转换表达式树从类型到另一种类型的与复杂的映射(Translating expression tree

2019-06-24 09:46发布

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灵感来自这个答案我想上一个模型类的属性映射到根据实际实体的表达式。 这些都是涉及到两个类： 

public class Customer
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Id { get; set; }
    public DateTime? BirthDate { get; set; }
    public int CustomerTypeId { get; set; }
}

public class CustomerModel
{
    ...
    public bool HasEvenId { get; set; }
}

我想转换可能表现的一个例子是： 

Expression<Func<CustomerModel, bool>> from = model => model.HasEvenId;
Expression<Func<Customer, bool>> to = entity => ((entity.Id % 2) == 0);

问题是，我不得不暴露通过ASP.NET的WebAPI的OData的终点，但我需要之前，我可以向他们作出实体的一些操作，模型类的因此需要以及需要​​翻译基于模型的表达式我无法接受在基于实体表达式的OData的查询，我会用它来查询EF4。

这是我到目前为止有： 

private static readonly Dictionary<Expression, Expression> Mappings = GetMappings();

private static Dictionary<Expression, Expression> GetMappings()
{
    var mappings = new Dictionary<Expression, Expression>();

    var mapping = GetMappingFor((CustomerModel model) => model.HasEvenId, (Customer customer) => (customer.Id%2) == 0);
    mappings.Add(mapping.Item1, mapping.Item2);

    return mappings;
}

private static Tuple<Expression, Expression> GetMappingFor<TFrom, TTo, TValue>(Expression<Func<TFrom, TValue>> fromExpression, Expression<Func<TTo, TValue>> toExpression)
{
    MemberExpression fromMemberExpression = (MemberExpression) fromExpression.Body;
    return Tuple.Create<Expression, Expression>(fromMemberExpression, toExpression);
}

public static Expression<Func<TTo, bool>> Translate<TFrom, TTo>(Expression<Func<TFrom, bool>> expression, Dictionary<Expression, Expression> mappings = null)
{
    if (expression == null)
        return null;

    string parameterName = expression.Parameters[0].Name;

    parameterName = string.IsNullOrWhiteSpace(parameterName) ? "p" : parameterName;

    var param = Expression.Parameter(typeof(TTo), parameterName);
    var subst = new Dictionary<Expression, Expression> { { expression.Parameters[0], param } };
    ParameterChangeVisitor parameterChange = new ParameterChangeVisitor(parameterName);
    if (mappings != null)
        foreach (var mapp in mappings)
            subst.Add(mapp.Key, parameterChange.Visit(mapp.Value));

    var visitor = new TypeChangeVisitor(typeof(TFrom), typeof(TTo), subst);
    return Expression.Lambda<Func<TTo, bool>>(visitor.Visit(expression.Body), param);
}

public IQueryable<CustomerModel> Get()
{
    var filterExtractor = new ODataFilterExtractor<CustomerModel>();
    Expression<Func<CustomerModel, bool>> expression = filterExtractor.Extract(Request);
    Expression<Func<Customer, bool>> translatedExpression = Translate<CustomerModel, Customer>(expression, Mappings);

    IQueryable<Customer> query = _context.Customers;

    if (translatedExpression != null)
        query = query.Where(translatedExpression);

    var finalQuery = from item in query.AsEnumerable() 
                     select new CustomerModel()
                        {
                            FirstName = item.FirstName, 
                            LastName = item.LastName, 
                            Id = item.Id, 
                            BirthDate = item.BirthDate, 
                            CustomerTypeId = item.CustomerTypeId,
                            HasEvenId = (item.Id % 2 ) == 0
                        };

    return finalQuery.AsQueryable();
}

哪里：

  • ODataFilterExtractor是提取我们收到RequestMessage的$过滤表达式一类;
  • ParameterChangeVisitor只是改变了所有的ParameterExpression于具有提供的字符串作为参数名新;

此外，我改变了这种方式上面链接的回答VisitMember方法： 

protected override Expression VisitMember(MemberExpression node)
{
    // if we see x.Name on the old type, substitute for new type
    if (node.Member.DeclaringType == _from)
    {
        MemberInfo toMember = _to.GetMember(node.Member.Name, node.Member.MemberType, BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic).SingleOrDefault();
        if (toMember != null)
        {
            return Expression.MakeMemberAccess(Visit(node.Expression), toMember);
        }
        else
        {
            if (_substitutions.Select(kvp => kvp.Key).OfType<MemberExpression>().Any(me => me.Member.Equals(node.Member)))
            {
                MemberExpression key = _substitutions.Select(kvp => kvp.Key).OfType<MemberExpression>().Single(me => me.Member.Equals(node.Member));
                Expression value = _substitutions[key];

                // What to return here?
                return Expression.Invoke(value);
            }
        }
    }
    return base.VisitMember(node);
}

感谢您的帮助。

Answer 1:

我把修改代码只是头发的自由，但这样做的伎俩， 

public class Customer
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int Id { get; set; }
    public DateTime? BirthDate { get; set; }
    public int CustomerTypeId { get; set; }
}

public class CustomerModel
{
    public string FullName { get; set; }
    public bool HasEvenId { get; set; }
}

sealed class AToBConverter<TA, TB> : ExpressionVisitor
{
    private readonly Dictionary<ParameterExpression, ParameterExpression> _parameters = new Dictionary<ParameterExpression, ParameterExpression>();
    private readonly Dictionary<MemberInfo, LambdaExpression> _mappings;

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node.Type == typeof(TA))
        {
            ParameterExpression parameter;
            if (!this._parameters.TryGetValue(node, out parameter))
            {
                this._parameters.Add(node, parameter = Expression.Parameter(typeof(TB), node.Name));
            }
            return parameter;
        }
        return node;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Expression == null || node.Expression.Type != typeof(TA))
        {
            return base.VisitMember(node);
        }
        Expression expression = this.Visit(node.Expression);
        if (expression.Type != typeof(TB))
        {
            throw new Exception("Whoops");
        }
        LambdaExpression lambdaExpression;
        if (this._mappings.TryGetValue(node.Member, out lambdaExpression))
        {
            return new SimpleExpressionReplacer(lambdaExpression.Parameters.Single(), expression).Visit(lambdaExpression.Body);
        }
        return Expression.Property(
            expression,
            node.Member.Name
        );
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
        return Expression.Lambda(
            this.Visit(node.Body),
            node.Parameters.Select(this.Visit).Cast<ParameterExpression>()
        );
    }

    public AToBConverter(Dictionary<MemberInfo, LambdaExpression> mappings)
    {
        this._mappings = mappings;
    }
}

sealed class SimpleExpressionReplacer : ExpressionVisitor
{
    private readonly Expression _replacement;
    private readonly Expression _toFind;

    public override Expression Visit(Expression node)
    {
        return node == this._toFind ? this._replacement : base.Visit(node);
    }

    public SimpleExpressionReplacer(Expression toFind, Expression replacement)
    {
        this._toFind = toFind;
        this._replacement = replacement;
    }
}

class Program 
{
    private static Dictionary<MemberInfo, LambdaExpression> GetMappings()
    {
        var mappings = new Dictionary<MemberInfo, LambdaExpression>();
        var mapping = GetMappingFor(model => model.HasEvenId, customer => (customer.Id % 2) == 0);
        mappings.Add(mapping.Item1, mapping.Item2);
        mapping = GetMappingFor(model => model.FullName, customer => customer.FirstName + " " + customer.LastName);
        mappings.Add(mapping.Item1, mapping.Item2);
        return mappings;
    }

    private static Tuple<MemberInfo, LambdaExpression> GetMappingFor<TValue>(Expression<Func<CustomerModel, TValue>> fromExpression, Expression<Func<Customer, TValue>> toExpression)
    {
        return Tuple.Create(((MemberExpression)fromExpression.Body).Member, (LambdaExpression)toExpression);
    }

    static void Main()
    {
        Expression<Func<CustomerModel, bool>> source = model => model.HasEvenId && model.FullName == "John Smith";
        Expression<Func<Customer, bool>> desiredResult = model => (model.Id % 2) == 0 && (model.FirstName + " " + model.LastName) == "John Smith";
        Expression output = new AToBConverter<CustomerModel, Customer>(GetMappings()).Visit(source);
        Console.WriteLine("The two expressions do {0}match.", desiredResult.ToString() == output.ToString() ? null : "not ");
        Console.ReadLine();
    }
}


Answer 2:

另一种解决方案将是使用AutoMapper映射复杂类型和它被执行之前用ExpressionTransformer修改所产生的表达的查询。 我会尽量用完整的示例来解释：

模型类

一些POCO类只用于保存数据。 

public enum CostUnitType
{
    None = 0,
    StockUnit = 1,
    MachineUnit = 2,
    MaintenanceUnit = 3
}

public class CostUnit
{
    public string CostUnitId { get; set; }
    public string WorkplaceId { get; set; }
    public string Name { get; set; }
    public string Description { get; set; }

    public CostUnitType Type { get; set; }

    public override string ToString()
    {
        return CostUnitId + " " + Name + " " + Type;
    }
}

public class StockUnit
{
    public string Id { get; set; }
    public string Name { get; set; }
}

public class MachineUnit
{
    public string Id { get; set; }
    public string Name { get; set; }
}

public class MaintenanceUnit
{
    public string Id { get; set; }
    public string Name { get; set; }
}

该ExpressionTransformer类

在大多数情况下，使用映射静态类是好的，但有时你需要映射同样类型和不同的配置，所以你需要明确使用IMappingEngine这样的： 

var configuration = new ConfigurationStore(new TypeMapFactory(), MapperRegistry.Mappers);
var engine = new MappingEngine(configuration);

创建MappingEngine的方法是不明显的。 我在源代码中挖找到它是怎么做。 

public static class ExpressionTransformer
{
    private static readonly MappingEngine Mapper;

    /// <summary>
    /// Initializes the <see cref="ExpressionTransformer"/> class.
    /// Creates an instance of AutoMapper. Initializes mappings.
    /// </summary>
    static ExpressionTransformer()
    {
        ConfigurationStore configurationStore = new ConfigurationStore(new TypeMapFactory(), MapperRegistry.Mappers);

        // Create mapping
        // Maps Id from StockUnit to CostUnitId from CostUnit
        configurationStore.CreateMap<StockUnit, CostUnit>()
            .ForMember(m => m.CostUnitId, opt => opt.MapFrom(src => src.Id));
        // Maps Id from MachineUnit to CostUnitId from CostUnit
        configurationStore.CreateMap<MachineUnit, CostUnit>()
            .ForMember(m => m.CostUnitId, opt => opt.MapFrom(src => src.Id));
        // Maps Id from MaintenanceUnit to CostUnitId from CostUnit
        configurationStore.CreateMap<MaintenanceUnit, CostUnit>()
            .ForMember(m => m.CostUnitId, opt => opt.MapFrom(src => src.Id));

        // Create instance of AutoMapper
        Mapper = new MappingEngine(configurationStore);
    }

    public static Expression<Func<TDestination, bool>> Tranform<TSource, TDestination>(Expression<Func<TSource, bool>> sourceExpression)
    {
        // Resolve mappings by AutoMapper for given types.
        var map = Mapper.ConfigurationProvider.FindTypeMapFor(typeof(TSource), typeof(TDestination));

        if (map == null)
        {
            throw new AutoMapperMappingException(string.Format("No Mapping found for {0} --> {1}.", typeof(TSource).Name, typeof(TDestination).Name));
        }

        // Transform from TSource to TDestination with specified mappings
        var visitor = new ParameterTypeVisitor<TSource, TDestination>(sourceExpression, map.GetPropertyMaps());
        var expression = visitor.Transform();

        return expression;
    }

    private class ParameterTypeVisitor<TSource, TDestination> : ExpressionVisitor
    {
        private readonly Dictionary<string, ParameterExpression> _parameters;
        private readonly Expression<Func<TSource, bool>> _expression;
        private readonly IEnumerable<PropertyMap> _maps;

        public ParameterTypeVisitor(Expression<Func<TSource, bool>> expression, IEnumerable<PropertyMap> maps)
        {
            _parameters = expression.Parameters
                .ToDictionary(p => p.Name, p => Expression.Parameter(typeof(TDestination), p.Name));

            _expression = expression;

            _maps = maps;
        }

        public Expression<Func<TDestination, bool>> Transform()
        {
            return (Expression<Func<TDestination, bool>>) Visit(_expression);
        }

        protected override Expression VisitMember(MemberExpression node)
        {
            if (node.Member.DeclaringType == typeof(TSource))
            {
                var memberName = node.Member.Name;
                var member = _maps.FirstOrDefault(p => typeof(TSource) == node.Expression.Type
                                                        && p.SourceMember.Name == memberName);
                if (member != null)
                {
                    // Return Property from TDestination
                    var expression = Visit(node.Expression);
                    return Expression.MakeMemberAccess(expression, member.DestinationProperty.MemberInfo);
                }
            }

            return base.VisitMember(node);
        }

        protected override Expression VisitParameter(ParameterExpression node)
        {
            var parameter = _parameters[node.Name];
            return parameter;
        }

        protected override Expression VisitLambda<T>(Expression<T> node)
        {
            var expression = Visit(node.Body);
            return Expression.Lambda(expression, _parameters.Select(x => x.Value));
        }
    }
}

用法

要转换我们只需要调用变换ExpressionTransformer类的方法表达

            Expression<Func<StockUnit, bool>> stockQuery = unit => unit.Id == "0815" && unit.Name == "ABC";

            // Call Transform<TSource, TDestination> method.
            Expression<Func<CostUnit, bool>> costUnitQuery = ExpressionTransformer.Tranform<StockUnit, CostUnit>(stockQuery);

结果



文章来源: Translating expression tree from a type to another type with complex mappings
标签： linq entity-framework expression-trees asp.net-web-api
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