Fixed a crash on Mono when marshalling types with no fields.

Signed-off-by: Dimitar Dobrev <dpldobrev@yahoo.com>

src/Generator/Passes/CheckAbiParameters.cs

@@ -12,6 +12,9 @@ namespace CppSharp.Passes
     /// and use that to call the function instead. In the case of parameters
     /// then the type of that parameter is converted to a pointer.
     /// 
+    /// Furthermore, there's at least one ABI (System V) that gives to empty structs
+    /// size 1 in C++ and size 0 in C. The former causes crashes in older versions of Mono.
+    /// 
     /// Itanium ABI reference (3.1.4 Return values):
     /// http://refspecs.linux-foundation.org/cxxabi-1.83.html#calls
     ///
@@ -20,6 +23,17 @@ namespace CppSharp.Passes
     /// </summary>
     public class CheckAbiParameters : TranslationUnitPass
     {
+        public override bool VisitClassDecl(Class @class)
+        {
+            if (!base.VisitClassDecl(@class))
+                return false;
+
+            if (@class.Fields.Count == 0)
+                @class.Layout.Size = @class.Layout.DataSize = 0;
+
+            return true;
+        }
+
         public override bool VisitFunctionDecl(Function function)
         {
             if (!VisitDeclaration(function))

tests/Basic/Basic.Tests.cs

@@ -516,5 +516,11 @@ public class BasicTests : GeneratorTestFixture
     {
         Assert.AreEqual(10, Foo.@unsafe);
     }
+
+    [Test]
+    public void TestMarshallingEmptyType()
+    {
+        var empty = new ReturnsEmpty().Empty;
+    }
 }
  

tests/Basic/Basic.cpp

@@ -425,3 +425,8 @@ int ChangedAccessOfInheritedProperty::getProperty()
 void ChangedAccessOfInheritedProperty::setProperty(int value)
 {
 }
+
+Empty ReturnsEmpty::getEmpty()
+{
+    return Empty();
+}

tests/Basic/Basic.h

@@ -787,3 +787,13 @@ protected:
     int getProperty();
     void setProperty(int value);
 };
+
+class DLL_API Empty
+{
+};
+
+class DLL_API ReturnsEmpty
+{
+public:
+    Empty getEmpty();
+};