Question

我已经看到了一些类似的问题，但是找不到一个如何解决这个问题的问题我希望有可能使用字符串作为属性名来订购集合。

型号：

public sealed class User : IdentityUser
{
    #region Constructors

    #endregion

    #region Properties

    [Required]
    [Display(Name = "First name")]
    public string FirstName { get; set; }

    [Required]
    [Display(Name = "Last name")]
    public string LastName { get; set; }

    [ForeignKey("Superior")]
    public string SuperiorId { get; set; }

    public User Superior{ get; set; }

    [NotMapped]
    public string FullName => this.LastName + " " + this.FirstName;

    #endregion
}

我想打个电话：

DbSet.Order("SuperiorUser.FullName", Asc/Desc)...

我的下面的功能适用于Order("FullName", Asc/Desc)，但是当我想要更深入的时候。

public static IOrderedQueryable<T> Order<T>(this IQueryable<T> source, string propertyName,
    ListSortDirection direction = ListSortDirection.Ascending)
{
    return ListSortDirection.Ascending == direction
        ? source.OrderBy(ToLambda<T>(propertyName))
        : source.OrderByDescending(ToLambda<T>(propertyName));
}

private static Expression<Func<T, object>> ToLambda<T>(string propertyName)
{
    var parameter = Expression.Parameter(typeof(T));
    var property = Expression.Property(parameter, propertyName);

    return Expression.Lambda<Func<T, object>>(property, parameter);
}

所以我让它看起来有点像这样

private static Expression<Func<T, object>> ToLambda<T>(string propertyName)
{
    var propertyNames = propertyName.Split('.');
    var type = typeof(T)
    ParameterExpression parameter;
    MemberExpression property;
    for (var propName in propertyNames)
    {
        parameter = Expression.Parameter(type);
        property = Expression.Property(parameter, propName);
        type = property.Type;
    }
    return Expression.Lambda<Func<T, object>>(property, parameter);
}

但不幸的是，这会返回错误The parameter '' was not bound in the specified LINQ to Entities query expression。我错过了什么？

Answer 1

您需要在循环中嵌套Property访问方法，然后将参数应用于lambda。

private static Expression<Func<T, object>> ToLambda<T>(string propertyName) {
    var propertyNames = propertyName.Split('.');
    var parameter = Expression.Parameter(typeof(T));
    Expression body = parameter;
    foreach (var propName in propertyNames)
        body = Expression.Property(body, propName);
    return Expression.Lambda<Func<T, object>>(body, parameter);
}

Answer 2

我曾经为IQueryables编写了一个扩展方法，用于按字符串：

定义的属性进行排序

public static IOrderedQueryable<T> SortBy<T>(this IQueryable<T> source, string propertyName)
{
    if (source == null)
    {
        throw new ArgumentNullException("source");
    }
    else
    {

        if (propertyName.EndsWith(" ASC", StringComparison.OrdinalIgnoreCase))
            propertyName = propertyName.Replace(" ASC", "");

        // DataSource control passes the sort parameter with a direction
        // if the direction is descending           
        int descIndex = propertyName.IndexOf(" DESC", StringComparison.OrdinalIgnoreCase);
        if (descIndex >= 0)
        {
            propertyName = propertyName.Substring(0, descIndex).Trim();
        }

        ParameterExpression parameter = Expression.Parameter(source.ElementType, String.Empty);
        MemberExpression property = Expression.Property(parameter, propertyName);
        LambdaExpression lambda = Expression.Lambda(property, parameter);

        string methodName = (descIndex < 0) ? "OrderBy" : "OrderByDescending";

        Expression methodCallExpression = Expression.Call(typeof(Queryable), methodName,
                                                                                new Type[] { source.ElementType, property.Type },
                                                                                source.Expression, Expression.Quote(lambda));

        return source.Provider.CreateQuery<T>(methodCallExpression) as IOrderedQueryable<T>;
    }
}

你可以调用LIST.AsQueryable（）。SortBy（＆＃34; Surname＆＃34;）。

EntityFramework LINQ使用字符串和嵌套反射的顺序

2 个答案: