LINQ 方法参数看不懂,该怎么处理

LINQ 方法参数看不懂
怎么我在编译器写代码LINQ方法参数提示看不懂啊，比如 GroupBy(Of TSource, TKey)(IQueryable(Of TSource), Expression(Of Func(Of TSource, TKey)))之类的，我都不知道到该传入什么参数？还有我想问下LINQ怎么左连接然后对数据读取？

[解决办法]
left join:

C# code

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);     } }
[解决办法]
Group by
http://code.msdn.microsoft.com/101-LINQ-Samples-3fb9811b