IEnumerable Concat Missing, does not contain a definition for 'Concat'

Question

I have the following Class which inherits IEnumerable

public class LinesEnumerable : IEnumerable<Point>
{
    protected readonly IPointSeries _pointSeries;
    protected readonly ICoordinateCalculator<double> _xCoordinateCalculator;
    protected readonly ICoordinateCalculator<double> _yCoordinateCalculator;
    protected readonly bool _isDigitalLine;

    /// <summary>
    /// Initializes a new instance of the <see cref="LinesEnumerable" /> class.
    /// </summary>
    /// <param name="pointSeries">The point series.</param>
    /// <param name="xCoordinateCalculator">The x coordinate calculator.</param>
    /// <param name="yCoordinateCalculator">The y coordinate calculator.</param>
    /// <param name="isDigitalLine">if set to <c>true</c> return a digital line .</param>
    public LinesEnumerable(IPointSeries pointSeries, ICoordinateCalculator<double> xCoordinateCalculator, ICoordinateCalculator<double> yCoordinateCalculator, bool isDigitalLine)
    {
        _pointSeries = pointSeries;
        _xCoordinateCalculator = xCoordinateCalculator;
        _yCoordinateCalculator = yCoordinateCalculator;
        _isDigitalLine = isDigitalLine;
    }

    /// <summary>
    /// Returns an enumerator that iterates through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="T:System.Collections.IEnumerator" /> object that can be used to iterate through the collection.
    /// </returns>
    public virtual IEnumerator<Point> GetEnumerator()
    {
        return _isDigitalLine ?
            (IEnumerator<Point>)new DigitalLinesIterator(_pointSeries, _xCoordinateCalculator, _yCoordinateCalculator) :
            new LinesIterator(_pointSeries, _xCoordinateCalculator, _yCoordinateCalculator);
    }

    /// <summary>
    /// Returns an enumerator that iterates through a collection.
    /// </summary>
    /// <returns>
    /// An <see cref="T:System.Collections.IEnumerator" /> object that can be used to iterate through the collection.
    /// </returns>
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

However, when I try to do the following:

linesEnumerable = linesEnumerable.Concat(new[] { new Point(viewportWidth, lastYCoordinate) });

it says 'System.Collections.IEnumerable' does not contain a definition for 'Concat' and the best extension method overload 'System.Linq.Queryable.Concat(System.Linq.IQueryable, System.Collections.Generic.IEnumerable)' has some invalid arguments

I already have System.Linq namespace added

Anybody know why this is happening?

Answer 1

The compiler will give this error when the two collections are of different types T. For example:

List<int> l1 = new List<int>();
List<string> l2 = new List<string>();

var l3 = l1.Concat(l2);
var l4 = l1.Union(l2);

The Concat and Union calls will result in the following compile-time errors respectively:

'List' does not contain a definition for 'Concat' and the best extension method overload 'Queryable.Concat(IQueryable, IEnumerable)' requires a receiver of type 'IQueryable'

'List' does not contain a definition for 'Union' and the best extension method overload 'Queryable.Union(IQueryable, IEnumerable)' requires a receiver of type 'IQueryable'

It is confusing because the message is misleading and VS intellisense recognizes the extension methods. The solution is that l1 and l2 must be lists of the same type T.

Answer 2

Well this won't help you but this question is the top result when you google the error so I'm going to say what my problem was.

I had the following classes:

class Group
class Row : Group
class Column : Group

I was trying to call Concat on an IEnumerable<Row> with an IEnumerable<Column> to get an IEnumerable<Group>. This doesn't work because you can't convert the columns to rows. The solution was to cast my IEnumerable<Row> to IEnumerable<Group>.

e.g. rows.Cast<Group>().Concat(columns)