Question

Somewhere deep inside a library in which I do lots of metaprogramming, there is a class that contains the following code:

// Initialized in the constructor
// Destroyed in IDisposable.Dispose
private IDataReader reader;

private T Retrieve<T>()
    where T : class, new() {

    if (!this.reader.Read())
        return null;

    // This does the same as typeof(T).GetProperties(), but ReflectionHelper
    // memoizes the properties of the types passed to GetProperties.
    PropertyInfo[] properties = ReflectionHelper.GetProperties(typeof(T));

    T result = new T();
    foreach (PropertyInfo property in properties)
        property.SetValue(result, this.reader[property.Name], null);
    return result;
}

public T RetrieveObject() {
    T result = this.RetrieveObject<T>();
    this.reader.NextResult();
    return result;
}

public List<T> RetrieveList<T>()
    where T : class, new() {

    List<T> result = new List<T>();
    for (T element; (element = this.Retrieve<T>(reader)) != null; )
        result.Add(element);
    this.reader.NextResult();
    return result;
}

Needless to say, the class is a wrapper around IDbConnection, IDbCommand and IDataReader, with the added benefit of allowing sane access to the results.

I want to add the following methods to the class:

public T RetrieveCompound<T, U>()
    where T : class, IHeader<U>, new()
    where U : class, new() {

    T header = this.RetrieveObject<T>();
    if (header != null)
        header.Details = this.RetrieveList<U>();
    return header;
}

public T RetrieveCompound<T, U, V>()
    where T : class, IHeader<U>, IHeader<V>, new()
    where U : class, new() {

    T header = this.RetrieveObject<T>();
    if (header != null) {
        ((IHeader<U>)header).Details = this.RetrieveList<U>();
        ((IHeader<V>)header).Details = this.RetrieveList<V>();
    }
    return header;
}

public T RetrieveCompound<T, U, V, W>()
    where T : class, IHeader<U>, IHeader<V>, IHeader<W>, new()
    where U : class, new() {

    T header = this.RetrieveObject<T>();
    if (header != null) {
        ((IHeader<U>)header).Details = this.RetrieveList<U>();
        ((IHeader<V>)header).Details = this.RetrieveList<V>();
        ((IHeader<W>)header).Details = this.RetrieveList<W>();
    }
    return header;
}

However, this has a very important pitfall: The interface IHeader<T> happens to be declared in a different assembly, and I do not want to force users of the IDataReader-wrapper class to add a reference to an assembly they might not need.

I know I could solve this using extension methods, but I am using .NET Framework 2.0. Is there another way to get around this problem?

Answer 1

You could add those methods to a derived type. That way, if you don’t need IHeader<T>-related functionality, you don’t need to reference the assembly.