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copied!<p>Previous answer deleted as I'd misread the code. (Using "default(bool)" anywhere is a bit odd, btw.)</p> <p>However, unless the variable is captured by a delegate etc, I'd expect them to <em>either</em> compile to IL which is effectively the same (in terms of both behaviour and performance).</p> <p>As ever, write the most <em>readable</em> code first. Micro-optimising things like this is asking for trouble. I agree with the others who have suggested that you restrict the scope of variables as much as you can - so if you need it after the loop, you haven't got any choice anyway; otherwise declare it inside.</p> <p>Okay, here's a test program:</p> <pre><code>using System; class Test { static void Main() {} static void DeclareInside() { for (int i=0; i < 10; i++) { bool x = false; for (int j=5; j < 20; j++) { if (i == j) { x = true; break; } if (x) { Console.WriteLine("Yes"); } } } } static void DeclareOutside() { bool x; for (int i=0; i < 10; i++) { x = false; for (int j=5; j < 20; j++) { if (i == j) { x = true; break; } if (x) { Console.WriteLine("Yes"); } } } } } </code></pre> <p>Generated IL (just <code>csc Test.cs</code>):</p> <pre><code>.method private hidebysig static void DeclareOutside() cil managed { // Code size 79 (0x4f) .maxstack 2 .locals init (bool V_0, int32 V_1, int32 V_2, bool V_3) IL_0000: nop IL_0001: ldc.i4.0 IL_0002: stloc.1 IL_0003: br.s IL_0045 IL_0005: nop IL_0006: ldc.i4.0 IL_0007: stloc.0 IL_0008: ldc.i4.5 IL_0009: stloc.2 IL_000a: br.s IL_0037 IL_000c: nop IL_000d: ldloc.1 IL_000e: ldloc.2 IL_000f: ceq IL_0011: ldc.i4.0 IL_0012: ceq IL_0014: stloc.3 IL_0015: ldloc.3 IL_0016: brtrue.s IL_001d IL_0018: nop IL_0019: ldc.i4.1 IL_001a: stloc.0 IL_001b: br.s IL_0040 IL_001d: ldloc.0 IL_001e: ldc.i4.0 IL_001f: ceq IL_0021: stloc.3 IL_0022: ldloc.3 IL_0023: brtrue.s IL_0032 IL_0025: nop IL_0026: ldstr "Yes" IL_002b: call void [mscorlib]System.Console::WriteLine(string) IL_0030: nop IL_0031: nop IL_0032: nop IL_0033: ldloc.2 IL_0034: ldc.i4.1 IL_0035: add IL_0036: stloc.2 IL_0037: ldloc.2 IL_0038: ldc.i4.s 20 IL_003a: clt IL_003c: stloc.3 IL_003d: ldloc.3 IL_003e: brtrue.s IL_000c IL_0040: nop IL_0041: ldloc.1 IL_0042: ldc.i4.1 IL_0043: add IL_0044: stloc.1 IL_0045: ldloc.1 IL_0046: ldc.i4.s 10 IL_0048: clt IL_004a: stloc.3 IL_004b: ldloc.3 IL_004c: brtrue.s IL_0005 IL_004e: ret } // end of method Test::DeclareOutside .method private hidebysig static void DeclareInside() cil managed { // Code size 79 (0x4f) .maxstack 2 .locals init (int32 V_0, bool V_1, int32 V_2, bool V_3) IL_0000: nop IL_0001: ldc.i4.0 IL_0002: stloc.0 IL_0003: br.s IL_0045 IL_0005: nop IL_0006: ldc.i4.0 IL_0007: stloc.1 IL_0008: ldc.i4.5 IL_0009: stloc.2 IL_000a: br.s IL_0037 IL_000c: nop IL_000d: ldloc.0 IL_000e: ldloc.2 IL_000f: ceq IL_0011: ldc.i4.0 IL_0012: ceq IL_0014: stloc.3 IL_0015: ldloc.3 IL_0016: brtrue.s IL_001d IL_0018: nop IL_0019: ldc.i4.1 IL_001a: stloc.1 IL_001b: br.s IL_0040 IL_001d: ldloc.1 IL_001e: ldc.i4.0 IL_001f: ceq IL_0021: stloc.3 IL_0022: ldloc.3 IL_0023: brtrue.s IL_0032 IL_0025: nop IL_0026: ldstr "Yes" IL_002b: call void [mscorlib]System.Console::WriteLine(string) IL_0030: nop IL_0031: nop IL_0032: nop IL_0033: ldloc.2 IL_0034: ldc.i4.1 IL_0035: add IL_0036: stloc.2 IL_0037: ldloc.2 IL_0038: ldc.i4.s 20 IL_003a: clt IL_003c: stloc.3 IL_003d: ldloc.3 IL_003e: brtrue.s IL_000c IL_0040: nop IL_0041: ldloc.0 IL_0042: ldc.i4.1 IL_0043: add IL_0044: stloc.0 IL_0045: ldloc.0 IL_0046: ldc.i4.s 10 IL_0048: clt IL_004a: stloc.3 IL_004b: ldloc.3 IL_004c: brtrue.s IL_0005 IL_004e: ret } // end of method Test::DeclareInside </code></pre> <p>The only differences are where the variables are located within the stack.</p>

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