为什么从IEnumerable投射 <string> 列表 <string> 失败？

Question

Why does the following not succeed in casting IEnumerable<string> to List<string> ?

var l = new List<Tuple<string, int>>();
l.Add(new Tuple<string, int>("a string", 1));
List<string> s = (List<string>)l.Select(x => x.Item1); // System.InvalidCastException
MessageBox.Show(s[0]);

Also, why is the exception not caught properly in Visual Studio? It appears in the debug window but doesn't stop execution of the program.

Answer 1

Select returns an IEnumerable<T> . If you want the results as a List<T> , use:

List<string> s = l.Select(x => x.Item1).ToList()

Internally the Select method is not generating a List at all; the elements are returned on the fly as the IEnumerable<T> is iterated.

I would except the Exception to be caught. My guess would be that you have a catch (perhaps not one that you added) that is picking it up somewhere along the way.

Answer 2

It's because your approach is all wrong; you can convert the result of a Select into a list - just not like that - simply because it is an IEnumerable not a List . If you want a list then do

var string_list = l.Select(x => x.Item1).ToList();

If you absolutely, concretely, 100%, certainly know that you only have one element[1] then do:

var l = new List<Tuple<string, int>>();
l.Add(new Tuple<string, int>("a string", 1));
MessageBox.Show( l.Select(x => x.Item1).First() );

If your list could contain more than one element then do

MessageBox.Show( String.Join(", ", l.Select(x => x.Item1)) );

If you want to work with LINQ effectively it's important to understand what's happening and to realize that what you are creating are somesort of result set - which is not the same as a primitive List and this is where IMO LINQ is incredibly powerful - you just need to understand it.

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1 - In which case why are you working with a list....