How to Solve the 'Entity Cannot be Constructed in a LINQ to Entities Query' Error in Entity Framework Core?

Troubleshooting the Entity Framework Core "Entity Cannot be Constructed in a LINQ to Entities Query" Error

When using Entity Framework Core, constructing a query that projects directly onto a mapped entity can lead to the frustrating "The entity cannot be constructed in a LINQ to Entities query" error. This typically happens when a custom select clause is used in a way that prevents the framework from properly building the entity within the query itself.

The Solution: Avoid Direct Entity Projection

The key to resolving this error is to avoid projecting directly to your entity type. Instead, utilize either anonymous types or Data Transfer Objects (DTOs).

1. Using Anonymous Types:

Anonymous types provide a simple way to structure the selected data. They're useful for quick queries where the data is only needed within the immediate scope.

public IQueryable<object> GetProducts(int categoryID) // Note: Return type is object
{
    return (from p in db.Products
            where p.CategoryID == categoryID
            select new { p.Name, p.Price, /* other properties */ });
}

Important: Remember that anonymous types are only accessible within the method where they are defined.

2. Using Data Transfer Objects (DTOs):

DTOs are custom classes designed to represent the data you want to retrieve. They offer more flexibility and reusability since they can be accessed outside the query's scope.

public class ProductDTO
{
    public string Name { get; set; }
    public decimal Price { get; set; }
    // ... other properties as needed
}

public List<ProductDTO> GetProducts(int categoryID)
{
    return (from p in db.Products
            where p.CategoryID == categoryID
            select new ProductDTO { Name = p.Name, Price = p.Price, /* other properties */ }).ToList();
}

By employing DTOs, you separate the data projection from the data retrieval, effectively circumventing the error and giving you greater control over the data structure returned by your query. This approach is generally preferred for better code organization and maintainability.

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