Convert Expression to Expression<Func<T, bool>>

Question

Is that possible to convert Expression to Expression<Func<T, bool>> if instance of Expression was created on T ?

At the end I have list List<Expression> and need to produce on Expression<Func<T, bool>> where each expression of List<Expression> is agregated with AND .

Answer 1

It's possible, but every expression in the list must actually be a Expression<Func<T, bool>> instance.

EDIT: It turns out that you use Kendo.Mvc.IFilterDescriptor.CreateFilterExpression which actually builds a MethodCallExpression s.

The following helper method should do the job (works with both lambda and method call expressions):

public static class Utils
{
    public static Expression<Func<T, bool>> And<T>(List<Expression> expressions)
    {
        var item = Expression.Parameter(typeof(T), "item");
        var body = expressions[0].GetPredicateExpression(item);
        for (int i = 1; i < expressions.Count; i++)
            body = Expression.AndAlso(body, expressions[i].GetPredicateExpression(item));
        return Expression.Lambda<Func<T, bool>>(body, item);
    }

    static Expression GetPredicateExpression(this Expression target, ParameterExpression parameter)
    {
        var lambda = target as LambdaExpression;
        var body = lambda != null ? lambda.Body : target;
        return new ParameterBinder { value = parameter }.Visit(body);
    }

    class ParameterBinder : ExpressionVisitor
    {
        public ParameterExpression value;
        protected override Expression VisitParameter(ParameterExpression node)
        {
            return node.Type == value.Type ? value : base.VisitParameter(node);
        }
    }
}

Answer 2

Yes; just call Expression.Lambda<Func<T, bool>>(..., parameter) , where ... is an expression composed of the expressions you want to combine.

You'd probably want list.Aggregate(Expressions.AndAlso) .

If your expressions don't all share the same ParameterExpression , you'll need to rewrite them to do so. (use ExpressionVisitor )