linq - 用于排序导航属性的动态 LINQ 表达式

Question

MVC3， Entity Framework 4.1 代码优先。

使用 2 个表

模型:

public class UniversityMaster
{
   [Key]
   public string UniversityId { get; set; }
   public string UniversityName { get; set; }

}

public class ProgramMaster
{
    [Key]
    public string ProgramId { get; set; }
    public string ProgramName { get; set; }
    public string UniversityId { get; set; }
    public virtual UniversityMaster University { get; set; } // navigation property

}

排序的动态表达式(只是为了避免 switch case 语句):

public virtual IQueryable< ProgramMaster > GetQueryableSort(string sortField="", string sortDirection="")
   {
      IQueryable<ProgramMaster> query = _dbSet;
      ParameterExpression pe = Expression.Parameter(typeof(ProgramMaster), string.Empty);
      MemberExpression property = Expression.PropertyOrField(pe, sortField);
     //get a exception here if the sort field is of navigation property                            (University.UniversityName)
      LambdaExpression lambda = Expression.Lambda(property, pe);
      if (sortDirection == "ASC")
         orderbydir = "OrderBy";
      else
         orderbydir = "OrderByDescending";
         MethodCallExpression call = Expression.Call(typeof(Queryable),
         orderbydir, new Type[] { typeof(TEntity), property.Type }, query.Expression,  Expression.Quote(lambda));

      var returnquery = (IOrderedQueryable<ProgramMaster>)query.Provider.CreateQuery< ProgramMaster >(call);
      return returnquery;
    }

该页面使用 webgrid 显示一个包含两列 Program Name 和 University Name 的网格。 Program Name 列的排序工作正常，但是如果按大学名称排序则会失败，因为此属性在 UniversityMaster 中并且 Expression.PropertyOrField 在 ProgramMaster 中搜索此属性。这是异常(exception):

University.UniversityName' is not a member of type 'App.Core.Model.ProgramMaster

我的问题是我如何使我的模型类的导航属性工作。

希望我能够解释这个场景。任何帮助表示赞赏。

Answer 1

那是因为 MemberExpression正在尝试调用名为 Univerty.UniversityName 的成员在参数上。您要做的是调用名为 Univerity 的成员在参数上，然后调用 UniversityName在那。实际上，您需要迭代地解析属性名称。

public virtual IQueryable< ProgramMaster > GetQueryableSort(string sortField = "", string sortDirection = "")
{
    IQueryable<ProgramMaster> query = _dbSet;

    var propertyNames = sortField.Split(".");

    ParameterExpression pe = Expression.Parameter(typeof(ProgramMaster), string.Empty);
    Expression property = pe;
    foreach(var prop in propertyName)
    {
        property = Expression.PropertyOrField(property, prop);
    }

    LambdaExpression lambda = Expression.Lambda(property, pe);

    if (sortDirection == "ASC")
        orderbydir = "OrderBy";
    else
        orderbydir = "OrderByDescending";

    MethodCallExpression call = Expression.Call(
        typeof(Queryable),
        orderbydir, 
        new Type[] { typeof(TEntity), property.Type }, 
        query.Expression, 
        Expression.Quote(lambda));

    var returnquery = (IOrderedQueryable<ProgramMaster>)query.Provider.CreateQuery<ProgramMaster>(call);

    return returnquery;
}