How to search a list for specific implementation of an interface.

Question

3.00/5 (1 vote)

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I'm trying to search a list for implementations of an interface, but cannot figure the syntax out for finding specific implementations.

What I have basically looks like this:

C#

public interface IVehicle
{
  void Speed()
  void Weight()
}

Public class Car : IVehicle
{
  //Methods implemented
}

Another example of an IVehicle would be

C#

public class Truck : IVehicle
{
  //Some Methods to implement
}

In this class I have a list of IVehicles that I would like to find Car from.

C#

Public class Road
{
  List<IVehicle> vehicles = new List<IVehicle>();

  void SomeMethod()
  {
    vehicles.Add(new Car);

    //How would I search for a Car.
  }
}

Any help would be greatly appreciated. What is off about my syntax? I've managed to pull Car out, if I make sure to hit the precise index, and the methods run correctly, but I'd like to not have to add in a classes to the List while insuring a precise order is followed. For instance, at one point I want to pull Car out but not Truck. Thanks in advance.

Posted 30-Nov-13 14:41pm

Zerophase

Updated 30-Nov-13 14:59pm

v2

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BillWoodruff 30-Nov-13 20:54pm

Please post code that you know can be compiled: there are several reasons your code will not compile now.

Since every instance of Class 'Car inherits from the interface 'IVehicle, then your question doesn't quite make sense to me: your question implies there could be a 'Car which does not inherit from 'IVehicle.

Please clarify your question.

Zerophase 30-Nov-13 21:01pm

I'm looking for the first instance of Car, (How I'm coding it there is only going to be one.) but I don't want to find instances of Trucks in this example.

BillWoodruff 1-Dec-13 1:38am

Shifting gears toward the "bigger picture:" have you considered possibly keeping separate generic Lists for each Type of Vehicle ?

Or, you might consider a Dictionary<vehicletype,List<vehicleType>>

If you need to frequently access instances of Classes that implement IVehicle, perhaps not having to bend-over to filter out Classes that are not the flavor you want would pay off in terms of performance ?

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tgrt · Accepted Answer · 2013-11-30T15:03:00

Solution 1

C#

vehicles.Find(v => v is Car).Speed();

Posted 30-Nov-13 15:03pm

tgrt

Comments

Zerophase 30-Nov-13 21:18pm

Thanks, that did it.

Sergey Alexandrovich Kryukov 30-Nov-13 21:40pm

It would work, but only for this exact type, Car, not for any descendants. As we don't know exact requiremens from OP (in OOP, "to find Car" is ambiguous, due to possible inheritance), I voted 5 anyway. (Please see my other comment below.)

All correct, my 5.

For OP, I want to note that the whole idea does look like a bad style of code design, violation of OOP.

—SA

BillWoodruff 30-Nov-13 23:29pm

Sergey, I think that determining if a given class is a descendant of another is the more interesting (potential) aspect of this question. I have never been able to get a solution using Linq to do that, although I am sure there is a way.

So, I might use something like this, if I know that the inheritance tree is only one-level deep:

foreach (IVehicle theVehicle in vehicles)
{
if (theVehicle.GetType().BaseType.Name == "Car")
{
theVehicle.Speed();
break;
}
}

Sergey Alexandrovich Kryukov 30-Nov-13 23:59pm

Comparing a type name is a really bad solution, much worse then "is Car", especially if you ignore namespaces (some other type can have identical simple name, and this is even a good style to have, in certain situations, identical simple names, I often use it).

But my statement was mistaken: I just forgot that "is" really means the same type or descendant. You can easily test it.

When you deal with Reflection and System.Type, there is another approach:
http://msdn.microsoft.com/en-us/library/system.type.isassignablefrom%28v=vs.110%29.aspx.

—SA

Sergey Alexandrovich Kryukov 1-Dec-13 0:01am

And if you want to compare exact type, you need to do:


it (myInstance.GetType() == typeof(Car)) {
    // then myInstance is of the exact type of Car, not descendant
}

BillWoodruff 1-Dec-13 1:17am

In the case of a top-level search, you are absolutely right !

But, to search for a Class that inherits, one-level down, this will not work:

// assume a Class 'Renault that inherits from 'Car, and assume
// that an instance of the 'Renault Class is at position
// #0 in the 'vehicles list

foreach (IVehicle theVehicle in vehicles)
{
if (theVehicle.GetType().BaseType == typeof(Car))
{
theVehicle.Speed();
break;
}
}

Yet, if you examine the Type information for an instance of Class 'Renault in the Command Window, you can see it does have a BaseType of Type 'Car !

Good discussion/code of/for depth search for ancestor here:

http://stackoverflow.com/questions/8868119/find-all-parent-types-both-base-classes-and-interfaces

Sergey Alexandrovich Kryukov 1-Dec-13 12:04pm

This is what I say, '==' on the System.Type checks up exact type. It should not work for inherited classes, quite apparently.
And, with either approach, one-level, two-levels — this is all the same, also obviously.
—SA

tgrt 1-Dec-13 23:14pm

Thank you. I agree the design might be a bit smelly, but then again this was just a manufactured example to illustrate the point. I'll give the OP the benefit of the doubt.

Sergey Alexandrovich Kryukov 2-Dec-13 1:12am

OP's design is probably bad, but there is nothing wrong with your solution.
Thank you,
—SA