C# Compile-Time Concatenation For String Constants

Question

Does C# do any compile-time optimization for constant string concatenation? If so, how must my code by written to take advantage of this?

Example: How do these compare at run time?

Console.WriteLine("ABC" + "DEF");

const string s1 = "ABC";
Console.WriteLine(s1 + "DEF");

const string s1 = "ABC";
const string s2 = s1 + "DEF";
Console.WriteLine(s2);

Answer 1

Yes, it does. You can verify this using by using ildasm or Reflector to inspect the code.

static void Main(string[] args) {
    string s = "A" + "B";
    Console.WriteLine(s);
}

is translated to

.method private hidebysig static void  Main(string[] args) cil managed {
    .entrypoint
    // Code size       17 (0x11)
    .maxstack  1
    .locals init ([0] string s)
    IL_0000:  nop
    IL_0001:  ldstr      "AB" // note that "A" + "B" is concatenated to "AB"
    IL_0006:  stloc.0
    IL_0007:  ldloc.0
    IL_0008:  call       void [mscorlib]System.Console::WriteLine(string)
    IL_000d:  nop
    IL_000e:  br.s       IL_0010
    IL_0010:  ret
} // end of method Program::Main

There is something even more interesting but related that happens. If you have a string literal in an assembly, the CLR will only create one object for all instances of that same literal in the assembly.

Thus:

static void Main(string[] args) {
    string s = "A" + "B";
    string t = "A" + "B";
    Console.WriteLine(Object.ReferenceEquals(s, t)); // prints true!
}

will print "True" on the console! This optimization is called string interning.