Question

我有表达

Expression<Func<Car, Driver, bool>> CanBeDrivenBy = 
    (car, driver) => car.Category == 'B' && driver.Age > 18;

我希望得到一些可以由一些司机驾驶的汽车

IQueryable<Cars> cars = ...;
Driver driver = ...;
cars.Where(CanBeDrivenBy);   // Fail, expecting Expression<Func<Car, bool>>

所以我需要将Expression<Func<Car, Driver, bool>>转换为Expression<Func<Car, bool>>（指定驱动程序）

是的，我可以使用

cars.Where(c => c.Category == 'B' && driver.Age > 18);

但是我需要能够动态改变表达式的解决方案。我需要传递Expression（使用实体框架）

Answer 1

这个作品

我编写了这个函数，通过指定第二个参数来减少从2到1的参数数量。

public static Expression<Func<T1, TResult>> Bind2nd<T1, T2, TResult>(Expression<Func<T1, T2, TResult>> source, T2 argument)
{
    Expression arg2 = Expression.Constant(argument, typeof(T2));
    var arg1 = Expression.Parameter(typeof(T1));
    return Expression.Lambda<Func<T1, TResult>>(Expression.Invoke(source, arg1, arg2), arg1);
}

用法：

IQueryable<Car> cars = ...;
Driver driver = ...;
cars.Where(Bind2nd(CanBeDrivenBy, driver));

arg1是来电之间的临时存储。

是否有任何系统等效功能？

Answer 2

您可以重复使用源表达式主体的修改版本

using System;
using System.Linq.Expressions;

public class Program
{
    public static Expression<Func<T1, TResult>> Bind2nd<T1, T2, TResult>(Expression<Func<T1, T2, TResult>> source, T2 argument)
    {
        Expression arg2 = Expression.Constant(argument, typeof (T2));
        Expression newBody = new Rewriter(source.Parameters[1], arg2).Visit(source.Body);
        return Expression.Lambda<Func<T1, TResult>>(newBody, source.Parameters[0]);
    }

    public static void Main(string[] args)
    {
        Expression<Func<string, string, int>> f = (a, b) => a.Length + b.Length;
        Console.WriteLine(f); // (a, b) => (a.Length + b.Length)

        Console.WriteLine(Bind2nd(f, "1")); // a => (a.Length + "1".Length)
    }

    #region Nested type: Rewriter

    private class Rewriter : ExpressionVisitor
    {
        private readonly Expression candidate_;
        private readonly Expression replacement_;

        public Rewriter(Expression candidate, Expression replacement)
        {
            candidate_ = candidate;
            replacement_ = replacement;
        }

        public override Expression Visit(Expression node)
        {
            return node == candidate_ ? replacement_ : base.Visit(node);
        }
    }

    #endregion
}

将2个参数Lambda表达式转换为1个参数Lambda表达式（指定一个参数）

2 个答案: