Using contravariance with Func<T, object> selector property

Question 1

After thorough investigation I found out the solution would require mixing covariance and contravariance which is not supported currently. The closest solution (which compiles) actually does not allow easy access to IDisplayColumn.Selector as T argument in IColumnSet.Columns will be visible as object not IDisplayColumn so it's not an option:

public interface IDisplayColumn<in T>
{
    string Title { get; set; }
    int Order { get; set; }
    Func<T, object> Selector { get; }
}

public class DisplayColumn<T> : IDisplayColumn<T>
{
    public string Title { get; set; }
    public int Order { get; set; }
    public Func<T, object> Selector { get; set; }
}

public interface IColumnSet<out T>
{
    Type TypeHandled { get; }
    IEnumerable<T> Columns { get; }
}

public class ColumnSet<T> : IColumnSet<IDisplayColumn<T>>
{
    public Type TypeHandled
    {
        get
        {
            return typeof(T);
        }
    }

    public IEnumerable<IDisplayColumn<T>> Columns { get; set; }
}

I ended up translating Func<,> using expressions when creating which is a one-time operation with minimal overhead of casting when using selector:

public interface IDisplayColumn
{
    string Title { get; set; }
    bool Visible { get; set; }
    int Order { get; set; }
    Func<object, object> Value { get; }
    T GetValue<T>(object source);
}

public class DisplayColumn<T>: IDisplayColumn
{
    public string Title { get; set; }
    public bool Visible { get; set; }
    public int Order { get; set; }
    public Func<object, object> Value { get; set; }
    public override string ToString()
    {
        return Title;
    }

    public TValue GetValue<TValue>(object source)
    {
        return (TValue)Convert.ChangeType(Value(source), typeof(TValue));
    }

    public Func<T, object> Selector
    {
        set
        {
            Value = value.ConvertObject<T>();
        }
    }
}

public interface IColumnSet
{
    Type TypeHandled { get; }
    IEnumerable<IDisplayColumn> Columns { get; }
}

public class ColumnSet<T>: IColumnSet
{
    public Type TypeHandled
    {
        get
        {
            return typeof(T);
        }
    }

    public IEnumerable<IDisplayColumn> Columns { get; set; }
}

public static Func<object, object> ConvertObject<T>(this Func<T, object> func)
{
    Contract.Requires(func != null);

    var param = Expression.Parameter(typeof(object));
    var convertedParam = new Expression[] { Expression.Convert(param, typeof(T)) };

    Expression call;
    call = Expression.Convert(
        func.Target == null
            ? Expression.Call(func.Method, convertedParam)
            : Expression.Call(Expression.Constant(func.Target), func.Method, convertedParam)
        , typeof(object));

    var delegateType = typeof(Func<,>).MakeGenericType(typeof(object), typeof(object));
    return (Func<object, object>)Expression.Lambda(delegateType, call, param).Compile();
}

And the example of usage:

private class TestObject1
{
    public int Id { get; set; }
    public string Name { get; set; }
}

IDisplayColumn objectColumn = new DisplayColumn<TestObject1> { Title = "Column 1", Selector = (x) => x.Name };

var columnSets = new List<IColumnSet>
{
    new ColumnSet<TestObject1>
    {
        Columns = new List<IDisplayColumn>
        {
            new DisplayColumn<TestObject1> { Title = "Column 1", Order = 3, Selector = x => x.Id },
            new DisplayColumn<TestObject1> { Title = "Column 2", Order = 2, Selector = x => x.Name },
            new DisplayColumn<TestObject1> { Title = "Column 3", Order = 1, Selector = x => x.Id.ToString(CultureInfo.InvariantCulture) + x.Name.ValueOrEmpty() },
        }
    }
};

So I will give myself the credit for this problem but if somebody can suggest a nicer solution using generics and variance, please feel free to post it as I will be happy to change the solution.

Question 2

If you make your ColumnSet generic as well, then you can specify the type used for the columns enumerable that it returns. The code below will compile, and I think achieve what you are after.

    public interface IDisplayColumn<in T>
{
    string Title { get; set; }
    int Order { get; set; }
    Func<T, object> Selector { get; }
}

public class DisplayColumn<T>: IDisplayColumn<T>
{
    public string Title { get; set; }
    public int Order { get; set; }
    public Func<T, object> Selector { get; set; }
}

public class ColumnSet<T>
{
    public Type TypeHandled { get; set; }
    public IEnumerable<IDisplayColumn<T>> Columns { get; set; }
}

public static class ColumnSetTest
{
    static ColumnSetTest()
    {
      IDisplayColumn<SettingsContext> testSingleColumn = new DisplayColumn<SettingsContext> { Title = "Test", Selector = x => x.Values };


        var columnSets = new List<ColumnSet<SettingsContext>>
        {
            new ColumnSet<SettingsContext>
            {
                TypeHandled = typeof(SettingsContext),
                Columns = new List<IDisplayColumn<SettingsContext>>
                {
                    new DisplayColumn<SettingsContext> {Title = "Column 1", Order = 1, Selector = x => x.IsReadOnly },
                    new DisplayColumn<SettingsContext> {Title = "Column 2", Order = 2, Selector = x => x.IsSynchronized },
                    new DisplayColumn<SettingsContext> {Title = "Column 3", Order = 3, Selector = x => x.Keys }
                }
            }
        };

} }