如何组合多个 c# Lambda 表达式（Expression<Func<T, T>>）

Question

我有以下函数，它实际上是 Z.EntityFramework.Plus 批量更新的包装器：

    public static int UpdateBulk<T>(this IQueryable<T> query, Expression<Func<T, T>> updateFactory) where T : IBaseEntity, new()
    {
        Expression<Func<T, T>> modifiedExpression = x => new T() { ModifiedBy = "Test", ModifiedDate = DateTime.Now };
        var combine = Expression.Lambda<Func<T, T>>(
            Expression.AndAlso(
                Expression.Invoke(updateFactory, updateFactory.Parameters),
                Expression.Invoke(modifiedExpression, modifiedExpression.Parameters)
            ),
            updateFactory.Parameters.Concat(modifiedExpression.Parameters)
        );  //This returns an error

        return query.Update(combine);
    }

这样称呼：

        decimal probId = ProbId.ParseDecimal();

        db.Problems
            .Where(e => e.ProbId == probId)
            .UpdateBulk(e => new Problem() {
                CatId = Category.ParseNullableInt(),
                SubCatId = SubCategory.ParseNullableInt(),
                ListId = Problem.ParseNullableInt()
            });

其中IBaseEntity定义如下：

public abstract class IBaseEntity
{
    public System.DateTime CreatedDate { get; set; }
    public string CreatedBy { get; set; }

    public System.DateTime ModifiedDate { get; set; }
    public string ModifiedBy { get; set; }

    public string DeletedBy { get; set; }
}

顺便说一句，“问题”类实现了 IBaseEntity。

我想要做的是自动将 ModifiedBy 和 ModifiedDate 附加到 UpdateBulk 函数中的 updateFactory，这样就不必在每次调用 UpdateBulk 时都这样做。

我尝试在上面的 UpdateBulk 函数中将解析的“updateFactory”表达式与“modifiedExpression”组合，但它返回错误：

二元运算符 AndAlso 没有为类型“问题”定义

是否可以像这样合并表达式，如果可以，我做错了什么？ 如果没有，如何将 ModifiedBy = "Test", ModifiedDate = DateTime.Now 合并到 updateFactory 表达式中？

谢谢， 棒

Answer 1

你不能使用AndAlso，因为那是为了BinaryExpression - Expression<Func<T,bool>>，在这种情况下你需要Expression Visitor，正如Marc Gravell所定义的here（所以他应该得到所有的功劳）

我正在使用相同的方法为您的情况提供解决方案，假设为 Problem class schema，粘贴 Linqpad 代码：

void Main()
{
  var final = UpdateBulk((Problem p) => new Problem{CatId = 1,SubCatId = 2, ListId=3});

 // final is of type Expression<Func<T,T>>, which can be used for further processing

  final.Dump();
}

public static Expression<Func<T, T>> UpdateBulk<T>(Expression<Func<T, T>> updateFactory) where T : IBaseEntity, new()
{
    Expression<Func<T, T>> modifiedExpression = x => new T() { ModifiedBy = "Test", ModifiedDate = DateTime.Now };

    var result = Combine(updateFactory, modifiedExpression);

    return result;
}


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);
}

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);
    }
}

public abstract class IBaseEntity
{
    public System.DateTime CreatedDate { get; set; }
    public string CreatedBy { get; set; }

    public System.DateTime ModifiedDate { get; set; }
    public string ModifiedBy { get; set; }

    public string DeletedBy { get; set; }
}

public class Problem : IBaseEntity
{
    public int CatId { get; set; }

    public int SubCatId { get; set; }

    public int ListId { get; set; }
}