C# Compile-Time Concatenation For String Constants
Does C开发者_JAVA百科# 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);
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.
According to Reflector:
Console.WriteLine("ABCDEF");
Console.WriteLine("ABCDEF");
Console.WriteLine("ABCDEF");
even in a Debug configuration.
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