如何向 DataReader 添加列

Question

我的目标是从数据源检索数据，向其中添加一些元数据并将其插入到另一个目标。

目标的架构比源（计算列）多四列。

我正在使用SqlBulkCopy，它需要一个包含所有列（包括计算的 4 个）的阅读器。

有没有办法手动向 DataReader 添加列？或者如果不可能有什么替代数据插入？

Answer 1

我需要这样做，并且还能够基于其他列创建列，并使列的值取决于读取器的行索引。这个类是IWrappedReader从 Dapper 实现的，如果这对你有用的话（对我来说）。该类并不完整，因为我没有实现所有IDataRecord字段，但您可以看看我是如何做的IDataRecord.GetInt32以查看简单的模式。

/// <inheritdoc />
public class WrappedDataReader : IDataReader
{
    private readonly IList<AdditionalField> _additionalFields;
    private readonly int _originalOrdinalCount;
    private IDbCommand _cmd;
    private int _currentRowIndex = -1; //The first Read() will make this 0
    private IDataReader _underlyingReader;

    public WrappedDataReader(IDataReader underlyingReader, IList<AdditionalField> additionalFields)
    {
        _additionalFields = additionalFields;
        _underlyingReader = underlyingReader;

        var schema = Reader.GetSchemaTable();
        if (schema == null)
        {
            throw new ObjectDisposedException(GetType().Name);
        }

        _originalOrdinalCount = schema.Rows.Count;
    }

    public object this[int i]
    {
        get { throw new NotImplementedException(); }
    }

    public IDataReader Reader
    {
        get
        {
            if (_underlyingReader == null)
            {
                throw new ObjectDisposedException(GetType().Name);
            }

            return _underlyingReader;
        }
    }

    IDbCommand IWrappedDataReader.Command
    {
        get
        {
            if (_cmd == null)
            {
                throw new ObjectDisposedException(GetType().Name);
            }

            return _cmd;
        }
    }

    void IDataReader.Close() => _underlyingReader?.Close();

    int IDataReader.Depth => Reader.Depth;

    DataTable IDataReader.GetSchemaTable()
    {
        var rv = Reader.GetSchemaTable();
        if (rv == null)
        {
            throw new ObjectDisposedException(GetType().Name);
        }

        for (var i = 0; i < _additionalFields.Count; i++)
        {
            var row = rv.NewRow();

            row["ColumnName"] = _additionalFields[i].ColumnName;
            row["ColumnOrdinal"] = GetAppendColumnOrdinal(i);
            row["DataType"] = _additionalFields[i].DataType;

            rv.Rows.Add(row);
        }

        return rv;
    }

    bool IDataReader.IsClosed => _underlyingReader?.IsClosed ?? true;

    bool IDataReader.NextResult() => Reader.NextResult();

    bool IDataReader.Read()
    {
        _currentRowIndex++;
        return Reader.Read();
    }

    int IDataReader.RecordsAffected => Reader.RecordsAffected;

    void IDisposable.Dispose()
    {
        _underlyingReader?.Close();
        _underlyingReader?.Dispose();
        _underlyingReader = null;
        _cmd?.Dispose();
        _cmd = null;
    }

    int IDataRecord.FieldCount => Reader.FieldCount + _additionalFields.Count;

    bool IDataRecord.GetBoolean(int i) => Reader.GetBoolean(i);

    byte IDataRecord.GetByte(int i) => Reader.GetByte(i);

    long IDataRecord.GetBytes(int i, long fieldOffset, byte[] buffer, int bufferoffset, int length) =>
        Reader.GetBytes(i, fieldOffset, buffer, bufferoffset, length);

    char IDataRecord.GetChar(int i) => Reader.GetChar(i);

    long IDataRecord.GetChars(int i, long fieldoffset, char[] buffer, int bufferoffset, int length) =>
        Reader.GetChars(i, fieldoffset, buffer, bufferoffset, length);

    IDataReader IDataRecord.GetData(int i) => Reader.GetData(i);

    string IDataRecord.GetDataTypeName(int i) => Reader.GetDataTypeName(i);

    DateTime IDataRecord.GetDateTime(int i) => Reader.GetDateTime(i);

    decimal IDataRecord.GetDecimal(int i) => Reader.GetDecimal(i);

    double IDataRecord.GetDouble(int i) => Reader.GetDouble(i);

    Type IDataRecord.GetFieldType(int i) => Reader.GetFieldType(i);

    float IDataRecord.GetFloat(int i) => Reader.GetFloat(i);

    Guid IDataRecord.GetGuid(int i) => Reader.GetGuid(i);

    short IDataRecord.GetInt16(int i) => Reader.GetInt16(i);

    int IDataRecord.GetInt32(int i)
    {
        return i >= _originalOrdinalCount ? (int) ExecuteAdditionalFieldFunc(i) : Reader.GetInt32(i);
    }

    long IDataRecord.GetInt64(int i) => Reader.GetInt64(i);

    string IDataRecord.GetName(int i)
    {
        return i >= _originalOrdinalCount ? _additionalFields[GetAppendColumnIndex(i)].ColumnName : Reader.GetName(i);
    }

    int IDataRecord.GetOrdinal(string name)
    {
        for (var i = 0; i < _additionalFields.Count; i++)
        {
            if (name.Equals(_additionalFields[i].ColumnName, StringComparison.OrdinalIgnoreCase))
            {
                return GetAppendColumnOrdinal(i);
            }
        }

        return Reader.GetOrdinal(name);
    }

    string IDataRecord.GetString(int i) => Reader.GetString(i);

    object IDataRecord.GetValue(int i)
    {
        return i >= _originalOrdinalCount ? ExecuteAdditionalFieldFunc(i) : Reader.GetValue(i);
    }

    int IDataRecord.GetValues(object[] values) => Reader.GetValues(values);

    bool IDataRecord.IsDBNull(int i)
    {
        return i >= _originalOrdinalCount ? ExecuteAdditionalFieldFunc(i) == null : Reader.IsDBNull(i);
    }

    object IDataRecord.this[string name]
    {
        get
        {
            var ordinal = ((IDataRecord) this).GetOrdinal(name);
            return ((IDataRecord) this).GetValue(ordinal);
        }
    }


    object IDataRecord.this[int i] => ((IDataRecord) this).GetValue(i);

    private int GetAppendColumnOrdinal(int index)
    {
        return _originalOrdinalCount + index;
    }

    private int GetAppendColumnIndex(int oridinal)
    {
        return oridinal - _originalOrdinalCount;
    }

    private object ExecuteAdditionalFieldFunc(int oridinal)
    {
        return _additionalFields[GetAppendColumnIndex(oridinal)].Func(_currentRowIndex, Reader);
    }

    public struct AdditionalField
    {
        public AdditionalField(string columnName, Type dataType, Func<int, IDataReader, object> func = null)
        {
            ColumnName = columnName;
            DataType = dataType;
            Func = func;
        }

        public string ColumnName;
        public Type DataType;
        public Func<int, IDataReader, object> Func;
    }
}

这是我编写的一个快速 NUnit 测试，用于展示其用法

[Test]
public void ReaderTest()
{
    using (var conn = new SqlConnection(ConnectionSettingsCollection.Default))
    {
        conn.Open();
        const string sql = @"
            SELECT 1 as OriginalField
            UNION
            SELECT -500 as OriginalField
            UNION
            SELECT 100 as OriginalField
        ";

        var additionalFields = new[]
        {
            new WrappedDataReader.AdditionalField("StaticField", typeof(int), delegate { return "X"; }),
            new WrappedDataReader.AdditionalField("CounterField", typeof(int), (i, reader) => i),
            new WrappedDataReader.AdditionalField("ComputedField", typeof(int), (i, reader) => (int) reader["OriginalField"] + 1000)
        };

        const string expectedJson = @"
            [
                {""OriginalField"":-500,""StaticField"":""X"",""CounterField"":0,""ComputedField"":500},
                {""OriginalField"":1,   ""StaticField"":""X"",""CounterField"":1,""ComputedField"":1001},
                {""OriginalField"":100, ""StaticField"":""X"",""CounterField"":2,""ComputedField"":1100}
            ]
        ";

        var actualJson = ToJson(new WrappedDataReader(new SqlCommand(sql, conn).ExecuteReader(), additionalFields));

        Assert.Zero(CultureInfo.InvariantCulture.CompareInfo.Compare(expectedJson, actualJson, CompareOptions.IgnoreSymbols));
    }
}

private static string ToJson(IDataReader reader)
{
    using (var strWriter = new StringWriter(new StringBuilder()))
    using (var jsonWriter = new JsonTextWriter(strWriter))
    {
        jsonWriter.WriteStartArray();

        while (reader.Read())
        {
            jsonWriter.WriteStartObject();

            for (var i = 0; i < reader.FieldCount; i++)
            {
                jsonWriter.WritePropertyName(reader.GetName(i));
                jsonWriter.WriteValue(reader[i]);
            }

            jsonWriter.WriteEndObject();
        }

        jsonWriter.WriteEndArray();

        return strWriter.ToString();
    }
}