Fix a bug when getting a string containing a substitution as an array element (#88)

Author: iron9light

src/Hocon.Tests/Substitution.cs

@@ -171,6 +171,35 @@ public void CanConcatenateQuotedString()
             Assert.Equal("Hello my name is Roger", Parser.Parse(hocon).GetString("a.c"));
         }
 
+        /*
+         * FACT:
+         * To get a string containing a substitution as an array element, 
+         * you must use value concatenation with the substitution in the unquoted portion
+         */
+        [Fact]
+        public void CanConcatenateUnquotedStringOfArrayElement()
+        {
+            var hocon = @"a {
+  name = Roger
+  c = [Hello my name is ${a.name}]
+}";
+            Assert.Equal("Hello my name is Roger", Parser.Parse(hocon).GetStringList("a.c")[0]);
+        }
+
+        /*
+         * FACT:
+         * or can use value concatenation of a substitution and a quoted string.
+         */
+        [Fact]
+        public void CanConcatenateQuotedStringOfArrayElement()
+        {
+            var hocon = @"a {
+  name = Roger
+  c = [""Hello my name is ""${a.name}]
+}";
+            Assert.Equal("Hello my name is Roger", Parser.Parse(hocon).GetStringList("a.c")[0]);
+        }
+
         /*
          * FACT:
          * Substitutions are resolved by looking up the path in the configuration. 

src/Hocon/Impl/HoconValue.cs

@@ -640,7 +640,10 @@ internal void ResolveValue(IHoconElement child)
                     $"Sibling type:{Type}, substitution type:{child.Type}");
             }
 
-            ((HoconField) Parent).ResolveValue(this);
+            if (Parent is HoconField hoconField)
+            {
+                hoconField.ResolveValue(this);
+            }
         }
 
         /// <summary>