在LINQ中获取动态OrderBy

Question

我在ASP.NET MVC 3站点中的很多页面中使用数据表.他们使用服务器端分页,现在我想基于列标题实现排序.数据表带有iSortCol_0 哪个是单击列的int值.

我不喜欢这种方法,因为查询最终会像:

if(iSortCol_0 == 0)
{    
    // query
    // OderBy(x => x.CarName)
}

然后对每列重复这一过程(加上每个列的else子句按降序排序).所以我改变了我的方法,现在将列名传递给服务器.

我想出了以下内容:

Expression<Func<vw_Car, string>> sortExpression1 = null;
Expression<Func<vw_Car, int>> sortExpression2 = null;

switch(columnToSort) 
{
    case "InvoiceNo": sortExpression1 = x => x.CarNo; break;
    case "ClientNumber": sortExpression1 = x => x.ForeName; break;
    case "ClientName": sortExpression1 = x => x.SurName; break;
    default: sortExpression2 =  x => x.Age.Value; break;
}

// start of query
.OrderByDir(sortDirection, sortExpression1 , sortExpression2)

现在OrderByDir看起来如下:

public static IOrderedQueryable<T> OrderByDir<T>(this IQueryable<T> source, string dir, Expression<Func<T, string>> column1, Expression<Func<T, int>> column2)
{
    if (column1 != null)
    {
        return dir == "asc" ? source.OrderBy(column1) : source.OrderByDescending(column1);
    }         
    if (column2 != null)
    {
        return dir == "asc" ? source.OrderBy(column2) : source.OrderByDescending(column2);
    }    
    return null;
}

这适用于它对类型string或类型的列进行排序int.我有另一列DateTime我想要排序,所以我需要写另一个sortExpression3,然后将其添加到我的OrderByDir.

但我真的不喜欢的实现-我曾试图写了一个对象作为第二个参数,而不是一个通用的排序表达式string,int,Datetime,但是当在地方有这个代码,我越来越无法投型"系统. DateTime'键入'System.Object'.LINQ to Entities仅支持转换实体数据模型基元类型.

任何一个关于可能更好的方法的想法？

Answer 1

此类扩展应符合您的需求:

public static class LinqExtensions
{
    public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder(source, property, "OrderBy");
    }

    public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string property)
    {
        return ApplyOrder(source, property, "OrderByDescending");
    }

    private static IOrderedQueryable<T> ApplyOrder<T>(IQueryable<T> source, string property, string methodName)
    {
        var props = property.Split('.');
        var type = typeof(T);
        var arg = Expression.Parameter(type, "x");
        Expression expr = arg;
        foreach (var prop in props)
        {
            // use reflection (not ComponentModel) to mirror LINQ
            var pi = type.GetProperty(prop);
            expr = Expression.Property(expr, pi);
            type = pi.PropertyType;
        }
        var delegateType = typeof(Func<,>).MakeGenericType(typeof(T), type);
        var lambda = Expression.Lambda(delegateType, expr, arg);

        var result = typeof(Queryable).GetMethods().Single(
                method => method.Name == methodName
                        && method.IsGenericMethodDefinition
                        && method.GetGenericArguments().Length == 2
                        && method.GetParameters().Length == 2)
                .MakeGenericMethod(typeof(T), type)
                .Invoke(null, new object[] { source, lambda });
        return (IOrderedQueryable<T>)result;
    }
}

调用示例:

query.OrderBy("Age.Value");