Question

This is a simplified example, to isolate the purpose of the question. In my actual scneario, the ColumnReader returned by GetColumnReader will actually do more work than merely ReadLine.

If I run the following program, I will get an error when I try to call Reader(), because of course the TextReader has already been disposed by the using statement.

public class Play{
    delegate string ColumnReader();
    static ColumnReader GetColumnReader(string filename){
        using (TextReader reader = new StreamReader(filename)){
            var headers = reader.ReadLine();
            return () => reader.ReadLine();
        }
    }
    public static void Main(string[] args){
        var Reader = GetColumnReader("Input.tsv");
        Console.WriteLine(Reader());
    }


}

Alternatively, I can remove the “using” and directly declare the TextReader, which would function, but now we no longer have a guarantee that the TextReader will be eventually closed.

Is there a way to add a “destructor” to the returned lambda function where I might be able to Dispose of the TextReader as soon as the lambda function goes out of scope (no more references)?

I also welcome other suggestions but wish to keep the basic closure structure (that is, fits into the scope of the question).

Answer 1

Essentially you need the scope of the disposable element outside of the delegate itself. In these situations I would make the delegate accept the disposable instance (I.e. TextReader) rather than a file name.