IQueryable OfType<T> where T is a runtime Type

问题

I need to be able to get something similar to the following to work:

Type type = ??? // something decided at runtime with .GetType or typeof;
object[] entityList = context.Resources.OfType<type>().ToList();

Is this possible? I am able to use .NET 4 if anything new in that allows this.

回答1:

You can call it by reflection:

MethodInfo method = typeof(Queryable).GetMethod("OfType");
MethodInfo generic = method.MakeGenericMethod(new Type[]{ type });
// Use .NET 4 covariance
var result = (IEnumerable<object>) generic.Invoke
      (null, new object[] { context.Resources });
object[] array = result.ToArray();

An alternative would be to write your own OfTypeAndToArray generic method to do both bits of it, but the above should work.

回答2:

Looks like you’ll need to use Reflection here...

public static IEnumerable<object> DyamicOfType<T>(
        this IQueryable<T> input, Type type)
{
    var ofType = typeof(Queryable).GetMethod("OfType",
                     BindingFlags.Static | BindingFlags.Public);
    var ofTypeT = ofType.MakeGenericMethod(type);
    return (IEnumerable<object>) ofTypeT.Invoke(null, new object[] { input });
}

Type type = // ...;
var entityList = context.Resources.DynamicOfType(type).ToList();

回答3:

what about ...

    public static IList OfTypeToList(this IEnumerable source, Type type)
    {
        if (type == null)
            throw new ArgumentNullException(nameof(type));
        return
            (IList) Activator.CreateInstance(
                typeof(List<>)
                   .MakeGenericType(type),
                typeof(System.Linq.Enumerable)
                   .GetMethod(nameof(System.Linq.Enumerable.OfType),
                              BindingFlags.Static | BindingFlags.Public)
                   .MakeGenericMethod(type)
                   .Invoke(null, new object[] { source }));
    }

回答4:

A solution to handle multiple types is

        public static IQueryable<TEntity> OfTypes<TEntity>(this DbSet<TEntity> query, IEnumerable<Type> types )  where TEntity : class
            {
                    if( types.Count() == 0 ) return query;

                    var lambda = GetOfOnlyTypesPredicate( typeof(TEntity), types.ToArray() );
                    return query.OfType<TEntity>().Where( lambda as Expression<Func<TEntity,bool>>);

            }


            public static LambdaExpression GetOfOnlyTypesPredicate( Type baseType, Type[] allowed )
            {
                    ParameterExpression param = Expression.Parameter( baseType, "typeonlyParam" );
                    Expression merged = Expression.TypeIs( param, allowed[0] );
                    for( int i = 1; i < allowed.Length; i++ )
                            merged = Expression.OrElse( merged, Expression.TypeIs( param, allowed[i] ));
                    return Expression.Lambda( merged, param );

回答5:

Purely on your question to use "Generics", No it is not possible.

Generics is a compile time feature, and not a runtime discovery. For runtime, you need to use Reflection or Dynamic.

回答6:

Based on @Jon Skeet's answer, here's a LINQ extension method:

public static class QueryableExtensions
{
    public static IQueryable<TSource> OfType<TSource>(this IQueryable<TSource> queryable,
        Type runtimeType)
    {
        var method = typeof(Queryable).GetMethod(nameof(Queryable.OfType));
        var generic = method.MakeGenericMethod(new[] { runtimeType });
        return (IQueryable<TSource>)generic.Invoke(null, new[] { queryable });
    }
}

回答7:

Resource[] entityList = context.Resources
                              .Where(t => t.GetType() == typeof(HumanResource))
                              .ToArray();

来源：https://stackoverflow.com/questions/3669760/iqueryable-oftypet-where-t-is-a-runtime-type