linq group by object | 易学教程

问题

I have an object which contains a list of another object as follow:

    class cl
    {
      List<a> a ;
      public List<a> listofA
      {
      get; set;
      }
    }

    class a
    {
    //other properties
       string comment ;
      public string comment
      {
      get; set;
      }
    }

Now how do i make a linq query to see if comment is of some string here is my query:

  var query = (from c in scope.Extent<cl>()
                         where  c.Date >= dateFrom && c.Date < dateTo
                         && c.Actions.Where(a => (a.comment== "") )
                         orderby c.Date.Value.Date
                         group c by c.Date.Value.Date into grpDate
                          select new { grpDate.Key, items = grpDate });

but i get error saying :

Error   15  Operator '&&' cannot be applied to operands of type 'bool' and 'System.Collections.Generic.IEnumerable<>
Error   13  Cannot convert lambda expression to type 'string' because it is not a delegate type

回答1:

The problem is you're using c.Actions.Where. This returns an IEnumerable<T>, not a bool, but you're doing a check for your where clause which expects a boolean value.

You can most likely solve this by using Any instead of Where:

 var query = (from c in scope.Extent<cl>()
                     where  c.Date >= dateFrom && c.Date < dateTo
                     && c.Actions.Any(a => (a.comment== "") )
                     orderby c.Date.Value.Date
                     group c by c.Date.Value.Date into grpDate
                      select new { grpDate.Key, items = grpDate });

回答2:

You are trying to use the result of c.Actions.Where(a => (a.comment== "") ), which is IEnumerable, as a bool. If I understand correctly, you may want to use Any instead of Where in this expression - or other aggregation function like All.

回答3:

c.Actions.Where(a => (a.comment== "") ) returns a list of actions, but you're using it as a boolean (with &&). You should either use .Any() to test whether there is something that matches, or refactor so that the true/false tests are separate from the Where()

I'd give more specific advice, but I'm not sure why you're comparing to an empty string.

来源：https://stackoverflow.com/questions/13403483/linq-group-by-object

标签

linq

lambda

group-by