IEnumerable doesn't have a Count method

I have the following method:

public bool IsValid
{
  get { return (GetRuleViolations().Count() == 0); }
}

public IEnumerable<RuleViolation> GetRuleViolations(){
  //code here
}

Why is it that when I do .Count() above it is underlined in red?

I got the following error:

Error 1 'System.Collections.Generic.IEnumerable' does not contain a definition for 'Count' and no extension method 'Count' accepting a first argument of type 'System.Collections.Generic.IEnumerable' could be found (are you missing a using directive or an assembly reference?) c:\users\a\documents\visual studio 2010\Projects\NerdDinner\NerdDinner\Models\Dinner.cs 15 47 NerdDinner

You add:

using System.Linq;

at the top of your source and make sure you've got a reference to the System.Core assembly.

Count() is an extension method provided by the System.Linq.Enumerable static class for LINQ to Objects, and System.Linq.Queryable for LINQ to SQL and other out-of-process providers.

EDIT: In fact, using Count() here is relatively inefficient (at least in LINQ to Objects). All you want to know is whether there are any elements or not, right? In that case, Any() is a better fit:

public bool IsValid
{
  get { return !GetRuleViolations().Any(); }
}

Any() or Count() methods in Linq work only for generic types.

IEnumerable<T>

If you have a simple IEnumerable without a type, try to use

IEnumerable<object> 

instead.