Lou Haskett - All Comments

re: How-To: "Cast" between List's

Bin — Sun, 10 Oct 2010 02:03:26 GMT

To David Muir,

Your GetDerivedTypeList can pass the compiler check. But when called, the derived list returns all null properties for the individual list items (i.e. x.PropertyA is null)

re: How-To: "Cast" between List's

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re: How-To: "Cast" between List's

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re: How-To: "Cast" between List's

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re: How-To: "Cast" between List's

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re: How-To: "Cast" between List's

David Muir (david@tutorscotland.com) — Tue, 18 May 2010 16:24:39 GMT

A problem that had me wondering for ages! The problem of generics and interfaces I guess. This is what I came up with as a solution. Hope it helps someone as I'm a bit out of date posting here.

public static IList<TDerived> GetDerivedTypeList<TBase, TDerived>(this IList<TBase> baseList)

where TDerived : class, TBase

where TBase : class

{

return baseList.Where(x => x != null).Select(x => x as TDerived).ToList();

}

re: How-To: "Cast" between List's

Sylvain AURAT — Thu, 11 Mar 2010 14:17:51 GMT

As we can see previously, the better way to resolve this problem is certainely to use the proxy pattern (www.dofactory.com/.../PatternProxy.aspx).

public static class CollectionUtil {

public static IList<TOutput> Cast<TInput, TOutput>(List<TInput> input) where TInput : TOutput {

if (typeof(TInput) == typeof(TOutput)) {

return (IList<TOutput>)input;

} else {

return new CastedListProxy<TInput, TOutput>(input);

}

internal class CastedListProxyEnumerator<TInput, TOutput>: IEnumerator<TOutput> where TInput : TOutput {

#region Fields declarations

private readonly IEnumerator<TInput> _enumerator;

#endregion

#region Constructor declarations

public CastedListProxyEnumerator(CastedListProxy<TInput, TOutput> proxy) {

_enumerator = proxy.Source.GetEnumerator();

}

#endregion

#region Properties declarations

public TOutput Current {

get { return _enumerator.Current; }

}

object IEnumerator.Current {

get { return Current; }

}

#endregion

#region Methods declarations

public bool MoveNext() {

return _enumerator.MoveNext();

}

public void Reset() {

_enumerator.Reset();

}

public void Dispose() {

_enumerator.Dispose();

}

#endregion

}

internal class CastedListProxy<TInput, TOutput> : IList<TOutput> where TInput : TOutput {

#region Constants declarations

private const string _READONLY_EXCEPTION_MESSAGE = "";

#endregion

#region Fields declarations

private readonly IList<TInput> _src;

#endregion

#region Constructors declarations

public CastedListProxy(IList<TInput> source) {

_src = source;

}

#endregion

#region Properties declarations

public IList<TInput> Source {

get { return _src; }

}

public int Count {

get { return _src.Count; }

}

public bool IsReadOnly {

get { return true; }

}

public TOutput this[int index] {

get { return _src[index]; }

set { throw new NotSupportedException(_READONLY_EXCEPTION_MESSAGE); }

}

#endregion

#region Methods declarations

public IEnumerator<TOutput> GetEnumerator() {

return new CastedListProxyEnumerator<TInput, TOutput>(this);

}

IEnumerator IEnumerable.GetEnumerator() {

return GetEnumerator();

}

public bool Contains(TOutput item) {

if (item is TInput) {

return _src.Contains((TInput)item);

} else {

return false;

}

public void CopyTo(TOutput[] array, int arrayIndex) {

foreach (TInput entry in _src) {

array[arrayIndex] = entry;

arrayIndex++;

}

public int IndexOf(TOutput item) {

throw new NotImplementedException();

}

public bool Remove(TOutput item) {