Why Does Constructing Entities in LINQ to Entities Projections Throw an Error?

Entity Framework Core: Projection Limitations in LINQ to Entities Queries

When using Entity Framework Core, directly constructing entity objects within the projection of a LINQ to Entities query often leads to the error: "The entity or complex type cannot be constructed in a LINQ to Entities query."

The Issue:

Consider this example:

public IQueryable<Product> GetProducts(int categoryID)
{
    return from p in db.Products
           where p.CategoryID == categoryID
           select new Product { Name = p.Name };
}

Executing this with productRepository.GetProducts(1).ToList(); will fail.

Why it Fails:

The problem stems from the fact that Entity Framework Core translates LINQ queries into database commands. The database server lacks the knowledge of your application's Product class and its constructor. Therefore, it cannot create instances of this class.

The Solution: Project to Anonymous Types or DTOs

The solution is to project your query results into either anonymous types or dedicated Data Transfer Objects (DTOs).

Method 1: Using Anonymous Types

This is a quick and simple solution for smaller queries:

var products = (from p in db.Products
               where p.CategoryID == categoryID
               select new { Name = p.Name }).ToList();

Method 2: Employing Data Transfer Objects (DTOs)

For more complex scenarios or when you need strongly-typed objects, create a DTO:

public class ProductDTO
{
    public string Name { get; set; }
    // Add 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 }).ToList();
}

By projecting to anonymous types or DTOs, you circumvent the need for the database to instantiate your entity classes, ensuring successful query execution.

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