Casting to abstract class or interface when generics are used

Question

I have this method Verify_X which is called during databind for a listbox selected value. The problem is the strongly typed datasource. I want to use the abstract class BaseDataSource or an interface to call the methods supported: Parameters[] and Select(), Instead of using the most specific implementation as seen below.

This is so one method can be used for all the different types of datasources I have instead of having a method for each. They all inherit the same way.

Here is the chain of inheritance / implementation

public class DseDataSource : ProviderDataSource<SCCS.BLL.Dse, DseKey>

public abstract class ProviderDataSource<Entity, EntityKey> : BaseDataSource<Entity, EntityKey>, ILinkedDataSource, IListDataSource
    where Entity : SCCS.BLL.IEntityId<EntityKey>, new()
    where EntityKey : SCCS.BLL.IEntityKey, new()

public abstract class BaseDataSource<Entity, EntityKey> : DataSourceControl, IListDataSource, IDataSourceEvents
    where Entity : new()
    where EntityKey : new()

The BaseDataSource has the methods and properties I need. DseDataSource is implemented the following way:

public class DseDataSource : ProviderDataSource<SCCS.BLL.Dse, DseKey>

I know it is possible to edit the class DseDataSource, add an interface to access Parameters[] and Select(), then program against that, which allows what I want, but this requires editing the NetTiers libraries and I am curious to see if this can be done since it seemed so difficult.

    public static string Verify_DSE(string valueToBind, DseDataSource dataSource)
    {
        if (ListContainsValue(dataSource.GetEntityList(), valueToBind)) return valueToBind;
        CustomParameter p = dataSource.Parameters["WhereClause"] as CustomParameter;
        if (p != null)
        {
            p.Value = "IsActive=true OR Id=" + valueToBind;
            dataSource.Select();
            return valueToBind;
        }
        return string.Empty;
    }

    private static bool ListContainsValue(IEnumerable list, string value)
    {
        if (value.Length == 0) return true;

        foreach (object o in list)
        {
            IEntity entity = o as IEntity;
            if (entity != null)
            {
                if (entity.Id.ToString() == value)
                    return true;
            }
        }
        return false;
    }

The end result would be code such as:

public static string Verify(string valueToBind, object dataSource)
{
//what is the correct way to convert from object
BaseDataSource baseInstance = dataSource as BaseDataSource;

if baseInstance != null)
{
 if (ListContainsValue(baseInstance.GetEntityList(), valueToBind)) return valueToBind;
 CustomParameter p = baseInstance.Parameters["WhereClause"] as CustomParameter;
 if (p != null)
 {
  p.Value = "IsActive=true OR Id=" + valueToBind;
  baseInstance.Select();
  return valueToBind;
 }
}

return string.Empty;
}

Answer 1

Because the code isn't usable "as is", it is very hard to discuss the actual problem... can you simplify the example to something that doesn't depend on external libraries / classes?

Can I confirm: is DseDataSource generated from NetTiers? In which case, does NetTiers generate "partial" classes? If so, you can add the interface-implementation in a second class file:

namespace YourNamespace {
    partial class DseDataSource : IYourInterface {
        // implement interface
    }
}

This adds functionality to the DseDataSource class without requiring you to edit the generated code.

Answer 2

If you are unable to alter the class definition or to use some sort of extension methods, you can use Reflection. Here is a sample that I worked up using assumptions about your code:

    public static string Verify(string valueToBind, object dataSource)
    {
        ////what is the correct way to convert from object
        //BaseDataSource baseInstance = dataSource as BaseDataSource;
        Type type = dataSource.GetType();
        MethodInfo select = type.GetMethod("Select");
        PropertyInfo parameters = type.GetProperty("Parameters");
        PropertyInfo parameterGetter = null;
        object parametersInstance = null;
        if (parameters != null)
        {
            parametersInstance = parameters.GetValue(dataSource, null);
            type = parametersInstance.GetType();
            parameterGetter = type.GetProperty("Item");
        }

        //if baseInstance != null)
        if (select != null && parameters != null && parameterGetter != null)
        {
                if (ListContainsValue(baseInstance.GetEntityList(), valueToBind)) return valueToBind;
                CustomParameter p = parameterGetter.GetValue(parametersInstance, new object[] {"WhereClause" }) as CustomParameter;

                if (p != null)
                {
                        p.Value = "IsActive=true OR Id=" + valueToBind;
                        select.Invoke(dataSource, null);
                        return valueToBind;
                }
        }

        return string.Empty;
    }

Answer 3

Thanks John you really put me on the right path there. I ended up with the following code:

    public string Verify(string valueToBind, object dataSource)
    {
        IListDataSource listDataSource = dataSource as IListDataSource;
        if (listDataSource != null)
        {
            if (ListContainsValue(listDataSource.GetEntityList(), valueToBind)) return valueToBind;
        }

        Type type = dataSource.GetType();
        MethodInfo select = type.GetMethod("Select", new Type[0]);
        PropertyInfo parameterCollectionInfo = type.GetProperty("Parameters");
        ParameterCollection pc = parameterCollectionInfo.GetValue(dataSource, null) as ParameterCollection;

        if (pc != null)
        {
            CustomParameter p = pc["WhereClause"] as CustomParameter;
            if (p != null)
            {
                p.Value = "IsActive=true OR Id=" + valueToBind;
                select.Invoke(dataSource, null);
                return valueToBind;
            }
        }

        return string.Empty;
    }