从函数返回匿名类型

Question

Can I use an anonymous type as a return type in a Function, and then stuff that returned value into an array or collection of some sort whilst also adding an additional field to the new array/collection? excuse my pseudocode...

private var GetRowGroups(string columnName)
{
var groupQuery = from table in _dataSetDataTable.AsEnumerable()
                             group table by new { column1 = table[columnName] }
                                 into groupedTable
                                 select new
                                 {
                                     groupName = groupedTable.Key.column1,
                                     rowSpan = groupedTable.Count()
                                 };
    return groupQuery;

}

private void CreateListofRowGroups()
{
    var RowGroupList = new List<????>();
    RowGroupList.Add(GetRowGroups("col1"));
    RowGroupList.Add(GetRowGroups("col2"));
    RowGroupList.Add(GetRowGroups("col3"));

}

Answer 1

No you can't return an anonymous type from the method. For more info read this MSDN doc. Use class or struct instead of an anonymous type.

You should read blog post - Horrible grotty hack: returning an anonymous type instance

If you are using framework 4.0 then you can return List<dynamic> but be careful to access the properties of anonymous object.

private List<dynamic> GetRowGroups(string columnName)
{
var groupQuery = from table in _dataSetDataTable.AsEnumerable()
                             group table by new { column1 = table[columnName] }
                                 into groupedTable
                                 select new
                                 {
                                     groupName = groupedTable.Key.column1,
                                     rowSpan = groupedTable.Count()
                                 };
    return groupQuery.ToList<dynamic>();
}

Answer 2

This is a very popular question . In general you cannot return an anonymous type due to the requirement of strong typing. However there are a couple of workarounds.

1. Create a simple type to represent the return value. (See here and here ). Make it simple by generating from usage .
2. Create a helper method to cast to the anonymous type using a sample instance for casting.

Answer 3

No, you can't return an anonymous type directly, but you can return it using an impromptu interface . Something like this:

public interface IMyInterface
{
    string GroupName { get;  }
    int RowSpan { get; }
}

private IEnumerable<IMyInterface> GetRowGroups()
{
    var list =
        from item in table
        select new
        {
            GroupName = groupedTable.Key.column1,
            RowSpan = groupedTable.Count()
        }
        .ActLike<IMyInterface>();

    return list;
}

Answer 4

Just use and ArrayList

    public static ArrayList GetMembersItems(string ProjectGuid)
    {
        ArrayList items = new ArrayList(); 

              items.AddRange(yourVariable 
                        .Where(p => p.yourproperty == something)
                        .ToList());
            return items;
    }

Answer 5

Use object , not var . You'll have to use reflection to access the properties outside the scope of the anonymous type though.

ie

private object GetRowGroups(string columnName) 
...
var RowGroupList = new List<object>();
...

Answer 6

From C#7 you can return a list of tuples:

private IEnumerable<(string, string)> GetRowGroups(string columnName)
{
    return from table in _dataSetDataTable.AsEnumerable()
           group table by new { column1 = table[columnName] }
           into groupedTable
           select (groupedTable.Key.column1, groupedTable.Count());
}

You can even give the members of the tuple names:

private IEnumerable<(string groupName, string rowSpan)> GetRowGroups(string columnName)
{
    return from table in _dataSetDataTable.AsEnumerable()
           group table by new { column1 = table[columnName] }
           into groupedTable
           select (groupedTable.Key.column1, groupedTable.Count());
}

However you need System.ValueTuple from the Nuget Package Manager.

Anyway I suggest to give things a clear name that includes what their purpose is, in particular when this is part of a public API. Having said this you should consider to create a class that holds those properties and return a list of that type.