;
using System.Diagnostics;
using System.Text;

class Test
{
    static readonly string[] Bits = { 
        "small string",
        "string which is a bit longer",
        "stirng which is longer again to force yet another copy with any luck"
    };

    static readonly int ExpectedLength = string.Join("", Bits).Length;

    static void Main()        
    {
        Time(StringBuilderTest);
        Time(ConcatenateTest);
    }

    static void Time(Action action)
    {
        GC.Collect();
        GC.WaitForPendingFinalizers();
        GC.Collect();
        // Make sure it's JITted
        action();
        Stopwatch sw = Stopwatch.StartNew();
        for (int i = 0; i < 10000000; i++)
        {
            action();
        }
        sw.Stop();
        Console.WriteLine("{0}: {1} millis", action.Method.Name,
                          (long) sw.Elapsed.TotalMilliseconds);
    }

    static void ConcatenateTest()
    {
        string x = "";
        foreach (string bit in Bits)
        {
            x += bit;
        }
        // Force a validation to prevent dodgy optimizations
        if (x.Length != ExpectedLength)
        {
            throw new Exception("Eek!");
        }
    }

    static void StringBuilderTest()
    {
        StringBuilder builder = new StringBuilder();
        foreach (string bit in Bits)
        {
            builder.Append(bit);
        }
        string x = builder.ToString();
        // Force a validation to prevent dodgy optimizations
        if (x.Length != ExpectedLength)
        {
            throw new Exception("Eek!");
        }
    }
}

Results on my machine (compiled with /o+ /debug-):

StringBuilderTest: 2245 millis
ConcatenateTest: 989 millis

I've run this several times, including reversing the order of the tests, and the results are consistent.