Is there any better way than the following?
Particularly, I want to replace Activator with something else.
public static List<T> ToList<T>(DataTable dt)
{
Type type = typeof(T);
List<T> list = new List<T>();
foreach (DataRow dr in dt.Rows)
{
object[] args = new object[1];
args[0] = dr;
list.Add((T)Activator.CreateInstance(type, args));
}
return list;
}
The first thing I want to mention is that you probably don't need a list. Odds are, an IEnumerable is enough. Even if you do need a list, it's trivial to convert an IEnumerable to a list.
With that in mind, this code is a nice generic way to accomplish it:
public static IEnumerable<T> ToEnumerable<T>(DataTable dt, Func<DataRow, T> translator)
{
foreach(DataRow dr in dt.Rows)
{
yield return translator(dr);
}
}
Hopefully you can see how re-usable this is. All you need to do is supply a function that knows how to convert an individual DataRow into your T type. That function might use Activator, but it doesn't have to. It might just use a normal constructor and set a few properties.
I don't really see any way to improve this code - why do you want to avoid Activator?
One option you could explore would be to create some sort of interface like this:
interface IFoo
{
void Initialize(DataRow dr);
}
And then implement this interface on any type that gets passed to this method. Then you would constrain your generic type parameter like this:
public static List<T> ToList<T>(DataTable dt)
where T : IFoo, new()
Then change the implementation of your method like this:
public static List<T> ToList<T>(DataTable dt)
where T : IFoo, new()
{
List<T> list = new List<T>();
foreach (DataRow dr in dt.Rows)
{
T t = new T();
t.Initialize(dr);
list.Add(t);
}
return list;
}
One thing I'd add to Andrew's answer is if you go that route you can (sorta) avoid the Activator class by constraining the generic method with a new() constraint.
public static List<T> ToList<T>(DataTable dt)
where T : IFoo, new()
{
...
foreach ( ... ) {
var foo = new T();
foo.Initialize(dataRow);
list.Add(foo);
}
...
}
The reason I say "sorta" is because C# actually just compiles that into an Activator.CreateInstance call at compile-time anyway. But it looks much cleaner.
Related
I'm trying to create an instance of a class that I can add to a list in a generic way..
I know the type of class that needs to be made and i've been able to make an object of the class using the code below but I have not found a way to create a cast that will allow me to add this to a list.. any ideas?
T is the same as objType
public static List<T> LoadList(string fileName, Type objType)
{
List<T> objList = new List<T>();
object o = Activator.CreateInstance(objType);
objList.Add((**o.GetType()**)o);
return objList;
}
if theres a better way of doing this too im open to ideas :)
Just use the non-generic API:
((IList)objList).Add(o);
I'm also assuming that type is a generic type-parameter; just to say: type is confusing for this; T or TSomethingSpecific would be more idiomatic.
Also: this line needs fixing:
List<type> objList = new List<type>(); // <=== not new List<Object>
Given that <type> is the same as objType, I'd suggest dropping the reflection and using the where T : new() type constraint:
public static List<T> LoadList(string fileName)
where T : new()
{
List<T> objList = new List<T>();
objList.add(new T());
return objList;
}
Edit:
If objType is a subclass of T, then I think something along these lines should work:
public static List<TListType, T> LoadList(string fileName)
where T : TListType, new()
{
List<TListType> objList = new List<TListType>();
objList.add(new T());
return objList;
}
You could user Zach Johnson's suggestion with the where constraint as it eliminates a duplicate specification of the type. However if the signature is set in stone, you could get by with a simple as cast, like:
public List<T> LoadList<T>(Type objType) where T : class
{
List<T> objList = new List<T>();
T item = (Activator.CreateInstance(objType) as T);
objList.Add(item);
return objList;
}
This also needs a where restriction, because in order to cast to a type, that type must be a reference type (aka class).
Two problems in one here ...
I have a set of DataRow wrappers (in VS2008) that inherit from a base class (called RecordBase). They all have a field called TableName. I wanted to make a generic enumerator that is an extension method to a DataSet. The specific TableName would select which table in the DataSet to enumerate. I'd like to write
public static IEnumerable<T> GetRecords<T>(this DataSet MySet) where T : RecordBase
{
foreach (DataRow row in MySet.Tables[T.TableName].Rows)
{
yield return new T(row);
}
}
Problem 1: I can’t find a way to have an overrideable static field, forcing me to create a dummy instance of the wrapper just to get the TableName.
Problem 2: Less serious, even though the wrappers (and the base) have a constructor that accepts a DataRow the compiler still insists that I use the parameterless constructor constraint.
All of which leaves me with code looking like
public static IEnumerable<T> GetRecords<T>(this DataSet MySet) where T : RecordBase, new()
{
string TableName = (new T()).TableName;
foreach (DataRow row in MySet.Tables[TableName].Rows)
{
T record = new T();
record.RowData = row;
yield return record;
}
}
Any ideas?
You can use an custom attribute for the table name and Activator to instantiate the type:
[Table("Customers")]
class Customer : RecordBase { }
//...
public static IEnumerable<T> GetRecords<T>(this DataSet MySet) where T : RecordBase
{
var attribT = typeof(TableAttribute);
var attrib = (TableAttribute) typeof(T).GetCustomAttributes(attribT,false)[0];
foreach (DataRow row in MySet.Tables[attrib.TableName].Rows)
{
yield return (T) Activator.CreateInstance(typeof(T),new[]{row});
}
}
I am trying to create a method that takes a DataTable or a DataRowCollection and converts it to an array of a generic type. Something like this:
public static T[] ConvertToArray<T>(DataTable dataTable)
{
List<T> result = new List<T>();
foreach (DataRow dataRow in dataTable.Rows)
result.Add((T)dataRow);
return result.ToArray();
}
The problem is this line
result.Add((T)dataRow);
which gives Cannot convert System.Data.DataRow to T.
If I do the same thing without using a generic type, and make sure the class of the objects have a defined custom conversion operator, the code works fine.
So the question is now, how do I pull this of using generics?
You could use an object that provides the conversion on a DataRow to your type :
public interface IDataRowConverter<T>
{
T Convert(DataRow row);
}
Provide your custom converter to your function :
public static T[] ConvertToArray<T>(DataTable dataTable, IDataRowConverter<T> converter)
{
List<T> result = new List<T>();
foreach (DataRow dataRow in dataTable.Rows)
result.Add(converter.Convert(dataRow));
return result.ToArray();
}
Then, implement the interface for your needed type :
public class MyObjectDataRowConverter : IDataRowConverter<MyObject>
{
public MyObject Convert(DataRow row)
{
MyObject myObject = new MyObject();
// Initialize object using the row instance
return myObject;
}
}
You can then call your function using this code :
MyObject[] objectArray =
ConvertToArray<MyObject>(datatable, new MyObjectDataRowConverter());
Couple options I found:
result = dataTable.Select()
or
var result = new System.Data.DataRow[dataTable.Rows.Count];
dataTable.Rows.CopyTo(result, 0);
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?
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.
The only constraint you can specify which allows for creation of new instances is new() - basically, a parameterless constructor. To circumvent this do either:
interface ISupportInitializeFromDataRow
{
void InitializeFromDataRow(DataRow dataRow);
}
protected List<T> PopulateCollection<T>(DataTable dt)
where T : BusinessBase, ISupportInitializeFromDataRow, new()
{
List<T> lst = new List<T>();
foreach (DataRow dr in dt.Rows)
{
T t = new T();
t.InitializeFromDataRow(dr);
lst.Add(t);
}
return lst;
}
Or
protected List<T> PopulateCollection<T>(DataTable dt, Func<DataRow, T> builder)
where T : BusinessBase
{
List<T> lst = new List<T>();
foreach (DataRow dr in dt.Rows)
{
T t = builder(dr);
lst.Add(t);
}
return lst;
}
You probably need to add the new generic constraint on T, as follows:
protected List<T> PopulateCollection<T>(DataTable dt) where T : BusinessBase, new()
...
I can't pass a DataRow into the constructor, but you can solve that by assigning it to a property of BusinessBase
A possible way is:
protected List<T> PopulateCollection<T>(DataTable dt) where T: BusinessBase, new()
{
List<T> lst = new List<T>();
foreach (DataRow dr in dt.Rows)
{
T t = new T();
t.DataRow = dr;
lst.Add(t);
}
return lst;
}
where T: BusinessBase
Should have have restriction of new() I think added
It is possible. I have exactly the same thing in my framework. I had exactly the same problem as you and this is how I solved it. Posting relevant snippets from the framework. If I remember correclty, the biggest problem was requirement to call parameterless constructor.
public class Book<APClass> : Book where APClass : APBase
private DataTable Table ; // data
public override IEnumerator GetEnumerator()
{
for (position = 0; position < Table.Rows.Count; position++)
yield return APBase.NewFromRow<APClass>(Table.Rows[position], this.IsOffline);
}
...
public class APBase ...
{
...
internal static T NewFromRow<T>(DataRow dr, bool offline) where T : APBase
{
Type t = typeof(T);
ConstructorInfo ci;
if (!ciDict.ContainsKey(t))
{
ci = t.GetConstructor(new Type[1] { typeof(DataRow) });
ciDict.Add(t, ci);
}
else ci = ciDict[t];
T result = (T)ci.Invoke(new Object[] { dr });
if (offline)
result.drCache = dr;
return result;
}
In this scenario, base class has static method to instantiate objects of its derived classes using constructor that accepts tablerow.
I have the following extension method, and would like to make it more generic so I don't have to implement it for every class in our domain.
public static IList<User> ToList(this DataTable table)
{
IList<User> users = new List<User>();
foreach (DataRow row in table.Rows)
users.Add(User.FromDataRow(row));
return users;
}
Is there any way to work around this frustrating limitation?
edit: the below paragraph is bollocks, but I'm keeping it so one of the answers makes sense to future readers:
User, as well as my other classes, implements IDataModel. IDataModel only requires 1 method, FromDataRow(DataRow row). Putting a where into the function prototype obviously doesn't help.
When you only need one method, think Func... perhaps a Func<DataRow, T>
public static IList<T> ToList<T>(this DataTable table,
Func<DataRow,T> converter)
{
IList<T> list = new List<T>();
foreach (DataRow row in table.Rows)
list.Add(converter(row));
return list;
}
Then call table.ToList<User>(User.FromDataRow)
In your example code, you're using a static method to create the user from the DataRow:
foreach (DataRow row in table.Rows)
users.Add(User.FromDataRow(row));
But, you can't use static methods to implement an interface.
Assuming that your interface looks like this:
public interface IDataModel {
void FromDataRow(DataRow row);
}
then your User class will have an instance method FromDataRow(), not a static one.
If your classes have parameterless constructors, then you could write this:
public static IList<T> ToList<T>(this DataTable table)
where T : IDataModel, new()
{
IList<T> results = new List<T>();
foreach (DataRow row in table.Rows)
{
T item = new T();
item.FromDataRow(row);
results.Add(item);
}
return users;
}
The IDataModel constraint on <T> requires the type to implement IDataModel.
The new() constraint on <T> requires the type to have a parameterless constructor.