Calling UpdateModel with a collection of complex data types reset all non-bound values?

asp.net-mvc, model-binding, updatemodel

Solution

UPDATE: I stepped through mvc source code (particularly `DefaultModelBinder` class) and here is what I found:

The class determines we are trying to bind a collection so it calls the method: `UpdateCollection(...)` which creates an inner `ModelBindingContext` that has a `null` `Model` property. Afterwards, that context is sent to the method `BindComplexModel(...)` which checks the `Model` property for `null` and creates a new instance of the model type if that is the case.

That's what causes the values to be reset.

And so, only the values that are coming through the form/query string/route data are populated, the rest remains in its initialized state.

I was able to make very few changes to `UpdateCollection(...)` to fix this problem.

Here is the method with my changes:

internal object UpdateCollection(ControllerContext controllerContext, ModelBindingContext bindingContext, Type elementType) {
IModelBinder elementBinder = Binders.GetBinder(elementType);

// build up a list of items from the request
List<object> modelList = new List<object>();
for (int currentIndex = 0; ; currentIndex++) {
    string subIndexKey = CreateSubIndexName(bindingContext.ModelName, currentIndex);
    if (!DictionaryHelpers.DoesAnyKeyHavePrefix(bindingContext.ValueProvider, subIndexKey)) {
        // we ran out of elements to pull
        break;
    }
    // **********************************************************
    // The DefaultModelBinder shouldn't always create a new
    // instance of elementType in the collection we are updating here.
    // If an instance already exists, then we should update it, not create a new one.
    // **********************************************************
    IList containerModel = bindingContext.Model as IList;
    object elementModel = null;
    if (containerModel != null && currentIndex < containerModel.Count)
    {
        elementModel = containerModel[currentIndex];
    }
     //*****************************************************
    ModelBindingContext innerContext = new ModelBindingContext() {
        Model = elementModel, // assign the Model property
        ModelName = subIndexKey,
        ModelState = bindingContext.ModelState,
        ModelType = elementType,
        PropertyFilter = bindingContext.PropertyFilter,
        ValueProvider = bindingContext.ValueProvider
    };
    object thisElement = elementBinder.BindModel(controllerContext, innerContext);

    // we need to merge model errors up
    VerifyValueUsability(controllerContext, bindingContext.ModelState, subIndexKey, elementType, thisElement);
    modelList.Add(thisElement);
}

// if there weren't any elements at all in the request, just return
if (modelList.Count == 0) {
    return null;
}

// replace the original collection
object collection = bindingContext.Model;
CollectionHelpers.ReplaceCollection(elementType, collection, modelList);
return collection;

}

Problem

Original source

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