C# list conversion in asp.net

Question

I have a business object class BusinessObject which implements an interface IAlternateInterface. I already have a method that will return a generic list of BusinessObject which has the objects I want but I want to get them as a list of IAlternateInterface. I tried to do something like the following psudo code but I am getting a "Can not convert source type ... to target type ..." message in Visual Studio. What is the best way to convert the list?

public List<BusinessObject> GetObjects(){
 //logic to get the list
}

public List<IAlternateInterface> GetInterfaceObjects(){
 return GetObjects();
}

Answer 1

You're looking for Enumerable.Cast<TResult>:

GetObjects().Cast<IAlternateInterface>.ToList();

The ToList at the end is only necessary if you need a list. Cast<TResult> returns an IEnumerable<TResult>

Answer 2

With LINQ Cast

public List<IAlternateInterface> GetInterfaceObjects(){
 return GetObjects().Cast<IAlternateInterface>().ToList();
}

Answer 3

The two answers given using Cast are fine for C# 3 and C# 4... but if you're using C# 4 (and .NET 4) you can also use generic variance to avoid one step:

public List<IAlternateInterface> GetInterfaceObjects(){
  return GetObjects().ToList<IAlternateInterface>();
}