Fix warnings about unnecessary using directives. (#1489)

src/Generator/Generators/CSharp/CSharpSources.cs

@@ -153,18 +153,21 @@ namespace CppSharp.Generators.CSharp
         public virtual void GenerateUsings()
         {
             PushBlock(BlockKind.Usings);
-            WriteLine("using System;");
+            var requiredNameSpaces = new List<string> {
-            WriteLine("using System.Runtime.InteropServices;");
+                "System",
-            WriteLine("using System.Security;");
+                "System.Runtime.InteropServices",
+                "System.Security",
+            };
 
             var internalsVisibleTo = (from m in Options.Modules
                                       where m.Dependencies.Contains(Module)
                                       select m.LibraryName).ToList();
+
             if (internalsVisibleTo.Any())
-                WriteLine("using System.Runtime.CompilerServices;");
+                requiredNameSpaces.Add("System.Runtime.CompilerServices");
 
-            foreach (var customUsingStatement in Options.DependentNameSpaces)
+            foreach (var @namespace in requiredNameSpaces.Union(Options.DependentNameSpaces).OrderBy(x => x))
-                WriteLine("using {0};", customUsingStatement);
+                WriteLine($"using {@namespace};");
 
             WriteLine("using __CallingConvention = global::System.Runtime.InteropServices.CallingConvention;");
             WriteLine("using __IntPtr = global::System.IntPtr;");

tests/NamespacesDerived/NamespacesDerived.cs

@@ -18,7 +18,7 @@ namespace CppSharp.Tests
             base.Setup(driver);
             driver.Options.GenerateDefaultValuesForArguments = true;
             driver.Options.GenerateClassTemplates = true;
-
+            driver.Options.DependentNameSpaces.Add("System.Runtime.CompilerServices");
             driver.Options.Modules[1].IncludeDirs.Add(GetTestsDirectory("NamespacesDerived"));
             var @base = "NamespacesBase";
             var module = driver.Options.AddModule(@base);
@@ -35,7 +35,6 @@ namespace CppSharp.Tests
             driver.Generator.OnUnitGenerated += o =>
             {
                 Block firstBlock = o.Outputs[0].RootBlock.Blocks[1];
-                firstBlock.WriteLine("using System.Runtime.CompilerServices;");
                 firstBlock.NewLine();
                 firstBlock.WriteLine("[assembly:InternalsVisibleTo(\"NamespacesDerived.CSharp\")]");
             };