c# - LINQ select 来自集合

Question

I'm attempting to write a simple Select method on a class that inherits from IList .

public class RowDataCollection : IList<RowData> {
  private List<RowData> rowList;

  internal RowDataCollection(List<RowData> data) {
    rowList = data;
  }
  // ...
}

public RowDataCollection Rows;

public RowDataCollection Select(string colName, object value) {
  List<RowData> rowList = from item in Rows
         where item[colName].Value == value
         select item;
  return new RowDataCollection(rowList);
}

Some problems I'm having:

First:

• VS2010 reports Cannot implicitly convert type 'IEnumerable<RowData>' to 'List<RowData>'. An explicit conversion exists (are you missing a cast?) Cannot implicitly convert type 'IEnumerable<RowData>' to 'List<RowData>'. An explicit conversion exists (are you missing a cast?)

OK, where does the CAST go?

Second:

• Someone could pass in an invalid colName value (ie String.IsNullOrEmpty(colName) ) or a null parameter (object value == null) .

How would I handle the way my function returns if the input parameters are invalid?

[Solved]

I edited my Select statement (even renamed it per the suggestions here). I had to use a switch to cast to the data type that the data was in, but it does work.

public RowDataCollection SelectRow(string colName, object value) {
  if (!String.IsNullOrEmpty(colName) && (value != null) && (0 < Rows.Count)) {
    switch (Rows[0][colName].GetValueType()) {
      case TableDataType.Boolean:
        return new RowDataCollection(Rows.Where(r => (bool)r[colName].Value == (bool)value).ToList());
      case TableDataType.Character:
        return new RowDataCollection(Rows.Where(r => (char)r[colName].Value == (char)value).ToList());
      case TableDataType.DateTime:
        return new RowDataCollection(Rows.Where(r => (DateTime)r[colName].Value == (DateTime)value).ToList());
      case TableDataType.Decimal:
        return new RowDataCollection(Rows.Where(r => (Decimal)r[colName].Value == (Decimal)value).ToList());
      case TableDataType.Integer:
        return new RowDataCollection(Rows.Where(r => (int)r[colName].Value == (int)value).ToList());
      case TableDataType.String:
        return new RowDataCollection(Rows.Where(r => r[colName].Value.ToString() == value.ToString()).ToList());
    }
  }
  return null;
}

[Solved (short version)]

Jon Skeet posted this about the same time I posted my solution, and (as always) his code is much nicer.

public RowDataCollection SelectRow(string colName, object value) {
  List<RowData> rowList = Rows.Where(r => r[colName].Value.Equals(value)).ToList();
  return new RowDataCollection(rowList);
}

@Jon Skeet: If I ever see your face in the same line at some software developer position I'm applying for, I'm just going to turn around and go home.

@Everyone : Thanks for all the help!

Answer 1

The result of a query like that isn't a List<T> , it's an IEnumerable<T> . If you want to convert that into a List<T> , just call ToList :

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

As it happens, you're only calling Where in your query. I would rewrite this as:

List<RowData> rowList = Rows.Where(item => item[colName].Value.Equals(value))
                            .ToList();

I'd also rename the method to something which is obviously filtering rather than projecting , given that the latter is the more common use of the term "select" in LINQ.

As for input parameters - I suggest you validate the arguments and throw an exception if they're not valid:

if (string.IsNullOrEmpty(colName))
{
    throw new ArgumentException("colName");
}

Answer 2

You can't directly cast an IEnumerable<RowData> to a List<RowData> , however, there does exist a convenience function Enumerable.ToList<T>() , used like so:

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

As for your second question, an exception would occur during the ToList() call as the LINQ expression is evaluated immediately. You have a few options, including throwing ArgumentException s or returning an empty list. It depends on your use cases. I'd suggest simply throwing an exception (assuming you have some HasColumn() method on your RowData class):

if (colName == null)
{
    throw new ArgumentNullException("colName");
}
else if (!Rows.All(row => row.HasColumn(colName)))
{
    throw new ArgumentException("No such column " + colName, "colName");
}

Per your edit, another approach, if a column missing is not necessarily a "problem":

...
// note the change to Any()
else if (!Rows.Any(row => row.HasColumn(colName))
{
    throw new ArgumentException("No such column " + colName, "colName");
}

List<RowData> rowList = (from item in Rows
                         where item.HasColumn(colName)
                            && item[colName].Value == value
                         select item).ToList();

Answer 3

You're getting the error message because LINQ Queries return IEnumerable, not List.

If you need a List, it's easy enough:

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

Answer 4

You have to convert IQueriable<> to List<>, by calling ToList();

  public RowDataCollection Select(string colName, object value) {
      List<RowData> rowList = from item in Rows
             where item[colName].Value == value
             select item;
      return new RowDataCollection(rowList.ToList());
    }