如何在实体框架中动态构造Where表达式? [英] How to construct Where Expression dynamically in Entity Framework?

问题描述

我看过

I've taken a look at this answer on how to dynamically create an OrderBy expression in Entity Framework. But I'd like to also build a dynamic Where expression. Something along the lines of this:

public IEnumerable<InventoryItem> GetAll(string filterBy, object value)
{
  var results = new List<InventoryItem>();
  using (var db = new InventoryDb())
  {
    if (QueryHelper.PropertyExists<InventoryItem>(filterBy)) 
    {
      var query = db.rri_InventoryItems.WhereByProperty(filterBy, value); 

      foreach(var item in query.Where(expr))
      {
        results.Add(ConvertItem(item));
      }   
    }            
  }
  return results;
}

传入属性以按过滤，并将值作为ab对象. Queryable有两种方法，其中Where都带有两个参数，所以我什至不确定哪一个是正确的.

Passing in the property to filter by and a value as ab object. Queryable has two methods for Where that both take two parameters, so I am not even sure which is the proper one.

在这一点上，我迷路了.我不确定如何重构原始的 OrderByProerty 方法以提供 WhereByProperty .我知道我这里完全是错误的.我不确定该怎么办.

And it's at this point I get a little more lost. I'm not sure how to refactor the original OrderByProerty method to provide a WhereByProperty. I know what I have here is completely wrong. I'm not sure what to do with it.

理想情况下，我想通过提供一组对象来进一步扩展此范围，这些对象可用于使用和和或运算符来构建查询.

Ideally, I'd want to extend this even more by providing a collection of objects that could be used to build a query with ands and or operators.

推荐答案

Queryable有两种方法，它们都使用两个参数，所以我什至不确定哪一个是正确的.

Queryable has two methods for Where that both take two parameters, so I am not even sure which is the proper one.

您需要一个接收Expression<Func<T, bool>> predicate的人.

You need the one that receives Expression<Func<T, bool>> predicate.

在这里，您可以动态地构建类似于(T item) => item.Property == value的谓词:

Here is how you can build dynamically a predicate similar to (T item) => item.Property == value:

public static partial class QueryableExtensions
{
    public static IQueryable<T> WhereEquals<T>(this IQueryable<T> source, string member, object value)
    {
        var item = Expression.Parameter(typeof(T), "item");
        var memberValue = member.Split('.').Aggregate((Expression)item, Expression.PropertyOrField);
        var memberType = memberValue.Type;
        if (value != null && value.GetType() != memberType)
            value = Convert.ChangeType(value, memberType);
        var condition = Expression.Equal(memberValue, Expression.Constant(value, memberType));
        var predicate = Expression.Lambda<Func<T, bool>>(condition, item);
        return source.Where(predicate);
    }
}

我试图以这种方式编写它，以便您可以逐步执行代码以了解它的作用.可能需要解释的唯一一行是:

I've tried to write it in such a way so you can step over the code in order to understand what it does. The only line that might need some explanation is:

var memberValue = member.Split('.').Aggregate((Expression)item, Expression.PropertyOrField);

这是处理诸如obj.Prop1.Prop2等嵌套属性的简单方法.如果不需要这种功能，则可以简单地使用此功能:

This is a simple way of handling nested properties like obj.Prop1.Prop2 etc. If you don't need such capability, you can simply use this instead:

var memberValue = Expression.PropertyOrField(item, member);

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