如何在实体框架中动态构造按表达式排序

本文关键字：表达式 排序 动态 实体 框架 | 更新日期: 2023-09-27 18:31:07

我使用以下方法来构造 Order By Expression。原始来源

它真的很光滑。缺点是它仅在属性为字符串类型时才有效。

如何使其接受不同的属性类型，而无需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（太多的if语句）

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);
}

缺点是我最终为每个数据类型提供了很多 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); 
}

如何在实体框架中动态构造按表达式排序

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);
}

选项 1 ：这可以使用表达式完成：检查此示例

public static IQueryable<T> OrderByPropertyOrField<T>(this IQueryable<T> queryable, string propertyOrFieldName, bool ascending = true)
{
        var elementType = typeof (T);
        var orderByMethodName = ascending ? "OrderBy" : "OrderByDescending";
        var parameterExpression = Expression.Parameter(elementType);
        var propertyOrFieldExpression = Expression.PropertyOrField(parameterExpression, propertyOrFieldName);
        var selector = Expression.Lambda(propertyOrFieldExpression, parameterExpression);
        var orderByExpression = Expression.Call(typeof (Queryable), orderByMethodName,
                                                new[] {elementType, propertyOrFieldExpression.Type}, queryable.Expression, selector);
        return queryable.Provider.CreateQuery<T>(orderByExpression);
}

选项 2（如果使用 EF 核心）：

 public static IQueryable<TEntity> ApplyOrderBy<TEntity>(
        this IQueryable<TEntity> query, string? orderBy, string orderDirection)
{
        if (orderBy is null) return query;
        
        query = orderDirection == "Asc"
            ? query.OrderBy(p => EF.Property<TEntity>(p!, orderBy))
            : query.OrderByDescending(p => EF.Property<TEntity>(p!, orderBy));
        return query;
 }
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......

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