Question

I am trying to filter a list of Chord objects (class) in C# using linq.

I have the following function in my class

public List<Chord> FilterDictionary_byKey<Chord>(string theKey)
{
    var filteredList = from item in _chords
                       where item.KeyName == theKey
                       select item;

    return (List<Chord>)filteredList;
}

in the above Chord is an object type, _chords is a class variable of List type.

then from my calling code I tried the following:

List<Chord> theChords = globalVariables.chordDictionary.FilterDictionary_byKey("A");

obviously trying to return a filtered list of Chord objects from the class

however, when i compile the compiler returns

Error 1 The type arguments for method ‘ChordDictionary.chordDictionary.FilterDictionary_byKey(string)’ cannot be inferred from the usage. Try specifying the type arguments explicitly. C:\Users\Daniel\Development\Projects\Tab Explorer\Tab Explorer\Forms\ScaleAndChordViewer.cs 75 37 Tab Explorer

Answer 1

The inferred type of a from…select LINQ query is actually an IEnumerable<T>, which is not a populated list or collection, but a sequence that will be enumerated on demand. Casting it to List<T> does not cause this enumeration; instead, you should call ToList() or ToArray().

To address the error you’re getting: Your method declaration should not have a generic type parameter, since neither its return type nor any of its parameters are generic. Just remove the <Chord> following the method name.

public List<Chord> FilterDictionary_byKey(string theKey)
{
    var filteredList = from item in _chords
                       where item.KeyName == theKey
                       select item;

    return filteredList.ToList();
}