问题:

I have a DataTable object. Every column is of type string.

Using LINQ, how can I get the maximum string length for every column?

回答1:

The maximum string length for the whole table (assuming at least one non-null value there, otherwise, Max will throw an exception):

int maxStringLength = dataTable.AsEnumerable()
                              .SelectMany(row => row.ItemArray.OfType<string>())
                              .Max(str => str.Length);

If you want maximum string length for each column, you could do (assuming at least one non-null value in each column, otherwise, Max will throw an exception):

List<int> maximumLengthForColumns = 
   Enumerable.Range(0, dataTable.Columns.Count)
             .Select(col => dataTable.AsEnumerable()
                                     .Select(row => row[col]).OfType<string>()
                                     .Max(val => val.Length)).ToList();

回答2:

With c# 6, you can prevent the exception by adding val?.Length

var maximumLengthForColumns =
                Enumerable.Range(0, dt.Columns.Count)
                .Select(col => dt.AsEnumerable()
                                     .Select(row => row[col]).OfType<string>()
                                     .Max(val => val?.Length )).ToList();