Question

I am a bit rusty on generics, trying to do the following, but the compiler complains:

protected List<T> PopulateCollection(DataTable dt) where T: BusinessBase {     List<T> lst = new List<T>();     foreach (DataRow dr in dt.Rows)     {         T t = new T(dr);         lst.Add(t);     }     return lst; } 

So as you can see, i am trying to dump contents of a Table into an object (via passing a DataRow to the constructor) and then add the object to collection. it complains that T is not a type or namespace it knows about and that I can’t use where on a non-generic declaration.

Is this not possible?

Answer 1

There are two big problems:

• You can’t specify a constructor constraint which takes a parameter
• Your method isn’t currently generic – it should be PopulateCollection<T> instead of PopulateCollection.

You’ve already got a constraint that T : BusinessBase, so to get round the first problem I suggest you add an abstract (or virtual) method in BusinessBase:

public abstract void PopulateFrom(DataRow dr); 

Also add a parameterless constructor constraint to T.

Your method can then become:

protected List<T> PopulateCollection(DataTable dt)     where T: BusinessBase, new() {     List<T> lst = new List<T>();     foreach (DataRow dr in dt.Rows)     {         T t = new T();         t.PopulateFrom(dr);         lst.Add(t);     }     return lst; } 

If you’re using .NET 3.5, you can make this slightly simpler using the extension method in DataTableExtensions:

protected List<T> PopulateCollection<T>(DataTable dt)     where T: BusinessBase, new() {     return dt.AsEnumerable().Select(dr =>      {          T t = new T();         t.PopulateFrom(dr);     }.ToList(); } 

Alternatively, you could make it an extension method itself (again, assuming .NET 3.5) and pass in a function to return instances:

static List<T> ToList<T>(this DataTable dt, Func<DataRow dr, T> selector)     where T: BusinessBase {     return dt.AsEnumerable().Select(selector).ToList(); } 

Your callers would then write:

table.ToList(row => new Whatever(row)); 

This assumes you go back to having a constructor taking a DataRow. This has the benefit of allowing you to write immutable classes (and ones which don’t have a parameterless constructor) it but does mean you can’t work generically without also having the ‘factory’ function.