将几个类似的SELECT表达式合并为一个表达式 [英] Combine several similar SELECT-expressions into a single expression

问题描述

如何将几个类似的SELECT表达式组合成一个表达式?

How to combine several similar SELECT-expressions into a single expression?

   private static Expression<Func<Agency, AgencyDTO>> CombineSelectors(params Expression<Func<Agency, AgencyDTO>>[] selectors)
    {

        // ???

        return null;
    }

    private void Query()
    {
        Expression<Func<Agency, AgencyDTO>> selector1 = x => new AgencyDTO { Name = x.Name };
        Expression<Func<Agency, AgencyDTO>> selector2 = x => new AgencyDTO { Phone = x.PhoneNumber };
        Expression<Func<Agency, AgencyDTO>> selector3 = x => new AgencyDTO { Location = x.Locality.Name };
        Expression<Func<Agency, AgencyDTO>> selector4 = x => new AgencyDTO { EmployeeCount = x.Employees.Count() };

        using (RealtyContext context = Session.CreateContext())
        {
            IQueryable<AgencyDTO> agencies = context.Agencies.Select(CombineSelectors(selector3, selector4));

            foreach (AgencyDTO agencyDTO in agencies)
            {
                // do something..;
            }
        }
    }

推荐答案

不简单；您需要重写所有表达式-好吧，严格来说，您可以回收其中的大多数表达式，但是问题是每个表达式中都有不同的x(即使它看起来相同)，因此您需要使用访问者将所有参数替换为 final x.幸运的是，这在4.0中还不错:

Not simple; you need to rewrite all the expressions - well, strictly speaking you can recycle most of one of them, but the problem is that you have different x in each (even though it looks the same), hence you need to use a visitor to replace all the parameters with the final x. Fortunately this isn't too bad in 4.0:

static void Main() {
    Expression<Func<Agency, AgencyDTO>> selector1 = x => new AgencyDTO { Name = x.Name };
    Expression<Func<Agency, AgencyDTO>> selector2 = x => new AgencyDTO { Phone = x.PhoneNumber };
    Expression<Func<Agency, AgencyDTO>> selector3 = x => new AgencyDTO { Location = x.Locality.Name };
    Expression<Func<Agency, AgencyDTO>> selector4 = x => new AgencyDTO { EmployeeCount = x.Employees.Count() };

    // combine the assignments from the 4 selectors
    var convert = Combine(selector1, selector2, selector3, selector4);

    // sample data
    var orig = new Agency
    {
        Name = "a",
        PhoneNumber = "b",
        Locality = new Location { Name = "c" },
        Employees = new List<Employee> { new Employee(), new Employee() }
    };

    // check it
    var dto = new[] { orig }.AsQueryable().Select(convert).Single();
    Console.WriteLine(dto.Name); // a
    Console.WriteLine(dto.Phone); // b
    Console.WriteLine(dto.Location); // c
    Console.WriteLine(dto.EmployeeCount); // 2
}
static Expression<Func<TSource, TDestination>> Combine<TSource, TDestination>(
    params Expression<Func<TSource, TDestination>>[] selectors)
{
    var zeroth = ((MemberInitExpression)selectors[0].Body);
    var param = selectors[0].Parameters[0];
    List<MemberBinding> bindings = new List<MemberBinding>(zeroth.Bindings.OfType<MemberAssignment>());
    for (int i = 1; i < selectors.Length; i++)
    {
        var memberInit = (MemberInitExpression)selectors[i].Body;
        var replace = new ParameterReplaceVisitor(selectors[i].Parameters[0], param);
        foreach (var binding in memberInit.Bindings.OfType<MemberAssignment>())
        {
            bindings.Add(Expression.Bind(binding.Member,
                replace.VisitAndConvert(binding.Expression, "Combine")));
        }
    }

    return Expression.Lambda<Func<TSource, TDestination>>(
        Expression.MemberInit(zeroth.NewExpression, bindings), param);
}
class ParameterReplaceVisitor : ExpressionVisitor
{
    private readonly ParameterExpression from, to;
    public ParameterReplaceVisitor(ParameterExpression from, ParameterExpression to)
    {
        this.from = from;
        this.to = to;
    }
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return node == from ? to : base.VisitParameter(node);
    }
}

此函数使用找到的第一个表达式中的构造函数，因此您可能要进行健全性检查，以确保所有其他表达式在各自的NewExpression中都使用平凡的构造函数.不过，我已将其留给读者.

This uses the constructor from the first expression found, so you might want to sanity-check that all of the others use trivial constructors in their respective NewExpressions. I've left that for the reader, though.

在评论中，@ Slaks指出，更多的LINQ可以使它更短.他当然是对的-尽管有点笨拙，但易于阅读:

In the comments, @Slaks notes that more LINQ could make this shorter. He is of course right - a bit dense for easy reading, though:

static Expression<Func<TSource, TDestination>> Combine<TSource, TDestination>(
    params Expression<Func<TSource, TDestination>>[] selectors)
{
    var param = Expression.Parameter(typeof(TSource), "x");
    return Expression.Lambda<Func<TSource, TDestination>>(
        Expression.MemberInit(
            Expression.New(typeof(TDestination).GetConstructor(Type.EmptyTypes)),
            from selector in selectors
            let replace = new ParameterReplaceVisitor(
                  selector.Parameters[0], param)
            from binding in ((MemberInitExpression)selector.Body).Bindings
                  .OfType<MemberAssignment>()
            select Expression.Bind(binding.Member,
                  replace.VisitAndConvert(binding.Expression, "Combine")))
        , param);        
}

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