C#•7mo ago

DDD Specification with AND Operator

Hi dear friends, Sorry because I know this topic is a bit of pain, I'm working with DDD, as you know DDD came with a useful pain solution called Specifications, Now I have the following interface that represents a Specification:

using System.Linq.Expressions;

namespace ExpenseService.Domain.Shared.Interfaces;

/// <summary>
/// Represents a specification that defines a condition that an entity must satisfy.
/// </summary>
/// <typeparam name="T">The type of entity.</typeparam>
public interface ISpecification<T> where T: class
{
    bool IsSatisfiedBy(T entity);

    Expression<Func<T, bool>> ToExpression();
    
    ISpecification<T> And(ISpecification<T> other);
}

using System.Linq.Expressions;

namespace ExpenseService.Domain.Shared.Interfaces;

/// <summary>
/// Represents a specification that defines a condition that an entity must satisfy.
/// </summary>
/// <typeparam name="T">The type of entity.</typeparam>
public interface ISpecification<T> where T: class
{
    bool IsSatisfiedBy(T entity);

    Expression<Func<T, bool>> ToExpression();
    
    ISpecification<T> And(ISpecification<T> other);
}

And I have created an Abstract class who represents this specification as the following:

using System.Linq.Expressions;
using ExpenseService.Domain.Shared.Interfaces;

namespace ExpenseService.Domain.Specifications;


public abstract class Specification<T> : ISpecification<T> where T : class
{
    
    public abstract Expression<Func<T, bool>> ToExpression();


    public virtual bool IsSatisfiedBy(T entity)
    {
        var predicate = ToExpression().Compile();
        return predicate(entity);
    }
    
    
    public ISpecification<T> And(ISpecification<T> other)
    {

    }
}

using System.Linq.Expressions;
using ExpenseService.Domain.Shared.Interfaces;

namespace ExpenseService.Domain.Specifications;


public abstract class Specification<T> : ISpecification<T> where T : class
{
    
    public abstract Expression<Func<T, bool>> ToExpression();


    public virtual bool IsSatisfiedBy(T entity)
    {
        var predicate = ToExpression().Compile();
        return predicate(entity);
    }
    
    
    public ISpecification<T> And(ISpecification<T> other)
    {

    }
}

The issue: The issue is that And method I don't know how to well implement it, I will do a brief explaination, All I want that method do is to take an ISpecification<T> and returns another/ a new ISpecification<T> that represents the First specification's expression combined with the second Specification's expression by AND operator. I have tried the Github Copilot but it suggests on me to create a new specification called AndSpecification and that method should returns a new instance of that AndSpecification. From your experience, what is the ideal/efficient implementation for that And method ?? Massive thanks in advance <3

1 Reply

M B V R K•7mo ago

public class AndSpecification<T>: Specification<T> where T: class
{
    private readonly ISpecification<T> _left;
    private readonly ISpecification<T> _right;

    public AndSpecification( ISpecification<T> left, ISpecification<T> right )
    {
        _left  = left;
        _right = right;
    }
    
  
    private new bool IsSatisfiedBy(T entity)
    {
        throw new NotImplementedException();
    }
    
    
    public override Expression<Func<T, bool>> ToExpression()
    {
        var leftExpression  = _left.ToExpression();
        var rightExpression = _right.ToExpression();
        
        var andExpression = Expression.AndAlso( leftExpression.Body, rightExpression.Body );
        
        return Expression.Lambda<Func<T, bool>>( andExpression, leftExpression.Parameters.Single() );
    }
    
    
    public override AndSpecification<T> And(ISpecification<T> other)
    {
        return new AndSpecification<T>( this, other );
    } 
    
}

public class AndSpecification<T>: Specification<T> where T: class
{
    private readonly ISpecification<T> _left;
    private readonly ISpecification<T> _right;

    public AndSpecification( ISpecification<T> left, ISpecification<T> right )
    {
        _left  = left;
        _right = right;
    }
    
  
    private new bool IsSatisfiedBy(T entity)
    {
        throw new NotImplementedException();
    }
    
    
    public override Expression<Func<T, bool>> ToExpression()
    {
        var leftExpression  = _left.ToExpression();
        var rightExpression = _right.ToExpression();
        
        var andExpression = Expression.AndAlso( leftExpression.Body, rightExpression.Body );
        
        return Expression.Lambda<Func<T, bool>>( andExpression, leftExpression.Parameters.Single() );
    }
    
    
    public override AndSpecification<T> And(ISpecification<T> other)
    {
        return new AndSpecification<T>( this, other );
    } 
    
}

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DDD Specification with AND Operator