How should I concatenate strings?

Question

Are there differences between these examples? Which should I use in which case?

var str1 = "abc" + dynamicString + dynamicString2;

var str2 = String.Format("abc{0}{1}", dynamicString, dynamicString2);

var str3 = new StringBuilder("abc").
    Append(dynamicString).
    Append(dynamicString2).
    ToString();

var str4 = String.Concat("abc", dynamicString, dynamicString2);

There are similar questions:

• Difference in String concatenation which only asks about the + operator, and it's not even mentioned in the answer that it is converted to String.Concat
• What's the best string concatenation method which is not really related to my question, where it is asking for the best, and not a comparation of the possible ways to concatenate a string and their outputs, as this question does.

This question is asking about what happens in each case, what will be the real output of those examples? What are the differences about them? Where should I use them in which case?

Answer 1

@ Jerod Houghtelling Answer

Actually String.Format uses a StringBuilder behind the scenes (use reflecton on String.Format if you want)

I agree with the following answer in general

Answer 2

@Xander. I believe you man. However my code shows sb is faster than string.format.

Beat this:

Stopwatch sw = new Stopwatch();
sw.Start();

for (int i = 0; i < 10000; i++)
{
    string r = string.Format("ABC{0}{1}{2}", i, i-10, 
        "dasdkadlkdjakdljadlkjdlkadjalkdj");
}

sw.Stop();
Console.WriteLine("string.format: " + sw.ElapsedTicks);

sw.Reset();
sw.Start();
for (int i = 0; i < 10000; i++)
{
    StringBuilder sb = new StringBuilder();
    string r = sb.AppendFormat("ABC{0}{1}{2}", i, i - 10,
        "dasdkadlkdjakdljadlkjdlkadjalkdj").ToString();
}

sw.Stop();
Console.WriteLine("AppendFormat: " + sw.ElapsedTicks);

Answer 3

It's important to understand that strings are immutable, they don't change. So ANY time that you change, add, modify, or whatever a string - it is going to create a new 'version' of the string in memory, then give the old version up for garbage collection. So something like this:

string output = firstName.ToUpper().ToLower() + "test";

This is going to create a string (for output), then create THREE other strings in memory (one for: ToUpper(), ToLower()'s output, and then one for the concatenation of "test").

So unless you use StringBuilder or string.Format, anything else you do is going to create extra instances of your string in memory. This is of course an issue inside of a loop where you could end up with hundreds or thousands of extra strings. Hope that helps