Expression Trees in NHibernate

Question

I have a method which have this signature

public static IList<T> GetBy<T>(System.Linq.Expressions.Expression<Func<T, bool>> expression)

I use to pass lambda expressions and make search restriction in nhibernate by retrieving data from expressiontree.

So when class user pass something like :

c => c.fullName == "John" && c.lastName == "Smith" && c.lastName != "" || c.fullName != ""  && c.Age > 18

I get to read this structure from expression tree, that way I have a full intellisense way to provide searching criteria

In other words: I need to pass searching criteria to data access layer (Nhibernate)

So I need to extract criteria from expression tree and then pass it to n hibernate by example :

c=>c.fullname = "John" 

I will extract the following information from the expression tree :

propertyname = fullname , value = "John" , restrictiontype = "equality" 

and then pass this info to nhibernate as following :

ICriteria crit = session.CreateCriteria(typeof(T));
                    crit.Add(Restrictions.Eq(propretyName, value));
    IList<T> list = crit.Add(List<T>())
                    return list;

Any way the problem is it's really hard to read from expressiontree, so I was wondering if you guys have any easy way of maybe iterating inside expressiontree to pull data, or maybe you guys have some code to retrieve data from ExpressionTree.

Answer 1

Here is some code that retrieves the information you mentioned. I’m sure you can extend this to include additional information you might be looking for.

public class Criterion
{
    public string PropertyName;
    public object Value;
    public ExpressionType RestrictionType;
}

[....]

public static IEnumerable<Criterion> GetCriteria<T>(Expression<Func<T, bool>> expression)
{
    return getCriteria<T>(expression.Body);
}
private static IEnumerable<Criterion> getCriteria<T>(Expression expression)
{
    if (expression is BinaryExpression)
    {
        var bin = (BinaryExpression) expression;
        if (bin.NodeType == ExpressionType.And || bin.NodeType == ExpressionType.AndAlso ||
            bin.NodeType == ExpressionType.Or || bin.NodeType == ExpressionType.OrElse)
            return getCriteria<T>(bin.Left).Concat(getCriteria<T>(bin.Right));

        if (bin.Left is MemberExpression)
        {
            var me = (MemberExpression) bin.Left;
            if (!(bin.Right is ConstantExpression))
                throw new InvalidOperationException("Constant expected in criterion: " + bin.ToString());
            return new[] { new Criterion {
                PropertyName = me.Member.Name,
                Value = ((ConstantExpression) bin.Right).Value,
                RestrictionType = bin.NodeType
            } };
        }

        throw new InvalidOperationException("Unsupported binary operator: " + bin.NodeType);
    }

    throw new InvalidOperationException("Unsupported expression type: " + expression.GetType().Name);
}