how can I use linq to return another list

Question

I have an IEnumerable and I want to use a linq statement to serialize each object in the IEnumerable and return another list. Here's my foreach I have:

List<EventData> payloads = new List<EventData>();

foreach (var request in requestList)
{
   string message = JsonConvert.SerializeObject(request);

   EventData data = new EventData(Encoding.UTF8.GetBytes(message));
   payloads.Add(data);
}

I was attempting to do a Select statement like this:

requestList.Select(request => { JsonConvert.SerializeObject(request); });

Error message is :

The type arguments for method 'Enumerable.Select<TSource, TResult>(IEnumerable, Func<TSource, TResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly

I get an error saying: The type arguments for method 'Enumerable.Select<TSource, TResult>(IEnumerable<TSource>, Func<TSource, TResult>)' cannot be inferred from the usage. Try specifying the type arguments explicitly — DannyD
– DannyD, Commented Sep 11, 2019 at 18:19
var list = requestList.Select(request => new EventData(Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(request)))).ToList(); — 15ee8f99-57ff-4f92-890c-b56153
– 15ee8f99-57ff-4f92-890c-b56153, Commented Sep 11, 2019 at 18:20
The problem is that the curly brackets mean you have a method body, but you're not returning anything from it. — juharr
– juharr, Commented Sep 11, 2019 at 18:22
Side note: typically variables of a collection type are named with the plural of the type they contain, so if requestList is a List<Request>, it would simply be named requests. — littlerufe
– littlerufe, Commented Sep 11, 2019 at 19:49

littlerufe · Accepted Answer · 2019-09-11 18:56:38Z

1

The problem with the original code was that there is a method body defined with curly braces, but it doesn't return anything. To fix this, you can simply take your existing code and put it into the select statement, but add a return statement from the block:

List<EventData> payloads = requestList.Select(request => 
{ 
    string message = JsonConvert.SerializeObject(request);
    EventData data = new EventData(Encoding.UTF8.GetBytes(message));
    return data;
}).ToList();

Or you could do the same thing without the method body approach, but this sometimes makes the code a little harder to read and debug, since you're doing everything in one line:

List<EventData> payloads = requestList
    .Select(request => 
        new EventData(Encoding.UTF8.GetBytes(JsonConvert.SerializeObject(request))))
    .ToList();

edited Sep 11, 2019 at 18:56

answered Sep 11, 2019 at 18:45

littlerufe

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