Why Does My ASP.NET MVC View Throw a 'Model Item Passed into the Dictionary is of Type X but Requires Y' Error?

Troubleshooting the "Model Item Passed into the Dictionary is of Type X but Requires Y" Error in ASP.NET MVC

This common ASP.NET MVC error, "The model item passed into the dictionary is of type 'X' but this dictionary requires a model item of type 'Y'," indicates a type mismatch between your view's expected model and the actual model data sent from your controller.

Root Causes and Solutions:

The problem usually arises from these scenarios:

1. Controller-View Model Mismatch: The controller action's returned model must precisely match the @model directive in your view. Using anonymous types or incorrect collections is a frequent culprit.

   • Example:

     <code class="language-csharp">// Incorrect: Anonymous type returned
     var model = db.Foos.Select(x => new { ID = x.ID, Name = x.Name });
     return View(model); // Error if view expects @model Foo</code>

     • Solution: Return the correct model type:

     <code class="language-csharp">var model = db.Foos.ToList(); // Or a single Foo object if needed
     return View(model);</code>

2. Partial View Model Conflicts: When using partial views with a complex main view model, the partial view might inherit the main model incorrectly.

   • Example: Main view uses @model Foo, partial view uses @model Bar.

   • Solution: Explicitly pass the correct model to the partial view:

     <code class="language-csharp">@Html.Partial("_BarPartial", Model.BarProperty)</code>

3. Layout Model Declarations: If your layout file declares a model, all views using that layout must also declare a compatible model (the same type or a derived type).

   • Solution: Either ensure model consistency across all views or avoid model declarations in layouts unless absolutely necessary. Consider using @Html.Action() with a [ChildActionOnly] method to render partial views with separate models within your layout.

In short, resolving this error requires carefully verifying that the model type passed from your controller precisely matches the type declared using the @model directive in your view, and that partial views receive the correct model objects. Pay close attention to anonymous types and model declarations in layouts.

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