Question

//will need these in a second
string a = "5";
string b = "7";
string c = "3";

So because C# will allocate more strings in memory

string mystring = "";
mystring += a;
mystring += b;
mystring += c;

is going to be slower than

StringBuilder sb = new StringBuilder();
sb.Append(a).Append(b).Append(c);

So then, what about:

string mystring = "";
mystring += a + b + c;

Is it just the += part that is slow, or is + also a culprit here?

Answer 1

+= is sintax sugare for s +a.

What about performance, on big numbers of strings StringBuilder is much much more performant.