c# - IEnumerable Concat Missing，不包含 'Concat' 的定义

Question

我有以下继承 IEnumerable 的类

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();
    }
}

但是，当我尝试执行以下操作时:

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

它说“System.Collections.IEnumerable”不包含“Concat”的定义和最佳扩展方法重载“System.Linq.Queryable.Concat(System.Linq.IQueryable, System.Collections.Generic.IEnumerable)” ' 有一些无效参数

我已经添加了 System.Linq 命名空间

有人知道为什么会这样吗？

Answer 1

当两个集合的T类型不同时，编译器会给出这个错误。例如:

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

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

Concat 和Union 调用将分别导致以下编译时错误:

'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'

这是令人困惑的，因为消息具有误导性并且 VS 智能感知识别扩展方法。解决方案是l1和l2必须是相同类型T的列表。