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理解了為甚麼會一直出現這個型別轉換的錯誤之後，接下來就是要動手，把原始碼調整成可以處理 Polygon 的形狀（？）。

觀察一下原始碼

輸出 Shapefile 所使用的方法是 Shapefile.WriteAllFeatures()，在 Shapefile.cs 中的原始碼如下：

public static void WriteAllFeatures(IEnumerable<IFeature> features, string shpPath, Encoding encoding = null, string projection = null)
{
    if (features == null)
        throw new ArgumentNullException(nameof(features));

    var firstFeature = features.FirstOrDefault();
    if (firstFeature == null)
        throw new ArgumentException(nameof(ShapefileWriter) + " requires at least one feature to be written.");

    var fields = firstFeature.Attributes.GetDbfFields();
    var shapeType = features.FindNonEmptyGeometry().GetShapeType();
    
    using (var shpWriter = OpenWrite(shpPath, shapeType, fields, encoding, projection))
    {
        shpWriter.Write(features);
    }
}

在第 13 行中所使用的 OpenWrite 方法：

public static ShapefileWriter OpenWrite(string shpPath, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null, string projection = null)
{
    if (type.IsPoint())
    {
        return new ShapefilePointWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsMultiPoint())
    {
        return new ShapefileMultiPointWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsPolyLine())
    {
        return new ShapefilePolyLineWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsPolygon())
    {
        return new ShapefilePolygonWriter(shpPath, type, fields, encoding, projection);
    }
    else
    {
        throw new FileLoadException("Unsupported shapefile type: " + type, shpPath);
    }
}

可以看到第 17 行，如果是 Polygon 就會建立一個新的 ShapefilePolygonWriter，而他是繼承 ShapefileWriter 的 MultiPolygon 型別。第 21 行所回傳的 ShpPolygonWriter 也是，繼承 ShpWriter，泛型的型別一樣是 MultiPolygon，可以參考 GitHub 的原始碼。

public class ShapefilePolygonWriter : ShapefileWriter<MultiPolygon>
{
    /// <inheritdoc/>
    public ShapefilePolygonWriter(Stream shpStream, Stream shxStream, Stream dbfStream, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null)
            : base(shpStream, shxStream, dbfStream, type, fields, encoding)
    { }

    /// <inheritdoc/>
    public ShapefilePolygonWriter(string shpPath, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null, string projection = null)
            : base(shpPath, type, fields, encoding, projection)
    { }

    /// <inheritdoc/>
    public ShapefilePolygonWriter(string shpPath, ShapeType type, params DbfField[] fields)
            : base(shpPath, type, fields)
    {
    }

    internal override ShpWriter<MultiPolygon> CreateShpWriter(Stream shpStream, Stream shxStream)
    {
        return new ShpPolygonWriter(shpStream, shxStream, ShapeType);
    }

我的想法很簡單，既然他把所有 Polygon 類型的 Geometry 都交由 MultiPolygonWriter 處理，那我就把 Polygon 單獨取出來，另外寫一個新的方式處理就好。

我不想寫一個新的怎麼辦?

針對我遇到的問題，我有在 GitHub 上提問，有人提供了另外一個解法：

var geometry = wktReader.Read(...);

// 指定 Geometry 的型別
var poly = (Polygon) geometry;

// 利用 Polygon 製作一個新的 MultiPolygon
var acceptableGeometry = poly.Factory.CreateMultiPolygon(new[] {poly});

Shapefile.WriteAllFeatures(...);

相較之下的確簡單了許多，但在實務上，如果需要和第三方接洽，可能就需要確認是否對方能接受 MultiPolygon 的格式。

寫一個新的 PolygonWriter

從我們剛剛所觀察的原始碼可以看到，需要修正的檔案有三個：

1. ShapeType.cs：把 IsPolygon() 和 IsMultiPolygon() 拆開
2. Shapefile.cs：依據 IsPolygon() 和 IsMultiPolygon() 分別回傳不同的 Writer
3. ShapefilePolygonWriter.cs：拆成兩種 Polygon Wrtier
4. ShpPolygonWriter.cs：拆成兩種 Polygon Wrtier

下面會附上需要新增的 Polygon 相關檔案，記得同時也要把原本的檔名修改成 MultiPolygon，編譯時才不會打架。修改完成的版本我有拉一個 fork 在我的 GitHub，有需要的話也可以直接前往下載。

ShapeType.cs

修改的重點在於把 Polygon 和 MultyPolygon 分開，實際操作上我比較不會使用到 PolygonM、PolygonZM 這兩個型別，有需要的話也可以自行拆解看看。

/// <summary>
/// Indicates if shape associated with this type is a Polygon.
/// </summary>
public static bool IsPolygon(this ShapeType type)
{
    // 修改成只符合 Polygon
    return type == ShapeType.Polygon;
}

/// <summary>
/// Indicates if shape associated with this type is a Multi Polygon.
/// </summary>
public static bool IsMultiPolygon(this ShapeType type)
{
    // 將其他 Polygon 類別歸到 MultiPolygon
    return type == ShapeType.PolygonM
        || type == ShapeType.PolygonZM
        || type == ShapeType.PolygonZ;
}

Shapefile.cs

public static ShapefileWriter OpenWrite(string shpPath, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null, string projection = null)
{
    if (type.IsPoint())
    {
        return new ShapefilePointWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsMultiPoint())
    {
        return new ShapefileMultiPointWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsPolyLine())
    {
        return new ShapefilePolyLineWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsPolygon())
    {
        // 新增 IsMultiPolygon() 的判斷，回傳對應的 Wtiter
        return new ShapefilePolygonWriter(shpPath, type, fields, encoding, projection);
    }
    else if (type.IsMultiPolygon())
    {
        // 新增 IsMultiPolygon() 的判斷，回傳對應的 Wtiter
        return new ShapefileMultiPolygonWriter(shpPath, type, fields, encoding, projection);
    }
    else
    {
        throw new FileLoadException("Unsupported shapefile type: " + type, shpPath);
    }
}

ShapefilePolygonWriter.cs

public class ShapefilePolygonWriter : ShapefileWriter<Polygon>  // 型別改成 Polygon
{
    /// <inheritdoc/>
    public ShapefilePolygonWriter(Stream shpStream, Stream shxStream, Stream dbfStream, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null)
        : base(shpStream, shxStream, dbfStream, type, fields, encoding)
    { }

    /// <inheritdoc/>
    public ShapefilePolygonWriter(string shpPath, ShapeType type, IReadOnlyList<DbfField> fields, Encoding encoding = null, string projection = null)
        : base(shpPath, type, fields, encoding, projection)
    { }

    /// <inheritdoc/>
    public ShapefilePolygonWriter(string shpPath, ShapeType type, params DbfField[] fields)
        : base(shpPath, type, fields)
    {
    }

    internal override ShpWriter<Polygon> CreateShpWriter(Stream shpStream, Stream shxStream)
    {
        // 回傳 Polygon 的 Writer
        return new ShpPolygonWriter(shpStream, shxStream, ShapeType);
    }
}

ShpPolygonWriter.cs

這邊要注意的是要如何把 Geometry 寫入。這邊使用的方法是參考 MultiPolygon，只是把迴圈的部分拿掉。

public class ShpPolygonWriter : ShpWriter<Polygon>
{
    /// <inheritdoc/>
    public ShpPolygonWriter(Stream shpStream, Stream shxStream, ShapeType type) : base(shpStream, shxStream, type)
    {
        if (!ShapeType.IsPolygon())
            ThrowUnsupportedShapeTypeException();
    }

    internal override void WriteGeometry(Polygon polygon, Stream stream)
    {
        var partsBuilder = new ShpMultiPartBuilder(1, 4);

        partsBuilder.AddPart(polygon.Shell.CoordinateSequence);
        partsBuilder.WriteParts(stream, HasZ, HasM);
        partsBuilder.UpdateExtent(Extent);
    }
}

References

• NetTopologySuite.IO.Esri 官方 GitHub