Cannot implicity convert type System.Collections.Generic.IEnumerable<B> to System.Collections.Generic.List<B>

问题

With the code below i get this error and need help with how to let method Load return List<B>

Cannot implicity convert type System.Collections.Generic.IEnumerable to System.Collections.Generic.List

public class A
    {
      public List<B> Load(Collection coll)
      {
        List<B> list = from x in coll select new B {Prop1 = x.title, Prop2 = x.dept};
        return list;
      }
    }

public class B
{
  public string Prop1 {get;set;}
  public string Prop2 {get;set;} 
}

回答1:

Your query returns an IEnumerable, while your method must return a List<B>.
You can convert the result of your query to a list via the ToList() extension method.

public class A
{
   public List<B> Load(Collection coll)
   {
       List<B> list = (from x in coll select new B {Prop1 = x.title, Prop2 = x.dept}).ToList();
       return list;
   }
}

The type of the list should be inferred automatically by the compiler. Were it not, you would need to call ToList<B>().

回答2:

You need to convert the enumeration to a list, there's an extension method to do that for you e.g. try this:

    var result = from x in coll select new B {Prop1 = x.title, Prop2 = x.dept};
    return result.ToList();

回答3:

You can't convert an object of a more generic type to a more specific.

Let's imagine, that we have a List of B and IEnumerable of B:

List<B> BList = ...
IEnumerable<B> BQuery = ...

You can do this:

IEnumerable<B> collection = BList;

But you can't do this:

List<B> collection = BQuery;

because collection is a more specific object, than IEnumerable.

So, you should use an extension method ToList() in your case:

(from x in coll
select new B 
  {
    Prop1 = x.title, 
    Prop2 = x.dept
  }
).ToList()

来源：https://stackoverflow.com/questions/13668648/cannot-implicity-convert-type-system-collections-generic-ienumerableb-to-syste