问题
What's the simplest way to code against a property in C# when I have the property name as a string? For example, I want to allow the user to order some search results by a property of their choice (using LINQ). They will choose the "order by" property in the UI - as a string value of course. Is there a way to use that string directly as a property of the linq query, without having to use conditional logic (if/else, switch) to map the strings to properties. Reflection?
Logically, this is what I'd like to do:
query = query.OrderBy(x => x."ProductId");
Update: I did not originally specify that I'm using Linq to Entities - it appears that reflection (at least the GetProperty, GetValue approach) does not translate to L2E.
回答1:
I would offer this alternative to what everyone else has posted.
System.Reflection.PropertyInfo prop = typeof(YourType).GetProperty("PropertyName");
query = query.OrderBy(x => prop.GetValue(x, null));
This avoids repeated calls to the reflection API for obtaining the property. Now the only repeated call is obtaining the value.
However
I would advocate using a PropertyDescriptor instead, as this will allow for custom TypeDescriptors to be assigned to your type, making it possible to have lightweight operations for retrieving properties and values. In the absence of a custom descriptor it will fall back to reflection anyhow.
PropertyDescriptor prop = TypeDescriptor.GetProperties(typeof(YourType)).Find("PropertyName");
query = query.OrderBy(x => prop.GetValue(x));
As for speeding it up, check out Marc Gravel's HyperDescriptor project on CodeProject. I've used this with great success; it's a life saver for high-performance data binding and dynamic property operations on business objects.
回答2:
I'm a little late to the party, however, I hope this can be of some help.
The problem with using reflection is that the resulting Expression Tree will almost certainly not be supported by any Linq providers other than the internal .Net provider. This is fine for internal collections, however this will not work where the sorting is to be done at source (be that SQL, MongoDb, etc.) prior to pagination.
The code sample below provides IQueryable extention methods for OrderBy and OrderByDescending, and can be used like so:
query = query.OrderBy("ProductId");
Extension Method:
public static class IQueryableExtensions
{
public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> source, string propertyName)
{
return source.OrderBy(ToLambda<T>(propertyName));
}
public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> source, string propertyName)
{
return source.OrderByDescending(ToLambda<T>(propertyName));
}
private static Expression<Func<T, object>> ToLambda<T>(string propertyName)
{
var parameter = Expression.Parameter(typeof(T));
var property = Expression.Property(parameter, propertyName);
var propAsObject = Expression.Convert(property, typeof(object));
return Expression.Lambda<Func<T, object>>(propAsObject, parameter);
}
}
Regards, Mark.
回答3:
I liked the answer from @Mark Powell, but as @ShuberFu said, it gives the error LINQ to Entities only supports casting EDM primitive or enumeration types.
Removing var propAsObject = Expression.Convert(property, typeof(object)); didn't work with properties that were value types, such as integer, as it wouldn't implicitly box the int to object.
Using Ideas from Kristofer Andersson and Marc Gravell I found a way to construct the Queryable function using the property name and have it still work with Entity Framework. I also included an optional IComparer parameter. Caution: The IComparer parameter does not work with Entity Framework and should be left out if using Linq to Sql.
The following works with Entity Framework and Linq to Sql:
query = query.OrderBy("ProductId");
And @Simon Scheurer this also works:
query = query.OrderBy("ProductCategory.CategoryId");
And if you are not using Entity Framework or Linq to Sql, this works:
query = query.OrderBy("ProductCategory", comparer);
Here is the code:
public static class IQueryableExtensions
{
public static IOrderedQueryable<T> OrderBy<T>(this IQueryable<T> query, string propertyName, IComparer<object> comparer = null)
{
return CallOrderedQueryable(query, "OrderBy", propertyName, comparer);
}
public static IOrderedQueryable<T> OrderByDescending<T>(this IQueryable<T> query, string propertyName, IComparer<object> comparer = null)
{
return CallOrderedQueryable(query, "OrderByDescending", propertyName, comparer);
}
public static IOrderedQueryable<T> ThenBy<T>(this IOrderedQueryable<T> query, string propertyName, IComparer<object> comparer = null)
{
return CallOrderedQueryable(query, "ThenBy", propertyName, comparer);
}
public static IOrderedQueryable<T> ThenByDescending<T>(this IOrderedQueryable<T> query, string propertyName, IComparer<object> comparer = null)
{
return CallOrderedQueryable(query, "ThenByDescending", propertyName, comparer);
}
/// <summary>
/// Builds the Queryable functions using a TSource property name.
/// </summary>
public static IOrderedQueryable<T> CallOrderedQueryable<T>(this IQueryable<T> query, string methodName, string propertyName,
IComparer<object> comparer = null)
{
var param = Expression.Parameter(typeof(T), "x");
var body = propertyName.Split('.').Aggregate<string, Expression>(param, Expression.PropertyOrField);
return comparer != null
? (IOrderedQueryable<T>)query.Provider.CreateQuery(
Expression.Call(
typeof(Queryable),
methodName,
new[] { typeof(T), body.Type },
query.Expression,
Expression.Lambda(body, param),
Expression.Constant(comparer)
)
)
: (IOrderedQueryable<T>)query.Provider.CreateQuery(
Expression.Call(
typeof(Queryable),
methodName,
new[] { typeof(T), body.Type },
query.Expression,
Expression.Lambda(body, param)
)
);
}
}
回答4:
Yes, I don't think there's another way than Reflection.
Example:
query = query.OrderBy(x => x.GetType().GetProperty("ProductId").GetValue(x, null));
回答5:
query = query.OrderBy(x => x.GetType().GetProperty("ProductId").GetValue(x, null));
Trying to recall exact syntax off the top of my head but I think that is correct.
回答6:
Reflection is the answer!
typeof(YourType).GetProperty("ProductId").GetValue(theInstance);
There's lots of things you can do to cache the reflected PropertyInfo, check for bad strings, write your query comparison function, etc., but at its heart, this is what you do.
回答7:
You can use dynamic Linq - check out this blog.
Also check out this StackOverFlow post...
回答8:
More productive than reflection extension to dynamic order items:
public static class DynamicExtentions
{
public static object GetPropertyDynamic<Tobj>(this Tobj self, string propertyName) where Tobj : class
{
var param = Expression.Parameter(typeof(Tobj), "value");
var getter = Expression.Property(param, propertyName);
var boxer = Expression.TypeAs(getter, typeof(object));
var getPropValue = Expression.Lambda<Func<Tobj, object>>(boxer, param).Compile();
return getPropValue(self);
}
}
Example:
var ordered = items.OrderBy(x => x.GetPropertyDynamic("ProductId"));
Also you may need to cache complied lambas(e.g. in Dictionary<>)
回答9:
Also Dynamic Expressions can solve this problem. You can use string-based queries through LINQ expressions that could have been dynamically constructed at run-time.
var query = query
.Where("Category.CategoryName == @0 and Orders.Count >= @1", "Book", 10)
.OrderBy("ProductId")
.Select("new(ProductName as Name, Price)");
来源：https://stackoverflow.com/questions/1689199/c-sharp-code-to-order-by-a-property-using-the-property-name-as-a-string