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Ardalis.Specification

repository·main·Indexed 25 days ago

https://github.com/ardalis/specification

A .NET library implementing the Specification pattern to encapsulate query logic into reusable objects. It reduces LINQ expression duplication in EF Core and repository abstractions. Features include SpecificationEvaluator for converting specifications to IQueryable, support for AsNoTracking, AsNoTrackingWithIdentityResolution, and AsSplitQuery, and extensibility via custom IEvaluator implementations and ISpecificationBuilder extensions.

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What's inside Ardalis.Specification

  1. Overview of Ardalis.Specification

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    Ardalis.Specification is a .NET library designed for building query specifications. It provides a way to encapsulate query logic into reusable specification objects, which can then be used to filter, sort, and paginate data when interacting with repositories or database contexts.

  2. What is the Specification pattern and when to use it

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    The Specification pattern is used to pull query-specific logic out of other parts of an application. It is particularly useful in two scenarios:

    1. Direct EF Core usage: It eliminates the need to repeat Where, Include, Select, and similar expressions across the codebase.
    2. Repository abstraction: It reduces the need for many custom Repository implementation classes or specialized query methods (e.g., GetCustomersByName). Instead, you use a few core repository methods that accept an ISpecification<T> to filter and shape data.

    By defining specifications in a centralized, easily discoverable location, you reduce bugs caused by duplicating lambda expressions.

  3. How to use the Select operator in Specification

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    The Query object does not contain a Select method. To use projections (the Select operator), you must inherit from Specification<T, TResult>, where TResult is the type you want to project into. This provides a strongly typed experience for both the builder and the evaluation process.

  4. Understand the benefits of the Specification pattern

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    The Specification pattern encapsulates query logic into dedicated classes. When used alongside the Repository pattern, it provides several advantages:

    • Centralized Logic: Keeps data access query logic in one place.
    • Domain Layer Integration: Allows query logic to reside within the domain layer.
    • Reusability: Common queries can be reused throughout the application.
    • Expressive Language: Provides meaningful names to common queries, improving the clarity of application behavior.
    • Repository Simplification: Prevents Repositories from becoming bloated with numerous custom query methods and avoids hiding ORM-specific data shaping features.

  5. Use conditional builder extensions for dynamic specifications

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    All builder extension methods in the specification library offer an overload that accepts a bool condition parameter. If the condition is false, the expression or value is not added to the specification state. This allows you to build dynamic specifications without manually checking conditions before calling builder methods, similar to WhereIf patterns in other LINQ libraries.

  6. Use the WithProjectionOf extension method to reuse query logic

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    The WithProjectionOf extension method allows you to create a new specification by reusing the filtering, ordering, and other query logic from an existing specification, but applying a projection (e.g., a Select or SelectMany clause) from a different specification.

    This method returns a new combined specification while leaving the original input specifications unchanged. It is useful for:

    • Projecting the same base query logic into different data shapes (e.g., different DTOs or ViewModels).
    • Applying a specific projection to different base query logic (e.g., different filtering criteria).
    // Example: Reusing filtering logic from a base spec with a projection from a DTO spec
var customerSpec = new CustomerSpec("John");
var customerDto1Spec = new CustomerToCustomerDto1Spec();

// Creates a new specification with CustomerSpec's filters and CustomerToCustomerDto1Spec's projection
var newSpec1 = customerSpec.WithProjectionOf(customerDto1Spec);

  7. How to reuse query logic without composite specifications

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    Composite specifications (using logical operators like AND, OR, NOT to combine specifications) are not supported by design due to the complexity of merging non-filtering features like includes, ordering, and paging.

    Instead of composition, use these patterns:

    1. Extension Methods: The recommended approach. Define extension methods for ISpecificationBuilder<T> to encapsulate reusable logic (filters, includes, etc.) and call them within your specifications.
    2. WithProjectionOf: A specific supported composition scenario that allows you to reuse all query logic (filtering, ordering, includes) from an existing specification while applying a different projection (Select or SelectMany) on top of it.

  8. Use PostProcessingAction for in-memory transformations

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    The PostProcessingAction allows you to define a delegate of type Func<IEnumerable<T>, IEnumerable<T>> that is applied to the result set after data is retrieved from the external source (e.g., a database).

    Use this feature when you need to perform transformations, complex filtering, or logic that cannot be translated into a database query by the underlying query provider (like Entity Framework). The action is stored in the specification state and is intended to be executed within the repository or data access layer after the initial query completes.

    public class CompanySpec : Specification<Company>
{
    public CompanySpec(int countryId)
    {
        Query.Where(x => x.CountryId == countryId)
             .Include(x => x.Stores);

        Query.PostProcessingAction(companies =>
        {
            // Your custom in-memory operation on the result set.
            return companies;
        });
    }
}

  9. Understand how specification caching works

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    It is important to understand that EnableCache and WithCacheKey only manage the metadata of the cache key within the specification's internal state.

    • Responsibility: The actual caching logic (storing, retrieving, and managing the cache) is implemented by the consuming infrastructure, such as a CachedRepository.
    • Limitations: These methods do not manage cache expiration, eviction, or tagging. For advanced cache management (like expiration policies), you must extend the specification builder.

  10. Benefits of using Select and SelectMany

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    Using Select and SelectMany provides several advantages for query efficiency:

    • Reduces data transfer: Only the required fields are returned.
    • Supports DTO projection: Easily map database entities to Data Transfer Objects.
    • Efficient queries: Projections are applied at the query level, meaning the data source only retrieves the selected fields.
    • Flattening: SelectMany is ideal for handling child entities and nested collections.

  11. How the Specification Pattern is used to define queries

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    The Specification Pattern is used to encapsulate LINQ logic within a specification object rather than scattering it throughout the codebase. This approach provides two main benefits:

    1. Encapsulation: It centralizes query logic, making it reusable and easier to maintain.
    2. Performance: By defining the exact data required for a query upfront, it ensures a single efficient query is executed, avoiding the performance overhead of lazy loading individual pieces of data as they are accessed.

    In the Ardalis.Specification package, this pattern is designed to work in conjunction with the Repository Pattern.

  12. Control Search logic with Grouping and Logical Operators

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    You can control how multiple Search predicates are combined using the group parameter:

    • OR Logic (Default): If you call multiple Search statements without specifying a group, they are grouped together and combined using OR logic. The entity is included if any condition matches.
    • AND Logic: To combine search conditions using AND logic, you must assign a different group integer to each Search statement. This forces the predicates into separate groups that are then joined by AND.

    Example: OR Logic (Default)

    Query
    .Search(x => x.Name, "%" + searchTerm + "%")
    .Search(x => x.Email, "%" + searchTerm + "%");

    Example: AND Logic (Different Groups)

    Query
    .Search(x => x.Name, "%" + name + "%", group: 1)
    .Search(x => x.Email, "%" + email + "%", group: 2);
    public class CustomerSpec : Specification<Customer>
{
    public CustomerSpec(string? name, string? email)
    {
        Query
            .Search(x => x.Name, "%" + name + "%", group: 1)
            .Search(x => x.Email, "%" + email + "%", group: 2);
    }
}