如何在实体框架中动态构造 Order By Expression？

Question

我使用了以下方法来构造Order By Expression。 Original Source

真的很漂亮。缺点是它只有在属性是 string 类型时才有效。

我怎样才能让它接受不同的属性类型而不为不同的数据类型创建一堆方法？

public static bool PropertyExists<T>(string propertyName)
{
    return typeof (T).GetProperty(propertyName, BindingFlags.IgnoreCase |
      BindingFlags.Public | BindingFlags.Instance) != null;
}

public static Expression<Func<T, string>> GetPropertyExpression<T>(string propertyName)
{
    if (typeof(T).GetProperty(propertyName, BindingFlags.IgnoreCase | 
        BindingFlags.Public | BindingFlags.Instance) == null)
    {
        return null;
    }

    var paramterExpression = Expression.Parameter(typeof(T));

    return (Expression<Func<T, string>>)Expression.Lambda(
        Expression.PropertyOrField(paramterExpression, propertyName), paramterExpression);
}

用法

// orderBy can be either Name or City.
if (QueryHelper.PropertyExists<Club>(orderBy)) 
{ 
   var orderByExpression = QueryHelper.GetPropertyExpression<Club>(orderBy); 
   clubQuery = clubQuery.OrderBy(orderByExpression); 
} 
else 
{ 
   clubQuery = clubQuery.OrderBy(c => c.Id); 
} 

问题

public class Club 
{ 
  public int Id { get; set; } 
  public string Name { get; set; } 
  public string City { get; set; } 
  public DateTime CreateDate { get; set; } <= this won't work
} 

我目前的方法（if 语句太多）

public static Expression<Func<TSource, TKey>> 
    GetPropertyExpression<TSource, TKey>(string propertyName)
{
    if (typeof (TSource).GetProperty(propertyName, BindingFlags.IgnoreCase | 
        BindingFlags.Public | BindingFlags.Instance) == null)
    {
        return null;
    }
    var paramterExpression = Expression.Parameter(typeof (TSource));
    return (Expression<Func<TSource, TKey>>) 
        Expression.Lambda(Expression.PropertyOrField(
           paramterExpression, propertyName), paramterExpression);
}

缺点是每种数据类型都有很多 if 语句。

if (QueryHelper.PropertyExists<Club>(orderBy)) 
{
   if(orderBy == "CreateDate")
   {       
      var orderByExpression = GetPropertyExpression<Club, DateTime>(orderBy);
      ...
   }
   else if(orderBy == "Name" || orderBy == "City")
   {
      var orderByExpression = GetPropertyExpression<Club, string>(orderBy);
      ...
   }
   ...
}
else 
{ 
   clubQuery = clubQuery.OrderBy(c => c.Id); 
} 

Answer 1

我在Jon Skeet 的旧答案的帮助下找到了解决方案。

public static class QueryHelper
{
    private static readonly MethodInfo OrderByMethod =
        typeof (Queryable).GetMethods().Single(method => 
        method.Name == "OrderBy" && method.GetParameters().Length == 2);

    private static readonly MethodInfo OrderByDescendingMethod =
        typeof (Queryable).GetMethods().Single(method => 
        method.Name == "OrderByDescending" && method.GetParameters().Length == 2);

    public static bool PropertyExists<T>(this IQueryable<T> source, string propertyName)
    {
        return typeof(T).GetProperty(propertyName, BindingFlags.IgnoreCase |
            BindingFlags.Public | BindingFlags.Instance) != null;
    }

    public static IQueryable<T> OrderByProperty<T>(
       this IQueryable<T> source, string propertyName)
    {
        if (typeof (T).GetProperty(propertyName, BindingFlags.IgnoreCase | 
            BindingFlags.Public | BindingFlags.Instance) == null)
        {
            return null;
        }
        ParameterExpression paramterExpression = Expression.Parameter(typeof (T));
        Expression orderByProperty = Expression.Property(paramterExpression, propertyName);
        LambdaExpression lambda = Expression.Lambda(orderByProperty, paramterExpression);
        MethodInfo genericMethod = 
          OrderByMethod.MakeGenericMethod(typeof (T), orderByProperty.Type);
        object ret = genericMethod.Invoke(null, new object[] {source, lambda});
        return (IQueryable<T>) ret;
    }

    public static IQueryable<T> OrderByPropertyDescending<T>(
        this IQueryable<T> source, string propertyName)
    {
        if (typeof (T).GetProperty(propertyName, BindingFlags.IgnoreCase | 
            BindingFlags.Public | BindingFlags.Instance) == null)
        {
            return null;
        }
        ParameterExpression paramterExpression = Expression.Parameter(typeof (T));
        Expression orderByProperty = Expression.Property(paramterExpression, propertyName);
        LambdaExpression lambda = Expression.Lambda(orderByProperty, paramterExpression);
        MethodInfo genericMethod = 
          OrderByDescendingMethod.MakeGenericMethod(typeof (T), orderByProperty.Type);
        object ret = genericMethod.Invoke(null, new object[] {source, lambda});
        return (IQueryable<T>) ret;
    }
}

用法

string orderBy = "Name";
if (query.PropertyExists(orderBy))
{
   query = query.OrderByProperty(orderBy);
   - OR - 
   query = query.OrderByPropertyDescending(orderBy);
}

Answer 2

恕我直言，我有一个更简单的解决方案：

public static IOrderedQueryable<TSource> Sort<TSource>(this IQueryable<TSource> source, bool ascending , string sortingProperty)
{
    if (ascending)
        return source.OrderBy(item => item.GetReflectedPropertyValue(sortingProperty));
    else
        return source.OrderByDescending(item => item.GetReflectedPropertyValue(sortingProperty));
}

private static object GetReflectedPropertyValue(this object subject, string field)
{
    return subject.GetType().GetProperty(field).GetValue(subject, null);
}

用法是：

myQueryableCollection.Sort(ascending: true, "Name")

当然，PropertyExist() 助手是一个很好的补充......