Support interface types in configuration values

Author: nblumhardt

src/Serilog.Settings.Configuration/Properties/AssemblyInfo.cs

@@ -19,5 +19,9 @@
 // COM, set the ComVisible attribute to true on that type.
 [assembly: ComVisible(false)]
 
-// The following GUID is for the ID of the typelib if this project is exposed to COM
-[assembly: Guid("21ff98ed-e68c-4a67-b241-c8d6122fad7d")]
+[assembly: InternalsVisibleTo("Serilog.Settings.Configuration.Tests, PublicKey=" +
+                              "0024000004800000940000000602000000240000525341310004000001000100fb8d13fd344a1c" +
+                              "6fe0fe83ef33c1080bf30690765bc6eb0df26ebfdf8f21670c64265b30db09f73a0dea5b3db4c9" +
+                              "d18dbf6d5a25af5ce9016f281014d79dc3b4201ac646c451830fc7e61a2dfd633d34c39f87b818" +
+                              "94191652df5ac63cc40c77f3542f702bda692e6e8a9158353df189007a49da0f3cfd55eb250066" +
+                              "b19485ec")]

src/Serilog.Settings.Configuration/Settings/Configuration/ConfigurationReader.cs

@@ -183,6 +183,13 @@ static void CallConfigurationMethods(Dictionary<string, Dictionary<string, strin
             }
         }
 
+        static readonly Dictionary<Type, Func<string, object>> ExtendedTypeConversions = new Dictionary<Type, Func<string, object>>
+            {
+                { typeof(Uri), s => new Uri(s) },
+                { typeof(TimeSpan), s => TimeSpan.Parse(s) }
+            };
+
+
         internal static object ConvertToType(string value, Type toType)
         {
             var toTypeInfo = toType.GetTypeInfo();
@@ -199,18 +206,34 @@ internal static object ConvertToType(string value, Type toType)
             if (toTypeInfo.IsEnum)
                 return Enum.Parse(toType, value);
 
-            var extendedTypeConversions = new Dictionary<Type, Func<string, object>>
-            {
-                { typeof(Uri), s => new Uri(s) },
-                { typeof(TimeSpan), s => TimeSpan.Parse(s) }
-            };
-
-            var convertor = extendedTypeConversions
+            var convertor = ExtendedTypeConversions
                 .Where(t => t.Key.GetTypeInfo().IsAssignableFrom(toTypeInfo))
                 .Select(t => t.Value)
                 .FirstOrDefault();
 
-            return convertor == null ? Convert.ChangeType(value, toType) : convertor(value);
+            if (convertor != null)
+                 return convertor(value);
+ 
+            if (toTypeInfo.IsInterface && !string.IsNullOrWhiteSpace(value))
+            {
+                var type = Type.GetType(value.Trim(), throwOnError: false);
+                if (type != null)
+                {
+                    var ctor = type.GetTypeInfo().DeclaredConstructors.FirstOrDefault(ci =>
+                    {
+                        var parameters = ci.GetParameters();
+                        return parameters.Length == 0 || parameters.All(pi => pi.HasDefaultValue);
+                    });
+ 
+                    if (ctor == null)
+                        throw new InvalidOperationException($"A default constructor was not found on {type.FullName}.");
+ 
+                    var call = ctor.GetParameters().Select(pi => pi.DefaultValue).ToArray();
+                    return ctor.Invoke(call);
+                }
+            }
+
+            return Convert.ChangeType(value, toType);
         }
 
         internal static IList<MethodInfo> FindSinkConfigurationMethods(IEnumerable<Assembly> configurationAssemblies)