Question

我使用以下方法构建按表达式排序。 Original Source

真的很光滑。缺点是它仅在Property为 string 类型时才有效。

如何在不为不同数据类型创建一堆方法的情况下接受不同的属性类型？

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
}

我当前的方法（如果陈述太多）

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);
}

缺点是我最终会为每种数据类型添加很多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>(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 (QueryHelper.PropertyExists<Club>(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()助手是很棒的补充...

如何在Entity Framework中动态构造Order By Expression？

用法

问题

我当前的方法（如果陈述太多）

2 个答案:

用法