public void Linq105()  
{ 
    string[] categories = new string[]{   
        "Beverages",  
        "Condiments",   
        "Vegetables",   
        "Dairy Products",  
        "Seafood" }; 
  
    List<Product> products = GetProductList(); 
  
  
  
    var q = 
        from c in categories 
        join p in products on c equals p.Category into ps 
        from p in ps.DefaultIfEmpty() 
        select new { Category = c, ProductName = p == null ? "(No products)" : p.ProductName }; 
  
    foreach (var v in q) 
    { 
        Console.WriteLine(v.ProductName + ": " + v.Category); 
    } 
}

------解决方案--------------------
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http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b