Question

Consider this C# snippet:

static string input = null; static string output = null;  static void Main(string[] args) {      input = 'input';      output = CallMe(input); }  public static string CallMe(string input) {      output = 'output';      return output; } 

Dissassembling using Reflector shows:

.method private hidebysig static void Main(string[] args) cil managed     {         .entrypoint         .maxstack 8         L_0000: nop          L_0001: ldstr 'input'         L_0006: stsfld string Reflector_Test.Program::input         L_000b: ldsfld string Reflector_Test.Program::input         L_0010: call string Reflector_Test.Program::CallMe(string)         L_0015: stsfld string Reflector_Test.Program::output         L_001a: ret      }   .method public hidebysig static string CallMe(string input) cil managed     {         .maxstack 1         .locals init (             [0] string CS$1$0000)         L_0000: nop          L_0001: ldstr 'output'         L_0006: stsfld string Reflector_Test.Program::output         L_000b: ldsfld string Reflector_Test.Program::output         L_0010: stloc.0          L_0011: br.s L_0013         L_0013: ldloc.0          L_0014: ret      } 

The piece that puzzles me is:

L_0010: stloc.0  L_0011: br.s L_0013 L_0013: ldloc.0  

It stores the item, branches to the next line (which would have been executed anyway) and then loads it again.

Is there a reason for this?

Answer 1

This only happens in Debug, not in Release. I suspect its to assist during debugging. It perhaps allows you to chuck breakpoints mid statement and see the return value.

Note the release version has much more concise IL:

.method private hidebysig static void Main(string[] args) cil managed {     .maxstack 8     L_0000: ldstr 'input'     L_0005: stsfld string Reflector_Test.Program::input     L_000a: ldsfld string Reflector_Test.Program::input     L_000f: call string Reflector_Test.Program::CallMe(string)     L_0014: stsfld string Reflector_Test.Program::output     L_0019: ret  }     .method public hidebysig static string CallMe(string input) cil managed {     .maxstack 8     L_0000: ldstr 'output'     L_0005: stsfld string Reflector_Test.Program::output     L_000a: ldsfld string Reflector_Test.Program::output     L_000f: ret  }