MVC Linq Query with dynamic column name in WHERE clause

Question

I have a number of calls for the same query but with slightly different WHERE clause, does anyone know if it's possible to pass a variable as the column name as I can't seem to acheive it.

I know the below isn't correct but just to give you an idea of what i'm trying to acheive.

public EmailUserListViewModel EmailUserListData(int CaseId, string ColumnName)
{
    CaseId = CaseId,
    EmailUserList = (from u in efContext.PgsUsers
                        where ColumnName == true
                        orderby u.FirstName, u.LastName
                        select new EmailUserListModel
                        {
                            UserId = u.Id,
                            Name = ((u.FirstName == null) ? "" : u.FirstName) 
                                   + ((u.LastName == null) ? "" : " " + u.LastName),
                            Email = u.Email,
                            Checked = false

                        }).ToList()
    };
}

Possible duplicate of [Dynamic WHERE clause in LINQ](http://stackoverflow.com/questions/848415/dynamic-where-clause-in-linq) — Nico, Jan 19 '17 at 12:44
You need to use the 'u' variable in the Where. So something like u == ColumnName — jdweng, Jan 19 '17 at 12:48

score 2 · Accepted Answer · answered Jan 19 '17 at 12:47

2

I suppose you could use Reflection to dynamically retrieve the value of the property

from u in efContext.PgsUsers where (typeof(PgsUser).GetProperty(ColumnName).GetValue(u) as bool) == true

or

from u in efContext.PgsUsers where (u.GetType().GetProperty(ColumnName).GetValue(u) as bool) == true

answered Jan 19 '17 at 12:47

Zaphod Fox

58
1
8

I've tried something similar to this and get the same error for them: `'System.Reflection.PropertyInfo GetProperty(System.String)' has no supported translation to SQL.` – Jammer Jan 19 '17 at 13:03
The Reflection filter is not SQL compatible, you have to apply it to a ToList()ed version of your query. – Zaphod Fox Jan 19 '17 at 13:09

score 1 · Answer 2 · answered Jan 19 '17 at 12:48

You could write such method:

public Expression<Func<T, bool>> getExpression<T>(string columnName)
{
    var param = Expression.Parameter(typeof(T));
    var equal = Expression.Equal(Expression.Property(param, columnName), Expression.Constant(true));
    return (Expression<Func<T, bool>>)Expression.Lambda(equal, param);
}

and use it in where:

where getExpression<YourType>("ColumnName")

MVC Linq Query with dynamic column name in WHERE clause

2 Answers2