Question

I have viewmodel (say memberviewmodel) and ef model (say member). how do i translate from

> GetByProperty(Expression<Func<MemberViewModel, bool>> whereCondition)

to

> GetByProperty(Expression<Func<Member, bool>> whereCondition)

any ideas?

My service exposes the method GetByProperty(Expression> whereCondition) but then within the service, it calls the business object which exposes a method GetByProperty(Expression> whereCondition).

Answer 1

To convert Expression<Func<MemberViewModel, bool>> to Expression<Func<Member, bool>> you’ll need to extend the ExpressionVisitor class. I’ve assumed MemberViewModel contains only fields and properties. Member of course needs to implement the same set of fields and properties. In this case the following should work:

public class ParameterModifier<TSrc, TDest> : ExpressionVisitor
{
    ParameterExpression parameter;

    public ParameterModifier()
    {
        parameter = Expression.Parameter(typeof(TDest), "member");
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        if (node.Type == typeof(TSrc))
        {
            return parameter;
        }
        else
            return base.VisitParameter(node);
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        if (node.Expression.Type == typeof(TSrc))
        {
            return Expression.PropertyOrField(Visit(node.Expression), node.Member.Name);
        }
        else
        {
            return base.VisitMember(node);
        }
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
        if (node.Parameters.Any(p => p.Type == typeof(TSrc)))
        {
            return Expression.Lambda(Visit(node.Body), node.Parameters.Select(p => p.Type == typeof(TSrc) ? parameter : p));
        }
        else
        {
            return base.VisitLambda<T>(node);
        }
    }
}

Before passing the predicate to the second method you can now convert it using the class above:

IEnumerable<MemberViewModel> GetByProperty(Expression<Func<MemberViewModel, bool>> whereCondition)
{
    var converter = new ParameterModifier<MemberViewModel, Member>();
    var convertedExpression = converter.Visit(original) as Expression<Func<Member, bool>>;
    IEnumerable<Member> members = GetByProperty(convertedExpression);
    return members.Select(new MemberViewModel(member));
}