Does C# optimize the concatenation of string literals?

Question

For instance, does the compiler know to translate

string s = "test " + "this " + "function";

to

string s = "test this function";

and thus avoid the performance hit with the string concatenation?

Answer 1

Yes. This is guaranteed by the C# specification. It's in section 7.18 (of the C# 3.0 spec):

Whenever an expression fulfills the requirements listed above, the expression is evaluated at compile-time. This is true even if the expression is a sub-expression of a larger expression that contains non-constant constructs.

(The "requirements listed above" include the + operator applied to two constant expressions.)

See also this question.

Answer 2

Just a side note on a related subject - the C# compiler will also 'optimize' multiple concatenations involving non-literals using the '+' operator to a single call to a multi-parameter overload of the String.Concat() method.

So

string result = x + y + z;

compiles to something equivalent to

string result = String.Concat( x, y, z);

rather than the more naive possibility:

string result = String.Concat( String.Concat( x, y), z);

Nothing earth-shattering, but just wanted to add this bit to the discussion about string literal concatenation optimization. I don't know whether this behavior is mandated by the language standard or not.

Answer 3

Yes.

C# not only optimizes the concatenation of string literals, it also collapses equivalent string literals into constants and uses pointers to reference all references to the same constant.

Answer 4

Yes - You can see this explicitly using ILDASM.

Example:

Here's a program that is similar to your example followed by the compiled CIL code:

Note: I am using the String.Concat() function just to see how the compiler treats the two different methods of concatenation.

Program

class Program
{
    static void Main(string[] args)
    {
        string s = "test " + "this " + "function";
        string ss = String.Concat("test", "this", "function");
    }
}

ILDASM

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       29 (0x1d)
  .maxstack  3
  .locals init (string V_0,
           string V_1)
  IL_0000:  nop
  IL_0001:  ldstr      "test this function"
  IL_0006:  stloc.0
  IL_0007:  ldstr      "test"
  IL_000c:  ldstr      "this"
  IL_0011:  ldstr      "function"
  IL_0016:  call       string [mscorlib]System.String::Concat(string,
                                                              string,
                                                              string)
  IL_001b:  stloc.1
  IL_001c:  ret
} // end of method Program::Main

Notice how at IL_0001 the compiler created the constant "test this function" as opposed to how the compiler treats the String.Concat() function - which creates a constant for each of the .Concat() params, then calls the .Concat() function.

Answer 5

From the horses mouth:

Concatenation is the process of appending one string to the end of another string. When you concatenate string literals or string constants by using the + operator, the compiler creates a single string. No run time concatenation occurs. However, string variables can be concatenated only at run time. In this case, you should understand the performance implications of the various approaches.

http://msdn.microsoft.com/en-us/library/ms228504.aspx

Answer 6

I had a similar question, but about VB.NET instead of C#. The simplest way of verifying this was to view the compiled assembly under Reflector.

The answer was that both the C# and VB.NET compiler optimise concatenation of string literals.

Answer 7

I believe the answer to that is yes, but you'd have to look at what the compiler spits out ... just compile, and use reflector on it :-)