IOC Container Circular Dependency Between Service and Factory

Question

I have the following classes/interfaces:

public class DataExpressionViewModelFactory : IDataExpressionViewModelFactory
{
    private readonly IDataExpressionService DataExpressionService;

    public DataExpressionViewModelFactory(IDataExpressionService dataExpressionService)
    {
        DataExpressionService = dataExpressionService;
    }

    public DataExpressionViewModel Create(DatabaseTableColumn column)
    {
        return new DataExpressionViewModel(DataExpressionService, column);
    }
}

public class DataExpressionService : IDataExpressionService
{
    private readonly IDataExpressionViewModelFactory DataExpressionViewModelFactory;

    ...
}

Using an IOC container ( Microsoft.Extensions.DependencyInjection ), there is a circular dependency between them. An IDataExpressionViewModelFactory requires an IDataExpressionService , and vice versa.

An IDataExpressionViewModelFactory creates DataExpressionViewModel s, passing in an IDataExpressionService . DataExpressionViewModel s use IDataExpressionService to validate data expression strings (user input) and create further DataExpressionViewModel s from those, as expressions can be nested within each other (a recursive model).

Is there a way I can resolve the circular dependency?

Answer 1

After refactoring the application to make IDataExpressionViewModel a model ( IDataExpression ), and splitting the service into a "writer" service and an "object creation" service, which removed the circular dependency, I came upon a solution that works for the original design from reading about delegate factories .

I was using delegates as factories for other entities, but I actually had two constructors for this particular entity and so used a factory class instead. Now knowing about delegate factories, I would state that the code smell (or one of them) in my original design was having the service instantiated within the factory itself.

To use a delegate factory to remove the circular dependency:

Delegate Factory Class

public delegate IDataExpressionViewModel CreateDataExpressionViewModel(DatabaseTableColumn column);
public delegate IDataExpressionViewModel CreateDataExpressionViewModelWithExpression(DatabaseTableColumn column, string expression);

public class DataExpressionViewModelDelegateFactory
{
    public readonly CreateDataExpressionViewModel;
    public readonly CreateDataExpressionViewModelWithExpression

    public DataExpressionViewModelDelegateFactory(
        CreateDataExpressionViewModel createDataExpressionViewModel,
        CreateDataExpressionViewModelWithExpression createDataExpressionViewModelWithExpression
    )
    {
        CreateDataExpressionViewModel = createDataExpressionViewModel;
        CreateDataExpressionViewModelWithExpression = createDataExpressionViewModelWithExpression;
    }
}

IOC Implementation (in App.xaml.cs)

services.AddTransient((provider) => new DataExpressionViewModelDelegateFactory(
    new CreateDataExpressionViewModel(
        (column) => new DataExpressionViewModel(
            provider.GetRequiredService<IDataExpressionService>(),
            column
        )
    ),
    new CreateDataExpressionViewModelWithExpression(
        (column, expression) => new DataExpressionViewModel(
            provider.GetRequiredService<IDataExpressionService>(),
            column,
            expression
        )
    )
));

The delegate factory is not dependent on the service, as the object creation dependencies on the service are resolved on creation by the ServiceProvider , so the dependency is removed.

As per the article I linked, if lots of delegates are required and it becomes too cumbersome, you can also use Func in the factory class rather than creating specific delegates for every method.