(Always helps when you post with the right account and therefore the right e-mail address to follow the thread... 😫)

To be honest, I'm not sure if this is a Ditto issue or a NuPickers issue or what, but I'm trying here first!

So, I've been using the code here on multiple projects with NuPickers (although this is my first using it on 7.6.3) https://github.com/leekelleher/umbraco-ditto/issues/175#issuecomment-264146559. With the site I'm setting up now, I'm getting the following error:

Unable to cast object of type 'nuPickers.Picker[]' to type 'System.Collections.Generic.IEnumerable`1[Client.Web.Models.FeatureFacility]'.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidCastException: Unable to cast object of type 'nuPickers.Picker[]' to type 'System.Collections.Generic.IEnumerable`1[Client.Web.Models.FeatureFacility]'.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 

[InvalidCastException: Unable to cast object of type 'nuPickers.Picker[]' to type 'System.Collections.Generic.IEnumerable`1[Client.Web.Models.FeatureFacility]'.]
    lambda_method(Closure , Object , Object ) +99
    Our.Umbraco.Ditto.PropertyInfoInvocations.SetValue(PropertyInfo property, Object instance, Object value) +219
    Our.Umbraco.Ditto.PublishedContentExtensions.ConvertContent(IPublishedContent content, Type type, CultureInfo culture, Object instance, IEnumerable`1 processorContexts, Action`1 onConverting, Action`1 onConverted) +2580
    Our.Umbraco.Ditto.PublishedContentExtensions.As(IPublishedContent content, Type type, CultureInfo culture, Object instance, IEnumerable`1 processorContexts, Action`1 onConverting, Action`1 onConverted) +706
    Our.Umbraco.Ditto.PublishedContentExtensions.As(IPublishedContent content, CultureInfo culture, T instance, IEnumerable`1 processorContexts, Action`1 onConverting, Action`1 onConverted) +110
    Client.Web.Controllers.BaseSurfaceController`1.Index(RenderModel model) +202
    lambda_method(Closure , ControllerBase , Object[] ) +94
    System.Web.Mvc.ReflectedActionDescriptor.Execute(ControllerContext controllerContext, IDictionary`2 parameters) +229
    System.Web.Mvc.ControllerActionInvoker.InvokeActionMethod(ControllerContext controllerContext, ActionDescriptor actionDescriptor, IDictionary`2 parameters) +90
    System.Web.Mvc.Async.AsyncControllerActionInvoker.<BeginInvokeSynchronousActionMethod>b__39(IAsyncResult asyncResult, ActionInvocation innerInvokeState) +39
    System.Web.Mvc.Async.WrappedAsyncResult`2.CallEndDelegate(IAsyncResult asyncResult) +79
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethod(IAsyncResult asyncResult) +42
    System.Web.Mvc.Async.AsyncInvocationWithFilters.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3d() +72
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +386
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +386
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +386
    System.Web.Mvc.Async.<>c__DisplayClass46.<InvokeActionMethodFilterAsynchronouslyRecursive>b__3f() +386
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeActionMethodWithFilters(IAsyncResult asyncResult) +42
    System.Web.Mvc.Async.<>c__DisplayClass2b.<BeginInvokeAction>b__1c() +38
    System.Web.Mvc.Async.<>c__DisplayClass21.<BeginInvokeAction>b__1e(IAsyncResult asyncResult) +186
    System.Web.Mvc.Async.AsyncControllerActionInvoker.EndInvokeAction(IAsyncResult asyncResult) +38
    System.Web.Mvc.Controller.<BeginExecuteCore>b__1d(IAsyncResult asyncResult, ExecuteCoreState innerState) +29
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +67
    System.Web.Mvc.Controller.EndExecuteCore(IAsyncResult asyncResult) +53
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +35
    System.Web.Mvc.Controller.EndExecute(IAsyncResult asyncResult) +38
    System.Web.Mvc.MvcHandler.<BeginProcessRequest>b__5(IAsyncResult asyncResult, ProcessRequestState innerState) +44
    System.Web.Mvc.Async.WrappedAsyncVoid`1.CallEndDelegate(IAsyncResult asyncResult) +67
    System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) +38
    System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +674
    System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +187

The property in the page's model:

public IEnumerable<FeatureFacility> Facilities { get; set; }

The class for the FeatureFacility:

[NuPicker]
public class FeatureFacility : PublishedContentModel
{
    public FeatureFacility(IPublishedContent content) : base(content)
    {
    }

    //[DittoIgnore]
    //public string ItemName => Content.GetSafeVortoString("itemName", Content.Name);
}

(At the moment, I have everything commented out because I wanted to make sure it wasn't my properties causing the problem)

The NuPicker attribute (copied directly from James's link at the top, which again, I've used before but not in 7.6.3...):

/// <summary>
/// Provides a unified way of converting the <see cref="Picker"/> type to the given <see cref="Type"/>
/// </summary>
public class NuPickerAttribute : DittoMultiProcessorAttribute
{
    /// <summary>
    /// Initializes a new instance of the <see cref="NuPickerAttribute"/> class.
    /// </summary>
    public NuPickerAttribute()
        : base(new DittoProcessorAttribute[] { new NuPickerConverterAttribute(), new UmbracoPickerAttribute() })
    {
    }

    /// <summary>
    /// Returns the correct value to pass to the <see cref="UmbracoPickerAttribute"/>
    /// </summary>
    private class NuPickerConverterAttribute : DittoProcessorAttribute
    {
        /// <inheritdoc/>
        public override object ProcessValue()
        {
            Picker picker = this.Value as Picker;
            if (picker != null && picker.PickedKeys.Any())
            {
                return picker.PickedKeys;
            }

            return this.Value;
        }
    }
}

Any insights? 🤔