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將參數替換為指向lambda表達式中的嵌套參數

[英]Replace parameter to point to nested parameter in lambda expression

由於前一個問題的一些答案,我可以成功地替換lambda表達式中的簡單參數類型,但我無法弄清楚如何將傳入的lambda中的參數替換為嵌套參數。

考慮以下對象:

public class DtoColour {

    public DtoColour(string name)
    {
        Name = name;
    }

    public string Name { get; set; }

    public ICollection<DtoFavouriteColour> FavouriteColours { get; set; }
}

public class DtoPerson
{
    public DtoPerson(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
        FavouriteColours = new Collection<DtoFavouriteColour>();
    }

    public string FirstName { get; private set; }

    public string LastName { get; private set; }

    public ICollection<DtoFavouriteColour> FavouriteColours { get; set; }
}

public class DtoFavouriteColour
{
    public DtoColour Colour { get; set; }

    public DtoPerson Person { get; set; }
}

public class DomainColour {

    public DomainColour(string name)
    {
        Name = name;
    }

    public string Name { get; set; }

    public ICollection<DomainPerson> People { get; set; }
}

public class DomainPerson {

    public DomainPerson(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
        Colours = new Collection<DomainColour>();
    }

    public string FirstName { get; private set; }

    public string LastName { get; private set; }

    public ICollection<DomainColour> Colours { get; set; }
}

和存儲庫:

public class ColourRepository {

    private IList<DtoColour> Colours { get; set; } 

    public ColourRepository()
    {
        var favColours = new Collection<DtoFavouriteColour>
        {
            new DtoFavouriteColour() { Person = new DtoPerson("Peter", "Parker") },
            new DtoFavouriteColour() { Person = new DtoPerson("John", "Smith") },
            new DtoFavouriteColour() { Person = new DtoPerson("Joe", "Blogs") }
        };
        Colours = new List<DtoColour>
        {
            new DtoColour("Red") { FavouriteColours = favColours },
            new DtoColour("Blue"),
            new DtoColour("Yellow")
        };
    }

    public IEnumerable<DomainColour> GetWhere(Expression<Func<DomainColour, bool>> predicate)
    {
        var coonvertedPred = MyExpressionVisitor.Convert(predicate);
        return Colours.Where(coonvertedPred).Select(c => new DomainColour(c.Name)).ToList();
    }
}

最后是一個表達式訪問者,它應該將謂詞轉換為Dto模型的正確謂詞

public class MyExpressionVisitor : ExpressionVisitor
{
    private ReadOnlyCollection<ParameterExpression> _parameters;

    public static Func<DtoColour, bool> Convert<T>(Expression<T> root)
    {
        var visitor = new MyExpressionVisitor();
        var expression = (Expression<Func<DtoColour, bool>>)visitor.Visit(root);
        return expression.Compile();
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        var param = _parameters?.FirstOrDefault(p => p.Name == node.Name);

        if (param != null)
        {
            return param;
        }

        if(node.Type == typeof(DomainColour))
        {
            return Expression.Parameter(typeof(DtoColour), node.Name);
        }

        if (node.Type == typeof(DomainPerson))
        {
            return Expression.Parameter(typeof(DtoFavouriteColour), node.Name);
        }

        return node;
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
        _parameters = VisitAndConvert<ParameterExpression>(node.Parameters, "VisitLambda");
        return Expression.Lambda(Visit(node.Body), _parameters);
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        var exp = Visit(node.Expression);

        if (node.Member.DeclaringType == typeof(DomainColour))
        {
            if (node.Type == typeof(ICollection<DomainPerson>))
            {
                return Expression.MakeMemberAccess(exp, typeof(DtoColour).GetProperty("FavouriteColours"));
            }

            return Expression.MakeMemberAccess(exp, typeof(DtoColour).GetProperty(node.Member.Name));
        }

        if (node.Member.DeclaringType == typeof(DomainPerson))
        {
            var nested = Expression.MakeMemberAccess(exp, typeof(DtoFavouriteColour).GetProperty("Person"));
            return Expression.MakeMemberAccess(nested, typeof(DtoPerson).GetProperty(node.Member.Name));
        }

        return base.VisitMember(node);
    }
}

目前我得到以下例外

[System.ArgumentException:類型'System.Collections.Generic.ICollection 1[ExpressionVisitorTests.DtoFavouriteColour]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable 1 1[ExpressionVisitorTests.DtoFavouriteColour]' cannot be used for parameter of type 'System.Collections.Generic.IEnumerable [ExpressionVisitorTests.DomainPerson]''方法'布爾值任意[DomainPerson](System.Collections.Generic.IEnumerable 1[ExpressionVisitorTests.DomainPerson], System.Func 2 [ExpressionVisitorTests.DomainPerson,System.Boolean])']

這是一個不起作用的dotnetfiddle

提前感謝您的幫助。

在進行了一些搜索之后,我遇到了John Skeet的 這個答案 ,這導致我想出了一個有效的解決方案,其中包括在ExpressionVisitor上添加一個VisitMethodCall方法的覆蓋,用一個新的MethodInfo替換正確的集合類型。

protected override Expression VisitMethodCall(MethodCallExpression node)
{
    if (node.Method.DeclaringType == typeof(Enumerable) && node.Arguments[0].Type == typeof(ICollection<DomainPerson>))
    {
        Expression obj = Visit(node.Object);
        IEnumerable<Expression> args = Visit(node.Arguments);
        if (obj != node.Object || args != node.Arguments)
        {
            var generic = typeof(Enumerable).GetMethods()
                            .Where(m => m.Name == node.Method.Name)
                            .Where(m => m.GetParameters().Length == node.Arguments.Count)
                            .Single();
            var constructed = generic.MakeGenericMethod(typeof(DtoFavouriteColour));
            return Expression.Call(obj, constructed, args);
        }
    }
    return node;
}

我還需要確保我對_parameters集合的引用沒有被訪問node.Body可能發生的對VisitLambda<T>的嵌套調用所取代。

protected override Expression VisitLambda<T>(Expression<T> node)
{
    var parameters = VisitAndConvert(node.Parameters, "VisitLambda");

    // ensure parameters set but dont let original reference 
    // be overidden by nested calls
    _parameters = parameters;

    return Expression.Lambda(Visit(node.Body), parameters);
}

請參閱dotnetfiddle了解完全可行的解決方案。

如果有人有更好/更優雅的解決方案,請為我添加一個答案。

你已經解決了具體的問題,所以我不能說我要提出的建議是更好/更優雅,但肯定是更通用(刪除了具體的類型/屬性/假設),因此可以重用於翻譯來自不同模型類型的類似表達式。

這是代碼:

public class ExpressionMap
{
    private Dictionary<Type, Type> typeMap = new Dictionary<Type, Type>();
    private Dictionary<MemberInfo, Expression> memberMap = new Dictionary<MemberInfo, Expression>();
    public ExpressionMap Add<TFrom, TTo>()
    {
        typeMap.Add(typeof(TFrom), typeof(TTo));
        return this;
    }
    public ExpressionMap Add<TFrom, TFromMember, TTo, TToMember>(Expression<Func<TFrom, TFromMember>> from, Expression<Func<TTo, TToMember>> to)
    {
        memberMap.Add(((MemberExpression)from.Body).Member, to.Body);
        return this;
    }
    public Expression Map(Expression source) => new MapVisitor { map = this }.Visit(source);

    private class MapVisitor : ExpressionVisitor
    {
        public ExpressionMap map;
        private Dictionary<Type, ParameterExpression> parameterMap = new Dictionary<Type, ParameterExpression>();
        protected override Expression VisitLambda<T>(Expression<T> node)
        {
            return Expression.Lambda(Visit(node.Body), node.Parameters.Select(Map));
        }
        protected override Expression VisitParameter(ParameterExpression node) => Map(node);
        protected override Expression VisitMember(MemberExpression node)
        {
            var expression = Visit(node.Expression);
            if (expression == node.Expression)
                return node;
            Expression mappedMember;
            if (map.memberMap.TryGetValue(node.Member, out mappedMember))
                return Visit(mappedMember);
            return Expression.PropertyOrField(expression, node.Member.Name);
        }
        protected override Expression VisitMethodCall(MethodCallExpression node)
        {
            if (node.Object == null && node.Method.IsGenericMethod)
            {
                // Static generic method
                var arguments = Visit(node.Arguments);
                var genericArguments = node.Method.GetGenericArguments().Select(Map).ToArray();
                var method = node.Method.GetGenericMethodDefinition().MakeGenericMethod(genericArguments);
                return Expression.Call(method, arguments);
            }
            return base.VisitMethodCall(node);
        }
        private Type Map(Type type)
        {
            Type mappedType;
            return map.typeMap.TryGetValue(type, out mappedType) ? mappedType : type;
        }
        private ParameterExpression Map(ParameterExpression parameter)
        {
            var mappedType = Map(parameter.Type);
            ParameterExpression mappedParameter;
            if (!parameterMap.TryGetValue(mappedType, out mappedParameter))
                parameterMap.Add(mappedType, mappedParameter = Expression.Parameter(mappedType, parameter.Name));
            return mappedParameter;
        }
    }
}

以及具體示例的用法:

public IEnumerable<DomainColour> GetWhere(Expression<Func<DomainColour, bool>> predicate)
{
    var map = new ExpressionMap()
        .Add<DomainColour, DtoColour>()
        .Add((DomainColour c) => c.People, (DtoColour c) => c.FavouriteColours.Select(fc => fc.Person))
        .Add<DomainPerson, DtoPerson>();
    var mappedPredicate = ((Expression<Func<DtoColour, bool>>)map.Map(predicate));
    return Colours.Where(mappedPredicate.Compile()).Select(c => new DomainColour(c.Name)).ToList();
}

如您所見,它允許您使用帶有lambda表達式的“fluent”語法定義從一種類型到另一種類型的簡單映射,並且可選地從一種類型的成員到另一種類型的成員/表達式(一旦它們兼容)。 沒有指定映射的成員按名稱映射,如原始代碼中所示。

定義映射后,當然實際處理由自定義ExpressionVisitor ,與您的類似。 區別在於它按類型映射和合並ParameterExpression ,並且還轉換每個靜態泛型方法 ,因此也應該與Queryable和類似工作。

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