Why Does The Compiler Add An Unnecessary Local Variable

Question

This C# code:

private void LoadAssignments(AssignmentType assignmentType, Collection<Assignment> assignments)
    {
        bool flag;
        DataTable lessons = this.GetResults(assignmentType);
        try
        {
            IEnumerator enumerator = lessons.Rows.GetEnumerator();
            try
            {
                while (true)
                {
                    flag = enumerator.MoveNext();
                    if (!flag)
                    {
                        break;
                    }
                    DataRow row = (DataRow)enumerator.Current;
                }
            }
            finally
            {
                IDisposable disposable = enumerator as IDisposable;
                flag = disposable == null;
                if (!flag)
                {
                    disposable.Dispose();
                }
            }
        }
        finally
        {
            flag = lessons == null;
            if (!flag)
            {
                lessons.Dispose();
            }
        }
    }

produces this CIL (.NET 4)

    .method private hidebysig 
    instance void LoadAssignments (
        valuetype TTReporterCore.AssignmentType assignmentType,
        class [mscorlib]System.Collections.ObjectModel.Collection`1<valuetype TTReporterCore.Assignment> assignments
    ) cil managed 
{
    .locals init (
        [0] bool flag,
        [1] class [System.Data]System.Data.DataTable lessons,
        [2] class [mscorlib]System.Collections.IEnumerator enumerator,
        [3] class [System.Data]System.Data.DataRow row,
        [4] class [mscorlib]System.IDisposable disposable,
        [5] bool flag1
    )

    IL_0000: nop
    IL_0001: ldarg.0
    IL_0002: ldarg.1
    IL_0003: call instance class [System.Data]System.Data.DataTable TTReporterCore.TTReader::GetResults(valuetype TTReporterCore.AssignmentType)
    IL_0008: stloc.1
    .try
    {
        IL_0009: nop
        IL_000a: ldloc.1
        IL_000b: callvirt instance class [System.Data]System.Data.DataRowCollection [System.Data]System.Data.DataTable::get_Rows()
        IL_0010: callvirt instance class [mscorlib]System.Collections.IEnumerator [System.Data]System.Data.InternalDataCollectionBase::GetEnumerator()
        IL_0015: stloc.2
        .try
        {
            IL_0016: nop
            IL_0017: br.s IL_0038
            .loop
            {
                IL_0019: nop
                IL_001a: ldloc.2
                IL_001b: callvirt instance bool [mscorlib]System.Collections.IEnumerator::MoveNext()
                IL_0020: stloc.0
                IL_0021: ldloc.0
                IL_0022: stloc.s flag1
                IL_0024: ldloc.s flag1
                IL_0026: brtrue.s IL_002b

                IL_0028: nop
                IL_0029: br.s IL_003d

                IL_002b: ldloc.2
                IL_002c: callvirt instance object [mscorlib]System.Collections.IEnumerator::get_Current()
                IL_0031: castclass [System.Data]System.Data.DataRow
                IL_0036: stloc.3
                IL_0037: nop

                IL_0038: ldc.i4.1
                IL_0039: stloc.s flag1
                IL_003b: br.s IL_0019
            }

            IL_003d: nop
            IL_003e: leave.s IL_0062
        }
        finally
        {
            IL_0040: nop
            IL_0041: ldloc.2
            IL_0042: isinst [mscorlib]System.IDisposable
            IL_0047: stloc.s disposable
            IL_0049: ldloc.s disposable
            IL_004b: ldnull
            IL_004c: ceq
            IL_004e: stloc.0
            IL_004f: ldloc.0
            IL_0050: stloc.s flag1
            IL_0052: ldloc.s flag1
            IL_0054: brtrue.s IL_0060

            IL_0056: nop
            IL_0057: ldloc.s disposable
            IL_0059: callvirt instance void [mscorlib]System.IDisposable::Dispose()
            IL_005e: nop
            IL_005f: nop

            IL_0060: nop
            IL_0061: endfinally
        }

        IL_0062: nop
        IL_0063: nop
        IL_0064: leave.s IL_007e
    }
    finally
    {
        IL_0066: nop
        IL_0067: ldloc.1
        IL_0068: ldnull
        IL_0069: ceq
        IL_006b: stloc.0
        IL_006c: ldloc.0
        IL_006d: stloc.s flag1
        IL_006f: ldloc.s flag1
        IL_0071: brtrue.s IL_007c

        IL_0073: nop
        IL_0074: ldloc.1
        IL_0075: callvirt instance void [System]System.ComponentModel.MarshalByValueComponent::Dispose()
        IL_007a: nop
        IL_007b: nop

        IL_007c: nop
        IL_007d: endfinally
    }

    IL_007e: nop
    IL_007f: ret
}

Why does the MSIL add the flag1, continue to perform the same logic to set flag, set flag1 to flag and finally, check for !flag1. This seems like a compiler inefficiency to me.

UPDATE: I was using Telerik's JustDecompile and while the results are quite different from ILDASM, the additional boolean is still created in debug mode.

Also, I modified the code by removing the boolean completely and the debug version still adds a boolean. I'm really looking for why the compiler does this.

Answer 1

It seems like a temporary local is created to hold the results of the comparison (i.e. disposable == null).

Trying this example:

class Program
{
   static void Main()
   {
       if (1 == 1) return;
   }
}

..is producing the following IL on my box (Microsoft (R) Visual C# 2010 Compiler version 4.0.30319.1):

.method private hidebysig static void  Main() cil managed
{
  .entrypoint
  .maxstack  1
  .locals init (bool V_0)
  IL_0000:  nop
  IL_0001:  ldc.i4.0
  IL_0002:  stloc.0
  IL_0003:  br.s       IL_0005
  IL_0005:  ret
}

local V_0 is created, even though it's actually not used. I believe it's an obvious optimisation even for 'non' optimized compilation :) ..or more likely: all necessary code is generated to enable debugging. I don't know how it might be used in a debugging session, but it's my best guess.

When compiled optimized (i.e. Release configuration) I don't see the extra local.