[英]C# Casting and Generics
我一直在努力處理一段C#代碼,盡管我找到了解決問題的方法,但這絕不是理想的方法(請參見下面的DoSomething_WorksButNotIdeal())。
我想做的是,而不是使用if,else語句(根據我要支持的類型,它可能會很大)只是具有通用類型轉換,但我無法使其正常工作。 我試圖在DoSomething_HelpMe()方法中對此進行演示。
無論如何要實現這一目標? 任何幫助是極大的贊賞。
public interface ITag
{
string TagName { get; }
Type Type { get; }
}
public interface ITag<T> : ITag
{
T InMemValue { get; set; }
T OnDiscValue { get; set; }
}
public class Tag<T> : ITag<T>
{
public Tag(string tagName)
{
TagName = tagName;
}
public string TagName { get; private set; }
public T InMemValue { get; set; }
public T OnDiscValue { get; set; }
public Type Type{ get{ return typeof(T);} }
}
public class MusicTrack
{
public MusicTrack()
{
TrackTitle = new Tag<string>("TrackTitle");
TrackNumber = new Tag<int>("TrackNumber");
Tags = new Dictionary<string, ITag>();
Tags.Add(TrackTitle.TagName, TrackTitle);
Tags.Add(TrackNumber.TagName, TrackNumber);
}
public IDictionary<string,ITag> Tags;
public ITag<string> TrackTitle { get; set; }
public ITag<int> TrackNumber { get; set; }
}
public static class Main
{
public static void DoSomething_WorksButNotIdeal()
{
MusicTrack track1 = new MusicTrack();
MusicTrack track2 = new MusicTrack();
// Set some values on the tracks
foreach (ITag tag in track1.Tags.Values)
{
Type type = tag.Type;
if (type == typeof(string))
{
((ITag<string>) tag).InMemValue = ((ITag<string>)track2.Tags[tag.TagName]).OnDiscValue;
}
else if (type == typeof(int))
{
((ITag<int>)tag).InMemValue = ((ITag<int>)track2.Tags[tag.TagName]).OnDiscValue;
}
else if (type == typeof(bool))
{
((ITag<bool>)tag).InMemValue = ((ITag<bool>)track2.Tags[tag.TagName]).OnDiscValue;
}
// etc etc
else
{
throw new Exception("Unsupported type.");
}
}
}
public static void DoSomething_HelpMe()
{
MusicTrack track1 = new MusicTrack();
MusicTrack track2 = new MusicTrack();
// Set some values on the tracks
foreach (ITag tag in track1.Tags.Values)
{
Type type = tag.Type;
// THIS OBVIOUSLY DOESN'T WORK BUT I'M JUST TRYING TO DEMONSTRATE WHAT
// I'D IDEALLY LIKE TO ACHIEVE
((ITag<typeof(type)>)tag).InMemValue = ((ITag<typeof(type)>)track2.Tags[tag.TagName]).OnDiscValue;
}
}
}
您無法擁有的任何原因:
public interface ITag
{
string TagName { get; }
Type Type { get; }
object InMemValue { get; set; }
object OnDiscValue { get; set; }
}
並使用ITag<T>
使其更具體?
public interface ITag<T> : ITag
{
new T InMemValue { get; set; }
new T OnDiscValue { get; set; }
}
然后,您的方法可以只使用ITag
。 您需要類似(int Tag<T>
)的東西:
object ITag.InMemValue
{
get { return InMemValue; }
set { InMemValue = (T)value; }
}
object ITag.OnDiscValue
{
get { return OnDiscValue; }
set { OnDiscValue = (T)value; }
}
(編輯)
另一個選擇是在非通用ITag
上的方法:
void CopyValueFrom(ITag tag);
(關於它復制到/從中復制的內容可能更具體)
您的具體實現( Tag<T>
)必須假定ITag
實際上是ITag<T>
並進行ITag<T>
:
public void CopyFromTag(ITag tag) {
ITag<T> from = tag as ITag<T>;
if(from==null) throw new ArgumentException("tag");
this.TheFirstProperty = from.TheSecondProperty;
}
解決該問題的最簡單方法是解析您擁有信息的類型,即在Tag<T>
實現內部,因此將以下內容添加到現有類型中(僅顯示添加內容!)
public interface ITag
{
void CopyFrom(bool sourceIsMem, ITag sourceTag, bool targetIsMem);
}
public class Tag<T> : ITag<T>
{
public void CopyFrom(bool sourceIsMem, ITag sourceTag, bool targetIsMem)
{
ITag<T> castSource = sourceTag as ITag<T>;
if (castSource == null)
throw new ArgumentException(
"Source tag is of an incompatible type", "sourceTag");
if (targetIsMem)
InMemValue = sourceIsMem ?
castSource.InMemValue : castSource.OnDiscValue;
else
OnDiscValue = sourceIsMem ?
castSource.InMemValue : castSource.OnDiscValue;
}
}
請注意,實際上您應該對sourceIsMem
和targetIsMem
使用enum
類型,因為在調用中bool
確實很丑陋且很難閱讀,如以下片段所示。
這是您現在使例行工作的方式:
public static void DoSomething_HelpMe()
{
MusicTrack track1 = new MusicTrack();
MusicTrack track2 = new MusicTrack();
// Set some values on the tracks
foreach (ITag tag in track1.Tags.Values)
tag.CopyFrom(false, track2.Tags[tag.TagName], true);
}
這是一種方法,它需要大量樣板文件,但允許您使用ITag
, ITag<T>
和Tag<T>
現有定義來ITag
ITag<T>
。 TagSetter
類以任何類型的ITag<T>
的類型安全方式從磁盤上的值設置內存中的值。
/// <summary>
/// Allows a tag of any type to be used to get a result of type TResult
/// </summary>
/// <typeparam name="TResult">The result type after using the tag</typeparam>
public interface ITagUser<TResult>
{
TResult Use<T>(ITag<T> tag);
}
/// <summary>
/// Allows a tag of any type to be used (with no return value)
/// </summary>
public interface ITagUser
{
void Use<T>(ITag<T> tag);
}
/// <summary>
/// Wraps a tag of some unknown type. Allows tag users (either with or without return values) to use the wrapped list.
/// </summary>
public interface IExistsTag
{
TResult Apply<TResult>(ITagUser<TResult> user);
void Apply(ITagUser user);
}
/// <summary>
/// Wraps a tag of type T, hiding the type itself.
/// </summary>
/// <typeparam name="T">The type of element contained in the tag</typeparam>
class ExistsTag<T> : IExistsTag
{
ITag<T> tag;
public ExistsTag(ITag<T> tag)
{
this.tag = tag;
}
#region IExistsTag Members
public TResult Apply<TResult>(ITagUser<TResult> user)
{
return user.Use(tag);
}
public void Apply(ITagUser user)
{
user.Use(tag);
}
#endregion
}
public interface ITag
{
string TagName { get; }
Type Type { get; }
}
public interface ITag<T> : ITag
{
T InMemValue { get; set; }
T OnDiscValue { get; set; }
}
public class Tag<T> : ITag<T>
{
public Tag(string tagName)
{
TagName = tagName;
}
public string TagName { get; private set; }
public T InMemValue { get; set; }
public T OnDiscValue { get; set; }
public Type Type { get { return typeof(T); } }
}
public class TagSetter : ITagUser
{
#region ITagUser Members
public void Use<T>(ITag<T> tag)
{
tag.InMemValue = tag.OnDiscValue;
}
#endregion
}
public class TagExtractor : ITagUser<ITag>
{
#region ITagUser<ITag> Members
public ITag Use<T>(ITag<T> tag)
{
return tag;
}
#endregion
}
public class MusicTrack
{
public MusicTrack()
{
TrackTitle = new Tag<string>("TrackTitle");
TrackNumber = new Tag<int>("TrackNumber");
Tags = new Dictionary<string, IExistsTag>();
Tags.Add(TrackTitle.TagName, new ExistsTag<string>(TrackTitle));
Tags.Add(TrackNumber.TagName, new ExistsTag<int>(TrackNumber));
}
public IDictionary<string, IExistsTag> Tags;
public ITag<string> TrackTitle { get; set; }
public ITag<int> TrackNumber { get; set; }
}
public static class Main
{
public static void DoSomething_WorksButNotIdeal()
{
MusicTrack track1 = new MusicTrack();
MusicTrack track2 = new MusicTrack();
TagSetter setter = new TagSetter();
TagExtractor extractor = new TagExtractor();
// Set some values on the tracks
foreach (IExistsTag tag in track1.Tags.Values)
{
tag.Apply(setter);
// do stuff using base interface if necessary
ITag itag = tag.Apply(extractor);
}
}
}
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