VersionedExecutor.cs

// Curiously Recurring Pattern
// http://blogs.msdn.com/b/ericlippert/archive/2011/02/03/curiouser-and-curiouser.aspx
public class VersionedExecutorBase<T, TFunc>
    where T : VersionedExecutorBase<T, TFunc>
{
    private readonly Func<Version> _versionProvider;
    private readonly Dictionary<Version, TFunc> _functions;
    private TFunc _default;

    public VersionedExecutorBase(Func<Version> versionProvider)
    {
        _versionProvider = versionProvider;

        _functions = new Dictionary<Version, TFunc>();
    }

    public virtual T ForPreviousVersions(TFunc func)
    {
        _default = func;

        return (T)this;
    }

    public virtual T ForVersion(Version version, TFunc func)
    {
        _functions.Add(version, func);

        return (T)this;
    }

    public virtual TFunc GetFunction()
    {
        var currentVersion = _versionProvider();

        if (currentVersion == null)
        {
            return _default;
        }

        var key = _functions.Keys
            .Where(x => x <= currentVersion)
            .OrderByDescending(x => x)
            .FirstOrDefault();

        return key != null ? _functions[key] : _default;
    }
}

public class VersionedExecutor<T> : VersionedExecutorBase<VersionedExecutor<T>, Func<T, Task>>
{
    public VersionedExecutor(Func<Version> versionProvider)
        : base(versionProvider)
    {
    }

    public virtual Task ExecuteAsync(T payload)
    {
        return GetFunction()(payload);
    }
}

public class VersionedExecutor<TIn, TOut> : VersionedExecutorBase<VersionedExecutor<TIn, TOut>, Func<TIn, Task<TOut>>>
{
    public VersionedExecutor(Func<Version> versionProvider)
        : base(versionProvider)
    {
    }

    public virtual Task<TOut> ExecuteAsync(TIn payload)
    {
        return GetFunction()(payload);
    }
}

VersionedExecutorDemo.cs

private VersionedExecutor<TIn, TOut> Versioner =>

    new VersionedExecutor<TIn, TOut>(() => Info.Version)

    .Default((TIn)payload =>
    {
        // Do something
        return (TOut)object;
    })

    .For(ApplicationVersions.Firefly2, (TIn)payload =>
    {
        // Do something
        return (TOut)object;
    })

    .For(ApplicationVersions.Firefly3, (TIn)payload =>

        // Do something
        return (TOut)object;
    );