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"));
}

4

6 Answers

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.

4

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>();
}

1

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;
}

1

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; }

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

i.e.

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

1

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.

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