Minor code readability refactor.

src/AST/Class.cs

@@ -96,17 +96,14 @@ namespace CppSharp.AST
         // True if the class is final / sealed.
         public bool IsFinal { get; set; }
 
-        private bool? isOpaque = null;
+        private bool? isOpaque;
 
         public bool IsInjected { get; set; }
 
         // True if the type is to be treated as opaque.
         public bool IsOpaque
         {
-            get
-            {
-                return isOpaque == null ? IsIncomplete && CompleteDeclaration == null : isOpaque.Value;
-            }
+            get => isOpaque ?? (IsIncomplete && CompleteDeclaration == null);
             set
             {
                 isOpaque = value;

src/CLI/CLI.cs

@@ -152,12 +152,12 @@ namespace CppSharp
                     options.HeaderFiles.Add(args);
                 else
                 {
-                    errorMessages.Add(string.Format("File '{0}' could not be found.", args));
+                    errorMessages.Add($"File '{args}' could not be found.");
                 }
             }
             catch(Exception)
             {
-                errorMessages.Add(string.Format("Error while looking for files inside path '{0}'. Ignoring.", args));
+                errorMessages.Add($"Error while looking for files inside path '{args}'. Ignoring.");
             }
         }