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POHow can I avoid this infinite loop?
 

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  1. POHow can I avoid this infinite loop?
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    <p>It feels like there must be some semi-simple solution to this, but I just can't figure it out.</p> <p>Edit: The previous example showed the infinite loop more clearly, but this gives a bit more context. Check out the pre-edit for a quick overview of the problem.</p> <p>The following 2 classes represent the View-Models of the Model View View-Model (<a href="http://msdn.microsoft.com/en-us/magazine/dd419663.aspx" rel="nofollow noreferrer">MVVM</a>) pattern.</p> <pre><code>/// &lt;summary&gt; /// A UI-friendly wrapper for a Recipe /// &lt;/summary&gt; public class RecipeViewModel : ViewModelBase { /// &lt;summary&gt; /// Gets the wrapped Recipe /// &lt;/summary&gt; public Recipe RecipeModel { get; private set; } private ObservableCollection&lt;CategoryViewModel&gt; categories = new ObservableCollection&lt;CategoryViewModel&gt;(); /// &lt;summary&gt; /// Creates a new UI-friendly wrapper for a Recipe /// &lt;/summary&gt; /// &lt;param name="recipe"&gt;The Recipe to be wrapped&lt;/param&gt; public RecipeViewModel(Recipe recipe) { this.RecipeModel = recipe; ((INotifyCollectionChanged)RecipeModel.Categories).CollectionChanged += BaseRecipeCategoriesCollectionChanged; foreach (var cat in RecipeModel.Categories) { var catVM = new CategoryViewModel(cat); //Causes infinite loop categories.AddIfNewAndNotNull(catVM); } } void BaseRecipeCategoriesCollectionChanged(object sender, NotifyCollectionChangedEventArgs e) { switch (e.Action) { case NotifyCollectionChangedAction.Add: categories.Add(new CategoryViewModel(e.NewItems[0] as Category)); break; case NotifyCollectionChangedAction.Remove: categories.Remove(new CategoryViewModel(e.OldItems[0] as Category)); break; default: throw new NotImplementedException(); } } //Some Properties and other non-related things public ReadOnlyObservableCollection&lt;CategoryViewModel&gt; Categories { get { return new ReadOnlyObservableCollection&lt;CategoryViewModel&gt;(categories); } } public void AddCategory(CategoryViewModel category) { RecipeModel.AddCategory(category.CategoryModel); } public void RemoveCategory(CategoryViewModel category) { RecipeModel.RemoveCategory(category.CategoryModel); } public override bool Equals(object obj) { var comparedRecipe = obj as RecipeViewModel; if (comparedRecipe == null) { return false; } return RecipeModel == comparedRecipe.RecipeModel; } public override int GetHashCode() { return RecipeModel.GetHashCode(); } } </code></pre> <p>.</p> <pre><code>/// &lt;summary&gt; /// A UI-friendly wrapper for a Category /// &lt;/summary&gt; public class CategoryViewModel : ViewModelBase { /// &lt;summary&gt; /// Gets the wrapped Category /// &lt;/summary&gt; public Category CategoryModel { get; private set; } private CategoryViewModel parent; private ObservableCollection&lt;RecipeViewModel&gt; recipes = new ObservableCollection&lt;RecipeViewModel&gt;(); /// &lt;summary&gt; /// Creates a new UI-friendly wrapper for a Category /// &lt;/summary&gt; /// &lt;param name="category"&gt;&lt;/param&gt; public CategoryViewModel(Category category) { this.CategoryModel = category; (category.DirectRecipes as INotifyCollectionChanged).CollectionChanged += baseCategoryDirectRecipesCollectionChanged; foreach (var item in category.DirectRecipes) { var recipeVM = new RecipeViewModel(item); //Causes infinite loop recipes.AddIfNewAndNotNull(recipeVM); } } /// &lt;summary&gt; /// Adds a recipe to this category /// &lt;/summary&gt; /// &lt;param name="recipe"&gt;&lt;/param&gt; public void AddRecipe(RecipeViewModel recipe) { CategoryModel.AddRecipe(recipe.RecipeModel); } /// &lt;summary&gt; /// Removes a recipe from this category /// &lt;/summary&gt; /// &lt;param name="recipe"&gt;&lt;/param&gt; public void RemoveRecipe(RecipeViewModel recipe) { CategoryModel.RemoveRecipe(recipe.RecipeModel); } /// &lt;summary&gt; /// A read-only collection of this category's recipes /// &lt;/summary&gt; public ReadOnlyObservableCollection&lt;RecipeViewModel&gt; Recipes { get { return new ReadOnlyObservableCollection&lt;RecipeViewModel&gt;(recipes); } } private void baseCategoryDirectRecipesCollectionChanged(object sender, NotifyCollectionChangedEventArgs e) { switch (e.Action) { case NotifyCollectionChangedAction.Add: var recipeVM = new RecipeViewModel((Recipe)e.NewItems[0], this); recipes.AddIfNewAndNotNull(recipeVM); break; case NotifyCollectionChangedAction.Remove: recipes.Remove(new RecipeViewModel((Recipe)e.OldItems[0])); break; default: throw new NotImplementedException(); } } /// &lt;summary&gt; /// Compares whether this object wraps the same Category as the parameter /// &lt;/summary&gt; /// &lt;param name="obj"&gt;The object to compare equality with&lt;/param&gt; /// &lt;returns&gt;True if they wrap the same Category&lt;/returns&gt; public override bool Equals(object obj) { var comparedCat = obj as CategoryViewModel; if(comparedCat == null) {return false;} return CategoryModel == comparedCat.CategoryModel; } /// &lt;summary&gt; /// Gets the hashcode of the wrapped Categry /// &lt;/summary&gt; /// &lt;returns&gt;The hashcode&lt;/returns&gt; public override int GetHashCode() { return CategoryModel.GetHashCode(); } } </code></pre> <p>I won't bother showing the Models (Recipe and Category) unless requested, but they basically take care of the business logic (for instance adding a recipe to a category will also add the other end of the link, i.e. if a category contains a recipe, then the recipe is also contained in that category) and basically dictate how things go. The ViewModels provide a nice interface for WPF databinding. That's the reason for the wrapper classes</p> <p>Since the infinite loop is in the constructor and it's trying to create new objects, I can't just set a boolean flag to prevent this because neither object ever gets finished being constructed. </p> <p>What I'm thinking is having (either as a singleton or passed in to the constructor or both) a <code>Dictionary&lt;Recipe, RecipeViewModel&gt;</code> and <code>Dictionary&lt;Category, CategoryViewModel&gt;</code> that will lazy-load the view models, but not create a new one if one already exists, but I haven't gotten around to trying to see if it'll work since it's getting late and I'm kinda tired of dealing with this for the past 6 hours or so.</p> <p>No guarantee the code here will compile since I took a bunch of stuff out that was unrelated to the problem at hand.</p>
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