c# - 替换参数以指向 lambda 表达式中的嵌套参数

Question

感谢 previous question 上的一些答案，我可以成功地替换 lambda 表达式中的简单参数类型但我不知道如何将参数从传入的 lambda 替换为嵌套参数。

考虑以下对象:

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; }
}

和一个存储库:

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();
    }
}

最后是一个表达式访问者，它将谓词转换为 Dto 模型的正确谓词

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);
    }
}

目前我得到以下异常

[System.ArgumentException: Expression of type 'System.Collections.Generic.ICollection1[ExpressionVisitorTests.DtoFavouriteColour]'
cannot be used for parameter of type
'System.Collections.Generic.IEnumerable1[ExpressionVisitorTests.DomainPerson]' of method 'Boolean Any[DomainPerson](System.Collections.Generic.IEnumerable1[ExpressionVisitorTests.DomainPerson],
System.Func2[ExpressionVisitorTests.DomainPerson,System.Boolean])']

这是一个dotnetfiddle它不起作用。

提前感谢您的帮助。

Answer 1

经过更多搜索，我找到了 this answer通过 John Skeet这让我想出了一个可行的解决方案，其中包括为 VisitMethodCall 添加一个覆盖。 ExpressionVisitor 上的方法替换原来的MethodInfo为正确的集合类型使用一个新的。

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;
}

我还需要确保我对 _parameters 的引用集合未被对 VisitLambda<T> 的嵌套调用所取代这可能在访问 node.Body 时发生.

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);
}

参见 dotnetfiddle用于完全工作的解决方案。

如果有人有更好/更优雅的解决方案请添加答案让我标记。