在列中找到最大数值时，如何忽略列中非数值的数据？

Question

I have a column in a datatable that normally consist of numerical values but there are rare occasions where letters will be present in the column. This is due to customize equipment taking measurements and when a measurement is out of range of its limits a word or abbreviation is recorded. With that being said I am having a problem getting the max numerical value from the column because one or more rows might contain letters.

I get the error message "Object cannot be cast from DBNull to other types "!!

  Decimal maxDataValue = Convert.ToDecimal(mainDataTable.Compute("max([Data])", string.Empty));
  Decimal roundMaxData = Math.Round(maxDataValue);

is it a way to work around rows who contain letters and find the max numerical value in the column?

Answer 1

The error you're seeing is saying that there are also null values in your data too. The other problem of having string data in the column you are trying to sum makes it difficult to use Compute directly. You could convert the DataTable to an Enumerable though and operate on it using Linq. Something like this could work...which returns 6.25 as the result.

var data = new DataTable();
data.Columns.Add("Data", typeof(string));
data.Rows.Add("1");
data.Rows.Add("2.0");
data.Rows.Add("3.25 kg");
data.Rows.Add(DBNull.Value);

var sum = data.AsEnumerable()
.Where(d => d["Data"] != DBNull.Value)
.Sum(d =>
{
    var columnData = d["Data"].ToString();
    var numericalString = new string(columnData.Where(c => char.IsDigit(c) || c == '.').ToArray());
    var numericalValue = decimal.Parse(numericalString);

    return numericalValue;
});