A method with an unspecified number of Generic Type parameters

Question

Is there a way to work with an unspecified number of Generic Type Parameters in a method?

Effectively what I want to do is something like this except for the obvious issue:

    private static void FetchChunk(int retries, KeyValuePair<List<dynamic>,Type>[] fetchfields)
    {
        try
        {
            foreach (var chunk in fetchfields)
            {
                var obj = chunk.Key;
                obj = DBRepository.GetAll<chunk.Value>().ToList();
            }
        }
        catch
        {
            //do stuff
        }

    }

However this fails because instead of Type, I need to use a Generic Type paramater such as T

The problem is however, I don't know how use generics in such a way to achieve what I am trying to achieve. Since I would want a unique generic type parameter for each KeyValuePair in the array.

This on the other hand, absolutely compiles, but doesn't do what I am intending which is each KeyValue pair would have it's own unique Generic Type parameters.

    private static void FetchChunk<T>(int retries, KeyValuePair<List<dynamic>, T>[] fetchfields)
    {
        try
        {
            foreach (var chunk in fetchfields)
            {
                var obj = chunk.Key;
                obj = DBRepository.GetAll<T>().ToList();
            }
        }
        catch
        {
            //do stuff
        }

The method signature of GetAll is

 public static IEnumerable<dynamic> GetAll(Type type, string columnNames = "*")

This doesn't work either:

                var type = chunk.Value.GenericTypeArguments[0];
                obj = DBRepository.GetAll<type>().ToList();

Is there a way to re-code this method in order to achieve my intentions?

I am not as familiar with reflection. l have used it before for other things, but nothing like this. Can you give an example? — Alexander Ryan Baggett, Apr 20 '17 at 16:56
I don't have the code myself. but the method signature looks like this : public static IEnumerable GetAll(string columnNames = "*"); — Alexander Ryan Baggett, Apr 20 '17 at 16:59
Can you change that code to be `public static IEnumerable GetAll(Type type, string columnNames = "*")` then use `type` in the method to get a non generic method for getting the result set. — Scott Chamberlain, Apr 20 '17 at 17:00
Its in a library that I don't have the code to. I can't modify it. — Alexander Ryan Baggett, Apr 20 '17 at 17:02
@EricLippert I believe I need to something that is the inverse of this: Type typeParameterType = typeof(T); — Alexander Ryan Baggett, Apr 20 '17 at 17:13

score 2 · Accepted Answer · edited May 23 '17 at 12:02

2

If you can use reflection in the wrapping method then as described in the following answer you should be able to construct the instance of the generic method using MakeGenericMethod and then call it with different set of types.

Code updated to match the latest comment from @AlexanderRyanBaggett

private static void FetchChunk(int retries, KeyValuePair<List<dynamic>, Type>[] fetchfields)
{
    try
    {
        foreach (var chunk in fetchfields)
        {
            var method = typeof(DBRepository).GetMethod("GetAll");
            var Generic = method.MakeGenericMethod(chunk.Value);
            chunk.Key.AddRange(Generic.Invoke(null, null) as IEnumerable<dynamic>);
        }
    }
    catch
    {
        //do stuff } }
    }
}

edited May 23 '17 at 12:02

Community

1
1

answered Apr 20 '17 at 18:15

Eugene Komisarenko

1,533
11
25

Let me try this. – Alexander Ryan Baggett Apr 20 '17 at 18:20
I have just fixed a bug in the Invoke.. please make sure you are using latest version. – Eugene Komisarenko Apr 20 '17 at 18:21
This is not quite what I am looking for, but It has the key to solution in it. – Alexander Ryan Baggett Apr 20 '17 at 18:38
This is what works : private static void FetchChunk(int retries, KeyValuePair,Type>[] fetchfields) { try { foreach (var chunk in fetchfields) { var method = typeof(DBRepository).GetMethod("GetAll"); var Generic = method.MakeGenericMethod(chunk.Value); chunk.Key.AddRange(Generic.Invoke(null, null)as IEnumerable); } } catch { //do stuff } } – Alexander Ryan Baggett Apr 20 '17 at 18:39
1

Update the answer with your latest code. I am glad it helped. – Eugene Komisarenko Apr 20 '17 at 18:44

A method with an unspecified number of Generic Type parameters

1 Answers1