Create Dynamic Func from Object

问题:

I have a criteria object in which I was to turn each property into a func, if it's value isn't null.

public class TestClassCriteria
{
    public bool? ColumnA { get; set; }
    public bool? ColumnB { get; set; }
}

This is what I have thus far, but I am pretty sure I am not defining the lambda correct. This is what I trying to achieve. funcs.Add(x => x.ColumnA == criteria.ColumnA).

var properties = criteria.GetType().GetProperties();
var funcs = new List<Func<dynamic, bool>>();

foreach (var property in properties)
{
    var propertyName = property.Name;

    funcs.Add(x => x.GetType().GetProperty(propertyName).Name == criteria.GetType().GetProperty(propertyName).Name);
}

It's not crashing or causing any error, it just isn't working.

Any help you can provide would be greatly appreciated.

回答1:

Do you want something like this?

    static List<Func<TEntity, TCriteria, bool>> GetCriteriaFunctions<TEntity, TCriteria>()
    {
        var criteriaFunctions = new List<Func<TEntity, TCriteria, bool>>();

        // searching for nullable properties of criteria
        var criteriaProperties = typeof(TCriteria)
            .GetProperties()
            .Where(p => p.PropertyType.IsGenericType && p.PropertyType.GetGenericTypeDefinition() == typeof(Nullable<>));

        foreach (var property in criteriaProperties)
        {
            // this is entity parameter
            var entityParameterExpression = Expression.Parameter(typeof(TEntity));
            // this is criteria parameter
            var criteriaParameterExpression = Expression.Parameter(typeof(TCriteria));
            // this is criteria property access: "criteria.SomeProperty"
            var criteriaPropertyExpression = Expression.Property(criteriaParameterExpression, property);
            // this is testing for equality between criteria property and entity property;
            // note, that criteria property should be converted first;
            // also, this code makes assumption, that entity and criteria properties have the same names
            var testingForEqualityExpression = Expression.Equal(
                Expression.Convert(criteriaPropertyExpression, property.PropertyType.GetGenericArguments()[0]), 
                Expression.Property(entityParameterExpression, property.Name));

            // criteria.SomeProperty == null ? true : ((EntityPropertyType)criteria.SomeProperty == entity.SomeProperty)
            var body = Expression.Condition(
                Expression.Equal(criteriaPropertyExpression, Expression.Constant(null)), 
                Expression.Constant(true),
                testingForEqualityExpression);

            // let's compile lambda to method
            var criteriaFunction = Expression.Lambda<Func<TEntity, TCriteria, bool>>(body, entityParameterExpression, criteriaParameterExpression).Compile();

            criteriaFunctions.Add(criteriaFunction);
        }

        return criteriaFunctions;
    }

Sample entity and sample criteria:

class CustomerCriteria
{
    public int? Age { get; set; }
    public bool? IsNew { get; set; }
}

class Customer
{
    public string Name { get; set; }
    public int Age { get; set; }
    public bool IsNew { get; set; }
}

Usage:

        var criteriaFunctions = GetCriteriaFunctions<Customer, CustomerCriteria>();
        var customer1 = new Customer { Name = "John", Age = 35, IsNew = false };
        var customer2 = new Customer { Name = "Mary", Age = 27, IsNew = true };
        var criteria1 = new CustomerCriteria { Age = 35 };
        var criteria2 = new CustomerCriteria { IsNew = true };

        Console.WriteLine("Is match: {0}", criteriaFunctions.All(f => f(customer1, criteria1)));
        Console.WriteLine("Is match: {0}", criteriaFunctions.All(f => f(customer2, criteria1)));
        Console.WriteLine("Is match: {0}", criteriaFunctions.All(f => f(customer1, criteria2)));
        Console.WriteLine("Is match: {0}", criteriaFunctions.All(f => f(customer2, criteria2)));

Instead of your code with dynamics, this code uses strongly typed member access, so, you could cache list of criteria for each pair "Entity - Criteria" and test instances form matching faster.

回答2:

Your current expression is comparing the property names, but I think you want to be comparing the property values:

var funcs = new List<Func<dynamic, bool>>();

foreach (var property in criteria.GetType().GetProperties())
{
    funcs.Add(x => x.GetType().GetProperty(property.Name).GetValue(x, null) == property.GetValue(criteria, null));
}

However, are you sure you need to do this? There would probably be a better way to refactor your code so that you don't need to use reflection to compare properties that happen to have the same name on two unrelated objects.