how to use multiple parameters when combining expressions using Expression.Lambda

Question

Ok so here's the problem I am creating an expression dynamically to execute against the database, that works fine. the problem appears when I add another expression using Expression.AndAlso, the expressions combine fine but the parameters don't work, my code is below.

Expression<Func<Invoice, bool>> condition = null;

ParameterExpression[] param = new ParameterExpression[sessionModel.FilterChildren.Count];

foreach (var filter in sessionModel.FilterChildren) {

   param[i] = Expression.Parameter(typeof(Invoice), filter.SysName);

   Type type = Type.GetType(filter.Type);

   if (i == 0)
     condition =
         Expression.Lambda<Func<Invoice, bool>>(
             Expression.Equal(
             Expression.Property(param[i], filter.SysName),
             Expression.Constant(filter.Value, type)
            ),
              param[i]);
    else {
      var newCond = Expression.Lambda<Func<Invoice, bool>>(
         Expression.Equal(
             Expression.Property(param[i], filter.SysName),
             Expression.Constant(filter.Value, type)
            ),
              param[i]);

       var test = Expression.AndAlso(condition.Body, newCond.Body);

       condition = Expression.Lambda<Func<Invoice, bool>>(test, param);
    }
    i++;
  }

it throws an error

Incorrect number of parameters supplied for lambda declaration

there are different parameters for each expression that will be added.

any ideas where i am going wrong?

Thanks

I've updated my question to show what is called just before :) — Ben Jones
– Ben Jones, Commented Aug 12, 2016 at 10:47
Well, I'd expect a NullReferenceException as you do Expression<Func<Invoice, bool>> condition = null; and then if if block is not triggered var test = Expression.AndAlso(condition.Body, newCond.Body); but I guess i is initially 0 as well, so that's not the case. Which line throws the exception? AndAlso part seems to be Ok to my inexperienced eyes tbh. — uTeisT
– uTeisT, Commented Aug 12, 2016 at 11:05
the issue is on the line condition = Expression.Lambda<Func<Invoice, bool>>(test, param); the and also seems to work fine — Ben Jones
– Ben Jones, Commented Aug 12, 2016 at 11:07

Ivan Stoev · Accepted Answer · 2016-08-12 11:33:25Z

4

how to use multiple parameters when combining expressions using Expression.Lambda

You shouldn't be using multiple parameters. The lambda expression

Expression<Func<Invoice, bool>> condition

implies single parameter of type Invoice.

To fix the issue, replace

ParameterExpression[] param = new ParameterExpression[sessionModel.FilterChildren.Count];

with

var param = Expression.Parameter(typeof(Invoice), "invoice");

and in the rest of the code, replace param[i] with param.

P.S. There is no real need to create lambda expressions for each filter because as you can see, when combining them you use only the Body. The whole procedure can be trimmed down to something like this:

Expression<Func<Invoice, bool>> condition = null;
if (sessionModel.FilterChildren.Any())
{
    var parameter = Expression.Parameter(typeof(Invoice), "invoice");
    var body = sessionModel.FilterChildren
        .Select(filter => Expression.Equal(
            Expression.Property(parameter, filter.SysName),
            Expression.Constant(filter.Value, Type.GetType(filter.Type))))
        .Aggregate(Expression.AndAlso);
    condition = Expression.Lambda<Func<Invoice, bool>>(body, parameter);
}

edited Aug 12, 2016 at 11:33

answered Aug 12, 2016 at 11:15

Ivan Stoev

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4 Comments

uTeisT Over a year ago

Just trying to learn, so it might sound stupid, but what's the reason behind this? Isn't param[i] also a ParameterExpression as expected in Expression.Lambda ?

Ivan Stoev Over a year ago

Yes, it is. But they are different objects and not considered to be one and the same. Even the names doesn't matter. So the real trick is to use one and the same parameter expression instance.

Ben Jones Over a year ago

makes sense :) Thanks for that...Legend

Ivan Stoev Over a year ago

Just added a simplified way of doing the same (for learning purposes :)

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