Fixes: U4-3591 Creating PropertyValueConverter for a v7 prop editor (created with manifest) ends up with YSOD when outputting value on frontend

src/Umbraco.Core/Models/PublishedContent/PublishedPropertyType.cs

@@ -82,24 +82,38 @@ namespace Umbraco.Core.Models.PublishedContent
         {
             var converters = PropertyValueConvertersResolver.Current.Converters.ToArray();
 
-            // todo: remove Union() once we drop IPropertyEditorValueConverter support.
-            _converter = null;
-            foreach (var converter in converters.Union(GetCompatConverters()).Where(x => x.IsConverter(this)))
-            {
-                if (_converter == null)
-                {
-                    _converter = converter;
-                }
-                else
-                {
-                    throw new InvalidOperationException(string.Format("Type '{2}' cannot be an IPropertyValueConverter"
-                        + " for property '{1}' of content type '{0}' because type '{3}' has already been detected as a converter"
-                        + " for that property, and only one converter can exist for a property.",
-                        ContentType.Alias, PropertyTypeAlias,
-                        converter.GetType().FullName, _converter.GetType().FullName));
-                }
-            }
+            var defaultConverters = converters
+                .Where(x => x.GetType().GetCustomAttribute<DefaultPropertyValueConverterAttribute>(false) != null)
+                .ToArray();
 
+            _converter = null;
+            
+            //get all converters for this property type
+            // todo: remove Union() once we drop IPropertyEditorValueConverter support.
+            var foundConverters = converters.Union(GetCompatConverters()).Where(x => x.IsConverter(this)).ToArray();
+            if (foundConverters.Length == 1)
+            {
+                _converter = foundConverters[0];
+            }
+            else if (foundConverters.Length > 1)
+            {
+                //more than one was found, we need to first figure out if one of these is an Umbraco default value type converter
+                var nonDefault = foundConverters.Except(defaultConverters).ToArray();
+                if (nonDefault.Length > 1)
+                {
+                    //this is not allowed, there cannot be more than 1 custom converter
+                    throw new InvalidOperationException(
+                        string.Format("Type '{2}' cannot be an IPropertyValueConverter"
+                                      + " for property '{1}' of content type '{0}' because type '{3}' has already been detected as a converter"
+                                      + " for that property, and only one converter can exist for a property.",
+                                      ContentType.Alias, PropertyTypeAlias,
+                                      nonDefault[1].GetType().FullName, nonDefault[0].GetType().FullName));
+                }
+
+                //there's only 1 custom converter registered that so use it
+                _converter = nonDefault[0];
+            }
+            
             // get the cache levels, quietely fixing the inconsistencies (no need to throw, really)
             _sourceCacheLevel = GetCacheLevel(_converter, PropertyCacheValue.Source);
             _objectCacheLevel = GetCacheLevel(_converter, PropertyCacheValue.Object);

src/Umbraco.Core/PropertyEditors/DefaultPropertyValueConverterAttribute.cs

@@ -0,0 +1,12 @@
+using System;
+
+namespace Umbraco.Core.PropertyEditors
+{
+    /// <summary>
+    /// Indicates that this is a default property value converter (shipped with Umbraco)
+    /// </summary>
+    [AttributeUsage(AttributeTargets.Class, AllowMultiple = false, Inherited = false)]
+    internal class DefaultPropertyValueConverterAttribute : Attribute
+    {
+    }
+}

src/Umbraco.Core/PropertyEditors/ValueConverters/JsonValueConverter.cs

@@ -6,6 +6,13 @@ using Umbraco.Core.Models.PublishedContent;
 
 namespace Umbraco.Core.PropertyEditors.ValueConverters
 {
+    /// <summary>
+    /// The default converter for all property editors that expose a JSON value type
+    /// </summary>
+    /// <remarks>
+    /// Since this is a default (umbraco) converter it will be ignored if another converter found conflicts with this one.
+    /// </remarks>
+    [DefaultPropertyValueConverter]
     [PropertyValueType(typeof(JToken))]
     [PropertyValueCache(PropertyCacheValue.All, PropertyCacheLevel.Content)]
     public class JsonValueConverter : PropertyValueConverterBase

src/Umbraco.Core/Umbraco.Core.csproj

@@ -331,6 +331,7 @@
     <Compile Include="Models\TaggableObjectTypes.cs" />
     <Compile Include="Models\TemplateNode.cs" />
     <Compile Include="Packaging\PackageBinaryInspector.cs" />
+    <Compile Include="PropertyEditors\DefaultPropertyValueConverterAttribute.cs" />
     <Compile Include="PropertyEditors\IValueEditor.cs" />
     <Compile Include="PropertyEditors\PropertyCacheValue.cs" />
     <Compile Include="PropertyEditors\PropertyValueCacheAttribute.cs" />