如何在实体框架中动态构造 Order By Expression? [英] How to construct Order By Expression dynamically in Entity Framework?
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问题描述
我使用以下方法构建Order By Expression.原始来源
它真的很光滑.缺点是它仅在 Property 为 string 类型时才有效.
It is really slick. The downside is it only works if Property is string type.
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?
How can I make it to accept different Property type without creating a bunch of methods for different data types?
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);
}
缺点是我最终会为每种数据类型编写大量 if 语句.
The downside is I end up with a lot of if statements for each datatype.
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);
}
推荐答案
我在 Jon Skeet 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);
}
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