Question

I have the name & type of an object and the method name.
Ideally I would like to instantiate the object & call the method. All is well with that, but I am getting an error trying to process the results.

private void GetData(DropDownList ddl)
    {
        ObjectDataSource ods = (ObjectDataSource)ddl.DataSourceObject;
        System.Reflection.Assembly assembly = typeof(ExistingObject).Assembly;

        Type type = assembly.GetType(ods.SelectMethod);
        var instance = Activator.CreateInstance(type);

        IterateCollection(instance, ddl);
    }

    private static void IterateCollection<T>(T instance, DropDownList ddl) 
    {
        ObjectDataSource ods = (ObjectDataSource)ddl.DataSourceObject;
        Type type = instance.GetType();
        MethodInfo methodInfo = type.GetMethod(ods.SelectMethod);

        LinkedList<T> col = (LinkedList<T>)methodInfo.Invoke(null, new object[] { (T)instance });

        foreach (T o in col)
        {
            string textfield = Convert.ToString(GetPropValue(o, ddl.DataTextField));
            string valuefield = Convert.ToString(GetPropValue(o, ddl.DataValueField));
            ListItem li = new ListItem(textfield, valuefield);
            if ((bool?)GetPropValue(o, "Active") != true)
                li.Attributes.CssStyle.Add("background-color", "gray");
            ddl.Items.Add(li);
        }
    }

I am getting an error on the “Invoke” line saying
System.InvalidCastException :
Unable to cast object of type 'System.Collections.Generic.LinkedList``1[Business.Objects.MyType]' to type 'System.Collections.Generic.ICollection1[System.Object]’.

I would like to be able to iterate through the collection. How can I do this?
How can I create a LinkedList or similar?

Thanks

Answer 1

It appears that the type T must always provide a method with the name stored in ods.SelectMethod and multiple properties. If T has to implement an interface containing this method, you can constrain IterateCollection<T> to that interface using:

   private static void IterateCollection<T>(T instance, DropDownList ddl)
       where T: IMyInterface 

Assuming you cannot do that, use a LinkedList<object>. You are invoking the methods using reflection so generics will not give you any syntactic assistance.