Performance creating concatenated strings

Question

Which way to create strings is more run time efficient in `C#` Number 1: ``` bool value = true; int channel = 1; String s = String.Format(":Channel{0}:Display {1}", channel, value ? "ON" : "OFF"); ``` Number 2: ``` bool value = true; String channel = "1"; string s = ":Channel" + channel + ":Display " + value ? "ON" : "OFF"; ```

Accepted Answer

I'm going to add an example now to illustrate how the compiler treats the two, because there seems to be a lot of confusion in some of the other answers (edit: note that much of these confusions have now been deleted/edited away):

```
bool value = true;
int channel = 1;
String s = String.Format(":Channel{0}:Display {1}", channel,
    value ? "ON" : "OFF");
```

The last line is compiled as:

```
String s = String.Format(":Channel{0}:Display {1}",
    new object[2] {(object)channel, value ? "ON" : "OFF")};
```

note interestingly the creation of an array and a "box" for `int channel`, and of course the extra parsing required to find `{0}` / `{1}`.

Now Number 2:

```
bool value = true;
String channel = "1";
string  s = ":Channel" + channel + ":Display " + (value ? "ON" : "OFF");
```

The last line is compiled as:

```
string s = string.Concat(":Channel", channel, ":Display ", value ? "ON" : "OFF");
```

here there is no array, no box (`channel` is now a string), and no parsing. The overload is `public static string string.Concat(string,string,string,string)`; `string.Concat` is implemented very efficiently, pre-allocating a right-sized string in advance then over-writing, etc.

In most code, either is fine. The second version is technically more efficient (no box, no array, no parsing), but is a major pain if you ever want to internationalize later. In most applications you will not notice any difference between the two. The parsing is fast, and the box / array are collected cheaply as gen-zero objects. But cheap is not free.

Performance creating concatenated strings

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