Minor refactorings.

src/Generator/Generators/C/CppHeaders.cs

@@ -101,7 +101,7 @@ namespace CppSharp.Generators.Cpp
             }
 
             foreach (var include in includes)
-                WriteLine(include);
+                WriteLine($"#include \"{include}\"");
         }
 
         private Namespace FindCreateNamespace(Namespace @namespace, Declaration decl)

src/Generator/Generators/CLI/CLISources.cs

@@ -21,7 +21,7 @@ namespace CppSharp.Generators.CLI
         {
         }
 
-        public override string FileExtension { get { return "cpp"; } }
+        public override string FileExtension => "cpp";
 
         public override void Process()
         {
@@ -45,7 +45,7 @@ namespace CppSharp.Generators.CLI
             WriteLine("using namespace System::Runtime::InteropServices;");
             foreach (var customUsingStatement in Options.DependentNameSpaces)
             {
-                WriteLine(string.Format("using namespace {0};", customUsingStatement)); ;
+                WriteLine($"using namespace {customUsingStatement};"); ;
             }
             NewLine();
             PopBlock();