LinqKit PredicateBuilder Examples

tinypond

I am trying to get the examples working found at http://www.albahari.com/nutshell/predicatebuilder.aspx

I have the following code:

public partial class PriceList
{
public string Name { get; set; }
public string Desc {get;set;}
public DateTime? ValidFrom {get;set;}
public DateTime? ValidTo{get;set;}

public static Expression> IsCurrent()
{
return p => (p.ValidFrom == null || p.ValidFrom <= DateTime.Now) && (p.ValidTo == null || p.ValidTo >= DateTime.Now);
}
}


void Main()
{
List plList = new List()
{
new PriceList(){ Name="Billy", Desc="Skilled", ValidFrom = DateTime.Now.AddDays(-10.0) , ValidTo= DateTime.Now.AddDays(1.0) },
new PriceList(){ Name="Jamie", Desc="Quick Study",ValidFrom =DateTime.Now.AddDays(-10.0) , ValidTo= DateTime.Now.AddDays(1.0) },
new PriceList(){Name= "Larry", Desc= "Rookie",ValidFrom = DateTime.Now.AddDays(-10.0) , ValidTo= DateTime.Now.AddDays(1.0) }
};

// Method 1
var myResultList = plList.Where ( p => (p.ValidFrom == null || p.ValidFrom <= DateTime.Now)
&& (p.ValidTo == null || p.ValidTo >= DateTime.Now)).Dump();

// Method 2

plList.Where(PriceList.IsCurrent()).Dump(); // This does not work currently


}

My question is between method 1 and method 2, the guts of the code are identical but when I call the Method 2 PriceList.IsCurrent I get the following error.

Any Suggestions would be greatly appreciated.

'System.Collections.Generic.List' does not contain a definition for 'Where' and the best extension method overload 'System.Linq.Queryable.Where(System.Linq.IQueryable, System.Linq.Expressions.Expression>)' has some invalid arguments

Instance argument: cannot convert from 'System.Collections.Generic.List' to 'System.Linq.IQueryable'

nescafe

List<T> has no Where(Expression) definition, you can work around this two ways:

1. The nutshell examples are based on db entities which are Linq.Table<T> classes. This class implements the IQueryable interface which has a Where(Expression) method:

  // Method 3

  plList
    .AsQueryable()
    .Where(PriceList.IsCurrent())
    .Dump();

2. As List<T>.Where() accepts a Func<T, bool> predicate, you can compile the Expression to a Func first:

  // Method 4

  var predicate = PriceList.IsCurrent().Compile();
  plList
    .Where(predicate)
    .Dump();

See also: http://stackoverflow.com/questions/1217749/how-to-convert-an-expressionfunct-bool-to-a-predicatet

In this example, you could have defined the function simpler this way:

  public static Func<PriceList, bool> IsCurrent()
  {
    return p => (p.ValidFrom == null || p.ValidFrom <= DateTime.Now) &&
                (p.ValidTo == null   || p.ValidTo >= DateTime.Now);
  }

No .Compile() or .AsQueryable() necessary

But you probably shouldn't, as the experts explain here:
http://stackoverflow.com/questions/793571/why-would-you-use-expressionfunct-rather-than-funct

tinypond

(nescafe ) Five star explanation, thanks again.