C# Linq where clause as a variable

You need to assembly an Expression<Func<T, bool>> and pass it to the Where() extension method:

Expression<Func<T, bool>> whereClause = a => a.zip == 23456;
var x = frSomeList.Where(whereClause);

EDIT: If you're using LINQ to Objects, remove the word Expression to create an ordinary delegate.

This:

var query = from something in someList where whereClause;

is shorthand for:

var query = someList.Where(something => whereClause);

Assuming someList is an IEnumerable<Address>, Where refers to the Enumerable.Where Extension Method. This method expects a Func<Address, bool> which you can define as follows:

Func<Address, bool> whereClause = address => address.Zip == 23456;
var query = someList.Where(whereClause);

As Richard has pointed out, the Dynamic Query Library can be used to build dynamic filter expressions. When using Linq-To-Objects make sure to convert your IEnumerable<T> to a IQueryable<T> first. Here is an (incomplete) example:

using System.Linq.Dynamic;

namespace System.Linq.Dynamic
{
  public class Example
  {
   // Assuming some value is assigned to below field somewhere... 
   private IEnumerable<Address> m_Addresses;

   public void FilterByZipCode(string zipCode)
   {
      var x = m_Addresses.AsQueryable().Where("Zip == @0", zipCode);
      dowork(x);
   }
  }

  public class Address
  {  
     public String Zip { get; set; }

     // More Properties...  
  }
}