The in keyword has three uses:

1. As part of the syntax in a foreach statement or as part of the syntax in a LINQ query

   foreach (var member in sequence)
   {
       // ...
   }
   

2. In the context of generic interfaces and generic delegate types signifies contravariance for the type parameter in question:

   public interface IComparer<in T>
   {
       // ...
   }
   

3. In the context of LINQ query refers to the collection that is being queried

   var query = from x in source select new { x.Name, x.ID, };