我如何才能保持稍后将传递到排序依据或LoadWith功能的lambda表达式的参考？ [英] How can i keep reference of a lambda expression that will be later passed to OrderBy or LoadWith functions?

问题描述

我想创建用于访问数据库的一般方法。这将意味着像参数：页面索引，项目每页显示的数量，订购选项，加载选项等

I am trying to create a general method for accessing the DB. This will imply parameters like: page index, number of items to display per page, ordering options, load options etc.

...

public IQuerable<T> GetAll(int pageIndex, int itemsToDisplayPerPage, System.Linq.Expressions.Expression<Func<T,object>>[] orderBy, System.Linq.Expressions.Expression<Func<T,object>>[] loadOptions)
{
  DataContext dc = null;
  IQuerable<T> result = null;
  // Make some initalizations
  ...
  foreach(var item in orderBy)
  {
    result = result.OrderBy(item);
  }

  System.Data.Linq.DataLoadOptions loadOptions = new System.Data.Linq.DataLoadOptions();
  foreach(var item in loadOptions)
  {
    loadOptions.LoadWith(item);
  }
  ...
}

...

的问题是，在 System.Linq.Expressions.Expression&下; FUNC< T，对象>方式> 类型不是将上面的两个例子可以通过任何lambda表达式的一个很好的一般表示。

The problem is that the System.Linq.Expressions.Expression< Func< T,object > > type is not a good general representation of any lambda expression that will be passed for the both examples above.

在订货会崩溃，因为的对象类型没有任何意义的顺序。同样在loadWith将无法正常工作。所以我不知道如何处理这个问题。有什么建议么？谢谢你。

On ordering will crash because of the object type that does not make any sense for ordering. Also on loadWith will not work. So i dont know how can i handle this problem. Any suggestions? Thank you.

推荐答案

不知道的LoadWith，因为我们使用LINQ到实体，但我们成功拿到排序依据的工作中存储库获取方法。客户端代码如下：

Not sure about the LoadWith, as we are using linq to entities, but we successfully got orderby to work in a repository Get method. The client code looks like this:

var results = _repository.GetAll(
    new GetAllCriteria()
        .OrderBy(x => x.Property1)
        .OrderBy(x => x.Property2)
);

我们不使用泛型方法库，只是还没有，这可能会在未来的重构。但是，标准的实施看起来是这样的：

We aren't using generics in repository methods just yet, that will probably come in a future refactor. But the criteria implementation looks like this:

public class GetAllCriteria
{
    public Dictionary<Expression<Func<CustomType, object>>, bool> ToBeOrderedBy 
    { 
        get; private set;
    }

    public GetAllCriteria OrderBy(
        Expression<Func<CustomType, object>> expression)
    {
        return OrderBy(expression, true);
    }

    public GetAllCriteria OrderByDescending(
        Expression<Func<CustomType, object>> expression)
    {
        return OrderBy(expression, false);
    }

    private GetAllCriteria OrderBy(
        Expression<Func<CustomType, object>> expression, bool isAscending)
    {
        if (expression != null)
        {
            if (ToBeOrderedBy == null)
                ToBeOrderedBy = new Dictionary<Expression<Func<CustomType, object>>, bool>();
            ToBeOrderedBy.Add(expression, isAscending);
        }
        return this;
    }   
}

然后，库订单像这样：

Then, the repository orders like so:

public Collection<CustomType> GetAll(GetAllCriteria criteria)
{
    var query = dbContext.CustomTypes.AsQueryable();

    // some code

    // apply order by
    if (criteria.ToBeOrderedBy != null && criteria.ToBeOrderedBy.Count > 0)
    {
        var firstOrderBy = criteria.ToBeOrderedBy.First();
        query = firstOrderBy.Value
            ? query.OrderBy(firstOrderBy.Key)
            : query.OrderByDescending(firstOrderBy.Key);

        query = criteria.ToBeOrderedBy.Skip(1).Aggregate(query, 
            (lastOrderBy, nextOrderBy) => nextOrderBy.Value
            ? ((IOrderedQueryable<CustomType>)lastOrderBy)
                .ThenBy(nextOrderBy.Key)
            : ((IOrderedQueryable<CustomType>)lastOrderBy)
                .ThenByDescending(nextOrderBy.Key));
    }

    // some more code

    var results = query.ToList();
    return results;
}

如果这个工程使用LINQ到实体，我会想象它应该使用LINQ工作为SQL。

If this works with linq to entities, I would imagine it should work with linq to sql.

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