Does the C# compiler remove an if that encapsulates a debug.writeline

Yes, it does, at least for the Debug call. I can’t see here if the JIT compiler also removed the evaluation of the if, but I guess it does since the equation doesn’t have any side effects.

However, you are better of keeping it safe by calling Debug.WriteLineIf, which doesn’t depend on the JIT compiler to remove the evaluation.

For completeness the proof for the compiler to remove the Debug.WriteLine.

The code in Release build:

.method public hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       17 (0x11)
  .maxstack  8
  IL_0000:  call       string [mscorlib]System.Console::ReadLine()
  IL_0005:  ldstr      "Ok"
  IL_000a:  call       bool [mscorlib]System.String::op_Inequality(string,
                                                                   string)
  IL_000f:  pop
  IL_0010:  ret
} // end of method Program::Main

Code in Debug build:

.method public hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       42 (0x2a)
  .maxstack  2
  .locals init ([0] string state,
           [1] bool V_1)
  IL_0000:  nop
  IL_0001:  call       string [mscorlib]System.Console::ReadLine()
  IL_0006:  stloc.0
  IL_0007:  ldloc.0
  IL_0008:  ldstr      "Ok"
  IL_000d:  call       bool [mscorlib]System.String::op_Inequality(string,
                                                                   string)
  IL_0012:  stloc.1
  IL_0013:  ldloc.1
  IL_0014:  brfalse.s  IL_0029
  IL_0016:  nop
  IL_0017:  ldstr      "Error occured: {0}"
  IL_001c:  ldloc.0
  IL_001d:  call       string [mscorlib]System.String::Format(string,
                                                              object)
  IL_0022:  call       void [System]System.Diagnostics.Debug::WriteLine(string)
  IL_0027:  nop
  IL_0028:  nop
  IL_0029:  ret
} // end of method Program::Main

As you see the Release mode has no call to Debug.WriteLine, where the Debug mode does.