Prefer simple if statements over switch with single case block

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.cs

@@ -988,6 +988,14 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return false;
 		}
 
+		public static void SingleIntervalIf(char c)
+		{
+			if (c >= 'A' && c <= 'Z') {
+				Console.WriteLine("alphabet");
+			}
+			Console.WriteLine("end");
+		}
+		
 		public static bool Loop8(char c, bool b, Func<char> getChar)
 		{
 			if (b) {

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.il

@@ -3345,6 +3345,38 @@
     IL_004f:  ret
   } // end of method Switch::SwitchlikeIf2
 
+  .method public hidebysig static void  SingleIntervalIf(char c) cil managed
+  {
+    // Code size       44 (0x2c)
+    .maxstack  2
+    .locals init (bool V_0)
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  ldc.i4.s   65
+    IL_0004:  blt.s      IL_000d
+
+    IL_0006:  ldarg.0
+    IL_0007:  ldc.i4.s   90
+    IL_0009:  cgt
+    IL_000b:  br.s       IL_000e
+
+    IL_000d:  ldc.i4.1
+    IL_000e:  nop
+    IL_000f:  stloc.0
+    IL_0010:  ldloc.0
+    IL_0011:  brtrue.s   IL_0020
+
+    IL_0013:  nop
+    IL_0014:  ldstr      "alphabet"
+    IL_0019:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_001e:  nop
+    IL_001f:  nop
+    IL_0020:  ldstr      "end"
+    IL_0025:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_002a:  nop
+    IL_002b:  ret
+  } // end of method Switch::SingleIntervalIf
+
   .method public hidebysig static bool  Loop8(char c,
                                               bool b,
                                               class [mscorlib]System.Func`1<char> getChar) cil managed

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.opt.il

@@ -2530,6 +2530,25 @@
     IL_0024:  ret
   } // end of method Switch::SwitchlikeIf2
 
+  .method public hidebysig static void  SingleIntervalIf(char c) cil managed
+  {
+    // Code size       31 (0x1f)
+    .maxstack  8
+    IL_0000:  ldarg.0
+    IL_0001:  ldc.i4.s   65
+    IL_0003:  blt.s      IL_0014
+
+    IL_0005:  ldarg.0
+    IL_0006:  ldc.i4.s   90
+    IL_0008:  bgt.s      IL_0014
+
+    IL_000a:  ldstr      "alphabet"
+    IL_000f:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_0014:  ldstr      "end"
+    IL_0019:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_001e:  ret
+  } // end of method Switch::SingleIntervalIf
+
   .method public hidebysig static bool  Loop8(char c,
                                               bool b,
                                               class [mscorlib]System.Func`1<char> getChar) cil managed

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.opt.roslyn.il

@@ -2691,6 +2691,25 @@
     IL_0024:  ret
   } // end of method Switch::SwitchlikeIf2
 
+  .method public hidebysig static void  SingleIntervalIf(char c) cil managed
+  {
+    // Code size       31 (0x1f)
+    .maxstack  8
+    IL_0000:  ldarg.0
+    IL_0001:  ldc.i4.s   65
+    IL_0003:  blt.s      IL_0014
+
+    IL_0005:  ldarg.0
+    IL_0006:  ldc.i4.s   90
+    IL_0008:  bgt.s      IL_0014
+
+    IL_000a:  ldstr      "alphabet"
+    IL_000f:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_0014:  ldstr      "end"
+    IL_0019:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_001e:  ret
+  } // end of method Switch::SingleIntervalIf
+
   .method public hidebysig static bool  Loop8(char c,
                                               bool b,
                                               class [mscorlib]System.Func`1<char> getChar) cil managed

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Switch.roslyn.il

@@ -3694,6 +3694,39 @@
     IL_0049:  ret
   } // end of method Switch::SwitchlikeIf2
 
+  .method public hidebysig static void  SingleIntervalIf(char c) cil managed
+  {
+    // Code size       46 (0x2e)
+    .maxstack  2
+    .locals init (bool V_0)
+    IL_0000:  nop
+    IL_0001:  ldarg.0
+    IL_0002:  ldc.i4.s   65
+    IL_0004:  blt.s      IL_0010
+
+    IL_0006:  ldarg.0
+    IL_0007:  ldc.i4.s   90
+    IL_0009:  cgt
+    IL_000b:  ldc.i4.0
+    IL_000c:  ceq
+    IL_000e:  br.s       IL_0011
+
+    IL_0010:  ldc.i4.0
+    IL_0011:  stloc.0
+    IL_0012:  ldloc.0
+    IL_0013:  brfalse.s  IL_0022
+
+    IL_0015:  nop
+    IL_0016:  ldstr      "alphabet"
+    IL_001b:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_0020:  nop
+    IL_0021:  nop
+    IL_0022:  ldstr      "end"
+    IL_0027:  call       void [mscorlib]System.Console::WriteLine(string)
+    IL_002c:  nop
+    IL_002d:  ret
+  } // end of method Switch::SingleIntervalIf
+
   .method public hidebysig static bool  Loop8(char c,
                                               bool b,
                                               class [mscorlib]System.Func`1<char> getChar) cil managed

ICSharpCode.Decompiler/IL/ControlFlow/SwitchDetection.cs

@@ -198,6 +198,10 @@ namespace ICSharpCode.Decompiler.IL.ControlFlow
 			if (ifCount < labelCount)
 				return false;
 
+			// don't create switch statements with only one non-default label (provided the if option is short enough)
+			if (analysis.Sections.Count == 2 && ifCount <= 2)
+				return false;
+			
 			// if there is no ILSwitch, there's still many control flow patterns that 
 			// match a switch statement but were originally just regular if statements,
 			// and converting them to switches results in poor quality code with goto statements