无法转换func类型 <T,bool> 布尔

Question

I have the following linq which i want to pass different conditions to the where clause,but i get error:

 var rslt = (from t in cr.faultStatAlt
                   join v in cr.errorCatGroup
                   on t.m_error_id equals v.m_error_id
                   join h in cr.masterAlarm on t.m_inv_error_details equals h.pc_group_pattern into ps
                   from h in ps.DefaultIfEmpty()
                   join s in cr.MasterDataTurbine on t.m_turbine_id equals s.m_turbine_id
                   where (whereClause)
                   group t.error_duration by v.m_error_subgroup_name into k
                   select new faulttest
                   {
                       m_error_subgroup = k.Key,
                       error_duration = Math.Round(k.Sum() / 3600, 2)


                   }).ToString();

the condition i want to make it dynamic is :

 Func<t_fault_stat_alt, bool> whereClause = t => t.m_date >= dt1 && t.m_date <= dt2 && t.m_grid_loss==true;

Answer 1

You can't pass a lambda into where in query syntax - you'll need to use method syntax instead. Something like

   var fltr = (from t in cr.faultStatAlt
               join v in cr.errorCatGroup
               on t.m_error_id equals v.m_error_id
               join h in cr.masterAlarm on t.m_inv_error_details equals h.pc_group_pattern into ps
               from h in ps.DefaultIfEmpty()
               join s in cr.MasterDataTurbine on t.m_turbine_id equals s.m_turbine_id)
              .Where (whereClause);

   var rslt = (from t in fltr
               group t.error_duration by v.m_error_subgroup_name into k
               select new faulttest
               {
                   m_error_subgroup = k.Key,
                   error_duration = Math.Round(k.Sum() / 3600, 2)


               }).ToString();

Explanation -

Query syntax is just a shortcut the compiler provides for you. The entire query is rewritten using LINQ method calls when it's compiled. For where clauses, the compiler requires a statement which evaluates to bool - the compiler converts that statement into a lambda.

You gave the compiler a lambda to start with ( Func<t_fault_stat_alt, bool> ) - it doesn't know how to convert that to a bool .

Answer 2

Even though the question already has been marked as correct, i need to note something important that may get lost in the comments.

If you use EntityFramework (or anything else that is accepting Expression rather then Func ), you need to use different types.

Given the folloging code where Tag is a random object containing the int field Id (+ its corresponding column) and cntxt is the EntityFramework DBContext , you would yield the following two SQL queries:

Func<Data.Net.Tag, bool> cond = (tag) => tag.Id == 1;
var query = cntxt.Tags.Where(cond);
query.ToArray();
// SELECT "Extent1"."Id", "Extent1"."Name" FROM "public"."Tags" AS "Extent1"

query = cntxt.Tags.Where((tag) => tag.Id == 1);
query.ToArray();
// SELECT "Extent1"."Id", "Extent1"."Name" FROM "public"."Tags" AS "Extent1" WHERE 1 = "Extent1"."Id"

Reason being that EntityFramework is using IQueriable to actually build those SQL queries. Due to IQueryable also implementing IEnumerable however, you may fall into the trap of expecting the same behavior for Func and Expression due to the looks. If one actually checks the methods themself, the difference gets clearer:

public static IQueryable<TSource> Where<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
public static IEnumerable<TSource> Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate);

To still get the same behavior (Filter-Method), you have to change the method head to something like this:

void MyWhere<T>(..., Expression<Func<T, bool>> cond)

For more on this, check out eg.: Why would you use Expression<Func<T>> rather than Func<T>? or google C# Expression :)