由于 DBNulls 而转换数据类型时出错

问：

我正在尝试通过从一个数据库调用存储过程来使用我的 c# 应用程序加载数据，然后需要将该数据插入到另一个数据库中。我打算使用它的 SQL 批量插入。我确实提出了 SSIS，但不幸的是，他们没有软件和基础设施。我需要转换某些列的数据类型，因为当我批量插入目标表时它会爆炸。我正在使用函数进行转换，但不幸的是在尝试转换值时出现错误。当我尝试在转换前检查值时，它似乎没有进入 If 条件块。有人能告诉我我哪里出了问题吗？DBNullDBNull

using (SqlConnection con = new SqlConnection(_SourceConnectionString))
{
    // con.Open();

    // Create a table with some rows. 
    DataTable table = new DataTable();

    var cmd = new SqlCommand("[EMR].[spOrderDailySummary]", con);
    var da = new SqlDataAdapter(cmd);

    cmd.CommandType = CommandType.StoredProcedure;
    cmd.Parameters.Add("@OrderDate", SqlDbType.DateTime).Value = _businessDate;

    try
    {
        // LogManager.Log(LogSeverity.Info, $"Initiating report {rgc.ReportType}.");
        da.Fill(table);

        table.ConvertColumnType("Original_Qty", typeof(int));
        table.ConvertColumnType("Remaining_Qty", typeof(int));
        table.ConvertColumnType("Price", typeof(decimal));
        table.ConvertColumnType("OrderDate", typeof(DateTime));
        table.ConvertColumnType("Valid_to_Value", typeof(DateTime));
        table.ConvertColumnType("ClientID", typeof(int));
        table.ConvertColumnType("Trigger_Price", typeof(decimal));
        table.ConvertColumnType("Investment_Decision_Within_Firm", typeof(int));
        table.ConvertColumnType("Order_Priority_Timestamp", typeof(DateTime));
        table.ConvertColumnType("Transaction_Time", typeof(DateTime));
        table.ConvertColumnType("NewOrdDateTime", typeof(DateTime));

        StartDataMigration(table);
    }
}

执行转换的函数

  public static void ConvertColumnType<T>(this DataTable dt, string columnName, Type newType) where T : Type, IConvertible
{
    using (DataColumn dc = new DataColumn(columnName + "_new", newType))
    {
        // Add the new column which has the new type, and move it to the ordinal of the old column
        int ordinal = dt.Columns[columnName].Ordinal;
        dt.Columns.Add(dc);
        dc.SetOrdinal(ordinal);

        // Get and convert the values of the old column, and insert them into the new
        foreach (DataRow dr in dt.Rows)
            if (dr[dc.ColumnName] != DBNull.Value)
            {
                dr[dc.ColumnName] = Convert.ChangeType(dr[columnName], newType);
            }

        // Remove the old column
        dt.Columns.Remove(columnName);

        // Give the new column the old column's name
        dc.ColumnName = columnName;
    }
}