c# - 扩展 Marc Gravell 的动态 Linq OrderBy

Question

我发现 Marc Gravell 的动态订单很棒:

Dynamic LINQ OrderBy on IEnumerable<T>

我已经把它放在类 LinqHelper 中了。在这个类中，我还创建了两个新类，因此在我的代码中我可以这样做:

var q = db.tblJobHeaders;

LinqHelper.OrderByCollection OBys = new LinqHelper.OrderByCollection();
OBys.AddOrderBy("some field", true);
OBys.AddOrderBy("anotherfield", false);
OBys.ExecuteOrderBys(q);

实现这一目标的类是:

/// <summary>
/// A collection of order bys
/// </summary>
public class OrderByCollection
{
    private ArrayList Orderings = new ArrayList();

    public OrderByCollection(){ }

    /// <summary>
    /// Add an order by to this collection
    /// </summary>
    public void AddOrderBy(string Field, bool Descending)
    {
        OrderByObj NewObj = new OrderByObj(Descending, Field);
        this.Orderings.Add(NewObj);
    }

    /// <summary>
    /// Executes the order bys
    /// </summary>
    public IOrderedQueryable<T> ExecuteOrderBys<T>(this IOrderedQueryable<T> source)
    {
        int ExecutionIndex = 0;
        foreach (OrderByObj O in this.Orderings)
        {
            if (ExecutionIndex == 0)
            {
                if (O.Descending)
                    source = LinqHelper.OrderByDescending(source, O.Field);
                else
                    source = LinqHelper.OrderBy(source, O.Field);
            }
            else
            {
                if (O.Descending)
                    source = LinqHelper.ThenByDescending(source, O.Field);
                else
                    source = LinqHelper.ThenBy(source, O.Field);
            }
            ExecutionIndex++;
        }
        return (IOrderedQueryable<T>)source;
    }
}

/// <summary>
/// An order by object
/// </summary>
private class OrderByObj
{
    public bool Descending { get; set; }
    public string Field { get; set; }

    public OrderByObj(bool IsDescending, string DatabaseField)
    {
        this.Descending = IsDescending;
        this.Field = DatabaseField;
    }
}

不过我对将 Linq 变量传递给函数还很陌生(这让我有点困惑)。我目前收到以下错误:

OBys.ExecuteOrderBys(q);

这给出了错误:

The type arguments for method 'LinqHelper.OrderByCollection.ExecuteOrderBys(System.Linq.IOrderedQueryable)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

如果有人能提供帮助，我对此有点困惑，我是否正确传递了 var q，然后正确返回了它？

Answer 1

我打赌 q 的类型是IQueryable<T>而不是 IOrderedQueryable<T> .只需更改签名就可以了，因为您从 OrderBy 开始.

然后你需要一个IOrderedQueryable<T>对于 ThenBy秒。你可以直接转换它，因为你确定你有一个 IOrderedQueryable<T>从之前调用 OrderBy或 ThenBy .

如果您不喜欢类型转换的想法，您需要进行一些更改:

public IOrderedQueryable<T> ExecuteOrderBys<T>(this IQueryable<T> source)
{
    if(!this.Orderings.Any())
        throw new InvalidOperationException("You need to add orderings");
    IOrderedQueryable<T> ordered;
    if (this.Orderings[0].Descending)
        ordered = LinqHelper.OrderByDescending(source, this.Orderings[0].Field);
    else
        ordered = LinqHelper.OrderBy(source, this.Orderings[0].Field);
    foreach(var ordering in this.Orderings.Skip(1))
    {
        if (ordering.Descending)
            ordered = LinqHelper.ThenByDescending(source, ordering.Field);
        else
            ordered = LinqHelper.ThenBy(source, ordering.Field);
    }
    return ordered;
}

请注意，如果您不添加任何顺序，您的代码将会失败，因为转换为 IOrderedQueryable<T>。到底。您可以将返回类型更改为 IQueryable<T> (它失去了稍后“附加”更多 OrderBys 的能力)，或者如果没有订单则抛出，就像我所做的那样。