fix #167 - Incorrect decompilation of null as extension method this parameter

ICSharpCode.Decompiler/Ast/AstMethodBodyBuilder.cs

@@ -906,7 +906,13 @@ namespace ICSharpCode.Decompiler.Ast
 					target = ((DirectionExpression)target).Expression;
 					target.Remove(); // detach from DirectionExpression
 				}
-				if (cecilMethodDef != null && cecilMethodDef.DeclaringType.IsInterface) {
+				
+				if (cecilMethodDef != null) {
+					// convert null.ToLower() to ((string)null).ToLower()
+					if (target is NullReferenceExpression)
+						target = target.CastTo(AstBuilder.ConvertType(cecilMethod.DeclaringType));
+					
+					if (cecilMethodDef.DeclaringType.IsInterface) {
 						TypeReference tr = byteCode.Arguments[0].InferredType;
 						if (tr != null) {
 							TypeDefinition td = tr.Resolve();
@@ -917,6 +923,7 @@ namespace ICSharpCode.Decompiler.Ast
 							}
 						}
 					}
+				}
 			} else {
 				target = new TypeReferenceExpression { Type = AstBuilder.ConvertType(cecilMethod.DeclaringType) };
 			}

ICSharpCode.Decompiler/Ast/Transforms/IntroduceExtensionMethods.cs

@@ -45,7 +45,11 @@ namespace ICSharpCode.Decompiler.Ast.Transforms
 					if (d != null) {
 						foreach (var ca in d.CustomAttributes) {
 							if (ca.AttributeType.Name == "ExtensionAttribute" && ca.AttributeType.Namespace == "System.Runtime.CompilerServices") {
-								mre.Target = invocation.Arguments.First().Detach();
+								var firstArgument = invocation.Arguments.First();
+								if (firstArgument is NullReferenceExpression)
+									firstArgument = firstArgument.ReplaceWith(expr => expr.CastTo(AstBuilder.ConvertType(d.Parameters.First().ParameterType)));
+								else
+									mre.Target = firstArgument.Detach();
 								if (invocation.Arguments.Any()) {
 									// HACK: removing type arguments should be done indepently from whether a method is an extension method,
 									// just by testing whether the arguments can be inferred