Identifying a custom indexer using reflection in C#

Question

I have a class with a custom indexer like so

public string this[VehicleProperty property]
{
  // Code
}

How can I identify the custom index

Answer 1

If there is only one indexer, you can use this:

var indexer = typeof(MyClass).GetProperties().First(x => x.GetIndexParameters().Length > 0));

If there are multiple indexers, you can select the overload you want by supplying the arguments like this:

var args = new[] { typeof(int) };
var indexer = typeof(MyClass).GetProperties().First(x => x.GetIndexParameters().Select(y => y.ParameterType).SequenceEqual(args));

You can create a helper extension like this:

//Usage      
var indexer = typeof(MyClass).GetIndexer(typeof(VehicleProperty));
//Class
public static class TypeExtensions
{
  public static PropertyInfo GetIndexer(this Type type, params Type[] arguments) => type.GetProperties().First(x => x.GetIndexParameters().Select(y => y.ParameterType).SequenceEqual(arguments));
}

Answer 2

To get a known indexer you can use:

var prop = typeof(MyClass).GetProperty("Item", new object[]{typeof(VehicleProperty)});
var value = prop.GetValue(classInstance, new object[]{ theVehicle });

or you can get the getter method of the indexer:

var getterMethod = typeof(MyClass).GetMethod("get_Item", new object[]{typeof(VehicleProperty)});
var value = getterMethod.Invoke(classInstance, new object[]{ theVehicle });

if the class has only one indexer, you can omit the type:

var prop = typeof(MyClass).GetProperty("Item", , BindingFlags.Public | BindingFlags.Instance);

I've added this answer for the ones who google search led them here.

Answer 3

Look for the DefaultMemberAttribute defined at type level.

(This used to be IndexerNameAttribute, but they seem to have dropped it)

Answer 4

    static void Main(string[] args) {

        foreach (System.Reflection.PropertyInfo propertyInfo in typeof(System.Collections.ArrayList).GetProperties()) {

            System.Reflection.ParameterInfo[] parameterInfos = propertyInfo.GetIndexParameters();
            // then is indexer property
            if (parameterInfos.Length > 0) {
                System.Console.WriteLine(propertyInfo.Name);
            }
        }


        System.Console.ReadKey();
    }

Answer 5

You can also look for index parameters, using the the PropertyInfo.GetIndexParameters method, if it returns more than 0 items, it's an indexed property:

foreach (PropertyInfo pi in typeof(MyClass).GetProperties())
{
    if (pi.GetIndexParameters().Length > 0)
    {
       // Indexed property...
    }
}