如何修复System.InvalidCastException：指定的强制转换无效

Question

I have two Data Tables and want to combine them to form one Data Table based on their individual ID's.

The column columnName should get summed up if the ID is duplicated across the two initial Data Tables.

When I run my code I get this error

How to fix System.InvalidCastException: Specified cast is not valid

DataTable SubData = dsResult.Tables[0];

var query = SubData.AsEnumerable().GroupBy(row => new
{
    ProductID = row.Field<Int64>("ProductID"),
    PrdCode = row.Field<string>("PrdCode")
}).Select(grp =>
{
    dynamic result = new ExpandoObject();
    var dict = result as IDictionary<string, object>;
    result.ProductID = grp.Key.ProductID;
    result.PrdCode = grp.Key.PrdCode;

    foreach (DataRow row in grp)
    {
       foreach (DataColumn column in SubData.Columns)
       {
          string columnName = column.ColumnName;

          if (columnName.Equals("ProductID") || columnName.Equals("PrdCode"))
          {
              continue;
          }

          if (!dict.Keys.Contains(columnName))
          {
              dict[columnName] = row[columnName];
          }
          else
          {
             if (row[columnName] is System.DBNull)
             {
                continue;
             }

             if (dict[columnName] is System.DBNull)
             {
                 dict[columnName] = row[columnName];
                 continue;
             }

             dict[columnName] = (Int64)dict[columnName] + (Int64)row[columnName];
          }
      }
  }

  return result;
});

foreach (var item in query)
{
    dtGraph.Rows.Add(item);
}

Answer 1

Verify the data in 'dict'. There must be a value which you can't convert to 'int'