Dynamically select column name using Linq to Entities (EF6)

Question

I want to encapsulate common scenarios when using EF6. Here's an example:

   public class StringRequest : DbRequestProperty
   {
      public string Name { get; set; }
      public bool? ExactMatch { get; set; }

      protected override bool IsValid()
      {
         return !string.IsNullOrWhiteSpace(Name);
      }

      private bool RequestExactMatch()
      {
         return ExactMatch.HasValue && ExactMatch.Value;
      }

      protected override IQueryable<T> Execute<T>(IQueryable<T> original, string propertyName)
      {
         return RequestExactMatch()
            ? original.Where(o => GetProperty<string>(o, propertyName) == Name)
            : original.Where(o => GetProperty<string>(o, propertyName).Contains(Name));
      }
   }

But GetProperty can't be converted to a query. So I'm thinking on selecting dynamically the column using "propertyName".

  protected override IQueryable<T> Execute<T>(IQueryable<T> original, string propertyName)
  {
     return RequestExactMatch()
        ? original.Where(o => GetColumnByName<string>(propertyName) == Name)
        : original.Where(o => GetColumnByName<string>(propertyName).Contains(Name));
  }

Is this possible? Thanks in advance.

score 7 · Accepted Answer · answered Jul 08 '15 at 21:35

7

You can dynamically create a Expression<Func<T, bool>> using the Expression class like this:

protected override IQueryable<T> Execute<T>(IQueryable<T> original, string propertyName)
{
   var parameter = Expression.Parameter(typeof(T));
   var property = Expression.PropertyOrField(parameter, propertyName);
   var constant = Expression.Constant(Name);

   Expression predicate;
   if(RequestExactMatch())  
   {     
      predicate = Expression.Equal(property, constant);    
   }
   else 
   {
      predicate = Expression.Call(property, "Contains", null, constant);    
   }

   var lambda = Expression.Lambda<Func<T, bool>>(predicate, parameter);

   return original.Where(lambda);
}

answered Jul 08 '15 at 21:35

Aducci

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Thank you sir. It worked perfectly. You helped me refactor tons of repeated code. – Chorinator Jul 08 '15 at 21:58
@Chorinator - I am glad I could help! – Aducci Jul 08 '15 at 21:58
What about dates. Is there a way to truncate the time part on the database part so I end up comparing only dates? I've tried using Convert and Cast but I get an exception saying that those functions don't exist on System.Date – Chorinator Jul 09 '15 at 00:48
@Chorinator - I believe there is a way, but ask another question – Aducci Jul 09 '15 at 04:11
I've found a way to do it, based on this post http://stackoverflow.com/a/1759993/261549 – Chorinator Jul 09 '15 at 16:25

Dynamically select column name using Linq to Entities (EF6)

1 Answers1