C# LINQ - Type Arguments Cannot Be Specified From Usage

Question

I was writing a function to parse an input from a string to get all the integers into a list. However, I got an an error:

The type arguments for method

System.Linq.Enumerable.SelectMany<TSource,TResult>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,System.Collections.Generic.IEnumerable<TResult>>)' cannot be inferred from the usage. Try specifying the type arguments explicitly.

Now I have tried to specify them but to not avail.

The logic of the code (using example input of "56#3=45#90") is:

So I am splitting "56#3=45#90" (on '=') into a string array of {"56#3", "45#90"} to be operated on by Select, which will split (on '#') into Enumerable( IEnumerable(string) ) containing "{"56", "3"},{"45", "90"} . Finally, the SelectMany is supposed to flatten the structure (and convert to int) into {56, 3, 45, 90} which should be able to be converted into a list.

I don't see any reason why there's something wrong with specifying type arguments.

// string 'a' contains "<int>#<int>=<int>#<int>" such as "56#3=45#90";
public static IList<int> ParseCKPInput(string a)  
{
        return (a.Split('=')).Select(n => n.Split('#')).SelectMany(n => Convert.ToInt32(n)).ToList();
}

I understand this can be done by regex (which I have implemented now); however, I want to improve my understanding of LINQ and figure out what is wrong with my original code. Thanks everyone!

Answer 1

The reason why .SelectMany(n => Convert.ToInt32(n)) doesn't work is because n will be a list, and you can't convert a list to an int.

The query should be :

return a.Split('=')
        .Select(n => n.Split('#'))
        .SelectMany(x => x)
        .Select(x => Convert.ToInt32(x))
        .ToList();

Or... there is no need to flatten this just pass all the split characters in at once:

return a.Split('=', '#')
        .Select(x => Convert.ToInt32(x))
        .ToList();