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

<分区>

我使用以下方法构造Order By Expression。 Original Source

它真的很光滑。缺点是它仅在 Property 为 string 类型时才有效。

如何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
}

我当前的方法(太多 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); 
}

最佳答案

我在 Jon Skeet 的帮助下找到了解决方案的旧答案。

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);
}

关于c# - 如何在Entity Framework中动态构造Order By Expression？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/34906437/

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