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[英]Create instance of a generic type that takes a parameter a secondary generic type
[英]Create instance of generic type whose constructor requires a parameter?
如果BaseFruit
有一個接受int weight
的構造函數,我可以用這樣的通用方法實例化一塊水果嗎?
public void AddFruit<T>()where T: BaseFruit{
BaseFruit fruit = new T(weight); /*new Apple(150);*/
fruit.Enlist(fruitManager);
}
在評論后面添加了一個示例。 似乎只有給BaseFruit
一個無參數的構造函數,然后通過成員變量填充所有內容,我才能做到這一點。 在我的真實代碼(不是關於水果)中,這是相當不切實際的。
-更新-
所以它似乎不能以任何方式通過約束來解決。 從答案中可以看出三個候選解決方案:
我傾向於認為反射是最不干凈的一種,但我無法在其他兩種之間做出決定。
另外一個更簡單的例子:
return (T)Activator.CreateInstance(typeof(T), new object[] { weight });
請注意,在 T 上使用 new() 約束只是為了讓編譯器在編譯時檢查公共無參數構造函數,用於創建類型的實際代碼是 Activator 類。
您需要確保自己了解現有的特定構造函數,並且這種要求可能是代碼異味(或者更確切地說,您應該在當前的 c# 版本中盡量避免這種情況)。
您不能使用任何參數化構造函數。 如果您有“ where T : new()
”約束,則可以使用無參數構造函數。
這是一種痛苦,但這就是生活:(
這是我想用“靜態接口”解決的問題之一。 然后您就可以約束 T 以包含靜態方法、運算符和構造函數,然后調用它們。
是的; 改變你的位置:
where T:BaseFruit, new()
但是,這只適用於無參數構造函數。 你必須有一些其他的方式來設置你的屬性(設置屬性本身或類似的東西)。
最簡單的解決方案Activator.CreateInstance<T>()
正如 Jon 指出的那樣,這是約束非無參數構造函數的生命。 然而,另一種解決方案是使用工廠模式。 這很容易限制
interface IFruitFactory<T> where T : BaseFruit {
T Create(int weight);
}
public void AddFruit<T>( IFruitFactory<T> factory ) where T: BaseFruit {
BaseFruit fruit = factory.Create(weight); /*new Apple(150);*/
fruit.Enlist(fruitManager);
}
另一種選擇是使用函數式方法。 傳入一個工廠方法。
public void AddFruit<T>(Func<int,T> factoryDel) where T : BaseFruit {
BaseFruit fruit = factoryDel(weight); /* new Apple(150); */
fruit.Enlist(fruitManager);
}
您可以使用反射來完成:
public void AddFruit<T>()where T: BaseFruit
{
ConstructorInfo constructor = typeof(T).GetConstructor(new Type[] { typeof(int) });
if (constructor == null)
{
throw new InvalidOperationException("Type " + typeof(T).Name + " does not contain an appropriate constructor");
}
BaseFruit fruit = constructor.Invoke(new object[] { (int)150 }) as BaseFruit;
fruit.Enlist(fruitManager);
}
編輯:添加了構造函數 == null 檢查。
編輯:使用緩存的更快變體:
public void AddFruit<T>()where T: BaseFruit
{
var constructor = FruitCompany<T>.constructor;
if (constructor == null)
{
throw new InvalidOperationException("Type " + typeof(T).Name + " does not contain an appropriate constructor");
}
var fruit = constructor.Invoke(new object[] { (int)150 }) as BaseFruit;
fruit.Enlist(fruitManager);
}
private static class FruitCompany<T>
{
public static readonly ConstructorInfo constructor = typeof(T).GetConstructor(new Type[] { typeof(int) });
}
作為 user1471935 建議的補充:
要使用帶有一個或多個參數的構造函數實例化泛型類,您現在可以使用 Activator 類。
T instance = Activator.CreateInstance(typeof(T), new object[] {...})
對象列表是您要提供的參數。 根據微軟的說法:
CreateInstance [...] 使用與指定參數最匹配的構造函數創建指定類型的實例。
還有一個通用版本的 CreateInstance ( CreateInstance<T>()
),但該版本也不允許您提供構造函數參數。
我創建了這個方法:
public static V ConvertParentObjToChildObj<T,V> (T obj) where V : new()
{
Type typeT = typeof(T);
PropertyInfo[] propertiesT = typeT.GetProperties();
V newV = new V();
foreach (var propT in propertiesT)
{
var nomePropT = propT.Name;
var valuePropT = propT.GetValue(obj, null);
Type typeV = typeof(V);
PropertyInfo[] propertiesV = typeV.GetProperties();
foreach (var propV in propertiesV)
{
var nomePropV = propV.Name;
if(nomePropT == nomePropV)
{
propV.SetValue(newV, valuePropT);
break;
}
}
}
return newV;
}
我以這種方式使用它:
public class A
{
public int PROP1 {get; set;}
}
public class B : A
{
public int PROP2 {get; set;}
}
代碼:
A instanceA = new A();
instanceA.PROP1 = 1;
B instanceB = new B();
instanceB = ConvertParentObjToChildObj<A,B>(instanceA);
您可以使用以下命令:
T instance = (T)typeof(T).GetConstructor(new Type[0]).Invoke(new object[0]);
請務必查看以下參考資料。
最近我遇到了一個非常相似的問題。 只是想與大家分享我們的解決方案。 我想從一個具有枚舉的 json 對象創建一個Car<CarA>
的實例:
Dictionary<MyEnum, Type> mapper = new Dictionary<MyEnum, Type>();
mapper.Add(1, typeof(CarA));
mapper.Add(2, typeof(BarB));
public class Car<T> where T : class
{
public T Detail { get; set; }
public Car(T data)
{
Detail = data;
}
}
public class CarA
{
public int PropA { get; set; }
public CarA(){}
}
public class CarB
{
public int PropB { get; set; }
public CarB(){}
}
var jsonObj = {"Type":"1","PropA":"10"}
MyEnum t = GetTypeOfCar(jsonObj);
Type objectT = mapper[t]
Type genericType = typeof(Car<>);
Type carTypeWithGenerics = genericType.MakeGenericType(objectT);
Activator.CreateInstance(carTypeWithGenerics , new Object[] { JsonConvert.DeserializeObject(jsonObj, objectT) });
如果您願意使用 ac# 預編譯器,您可以解決這個問題,以便它確實有編譯時間限制:
// Used attribute
[AttributeUsage(AttributeTargets.Parameter)]
class ResolvedAsAttribute : Attribute
{
public string Expression;
public ResolvedAsAttribute(string expression)
{
this.Expression = expression;
}
}
// Fruit manager source:
class FruitManager {
...
public void AddFruit<TFruit>([ResolvedAs("(int p) => new TFruit(p)")] Func<int,TFruit> ctor = null)where TFruit: BaseFruit{
BaseFruit fruit = ctor(weight); /*new Apple(150);*/
fruit.Enlist(fruitManager);
}
}
// Fruit user source:
#ResolveInclude ../Managers/FruitManager.cs
...
fruitManager.AddFruit<Apple>();
...
然后,您的預編譯器會將 Fruit 用戶源代碼轉換為:
...
fruitManager.AddFruit<Apple>((int p) => new Apple(p));
...
使用 Roslyn,你的預編譯器可能看起來像這樣(這里有改進的空間):
using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using Microsoft.CodeAnalysis;
using Microsoft.CodeAnalysis.CSharp;
using Microsoft.CodeAnalysis.CSharp.Syntax;
using Microsoft.CodeAnalysis.CSharp.Symbols;
using System.Threading;
using System.Text.RegularExpressions;
public class CsResolveIncludeAnalyser : CSharpSyntaxWalker
{
private List<(string key, MethodDeclarationSyntax node)> methodsToResolve = new List<(string key, MethodDeclarationSyntax node)>();
public List<(string key, MethodDeclarationSyntax node)> Analyse(string source)
{
var tree = CSharpSyntaxTree.ParseText(source);
var syntaxRoot = tree.GetRoot();
Visit(tree.GetRoot());
return methodsToResolve;
}
public override void VisitMethodDeclaration(MethodDeclarationSyntax methodDeclaration)
{
base.VisitMethodDeclaration(methodDeclaration);
if (methodDeclaration.ParameterList.Parameters.Count > 0)
{
foreach (var parm in methodDeclaration.ParameterList.Parameters)
{
var parmHasResolvedAs = parm.AttributeLists.Where((el) => el.Attributes.Where((attr) => attr.Name is IdentifierNameSyntax && ((IdentifierNameSyntax)attr.Name).Identifier.Text.Contains("ResolvedAs")).Any()).Any();
if (parmHasResolvedAs)
{
var name = methodDeclaration.Identifier.ValueText;
methodsToResolve.Add((name, methodDeclaration));
return;
}
}
}
}
}
public class CsSwiftRewriter : CSharpSyntaxRewriter
{
private string currentFileName;
private bool withWin32ErrorHandling;
private Dictionary<string,MethodDeclarationSyntax> methodsToResolve = new Dictionary<string, MethodDeclarationSyntax>();
private Dictionary<string, MethodDeclarationSyntax> getMethodsToResolve(string source, string fileName)
{
Dictionary<string, MethodDeclarationSyntax> methodsToResolve = new Dictionary<string, MethodDeclarationSyntax>();
var path = Path.GetDirectoryName(fileName);
var lines = source.Split(new[] { '\r', '\n' });
var resolveIncludes = (from el in lines where el.StartsWith("#ResolveInclude") select el.Substring("#ResolveInclude".Length).Trim()).ToList();
var analyser = new CsResolveIncludeAnalyser();
foreach (var resolveInclude in resolveIncludes)
{
var src = File.ReadAllText(path + "/" + resolveInclude);
var list = analyser.Analyse(src);
foreach (var el in list)
{
methodsToResolve.Add(el.key, el.node);
}
}
return methodsToResolve;
}
public static string Convert(string source, string fileName)
{
return Convert(source, fileName, false);
}
public static string Convert(string source, string fileName, bool isWithWin32ErrorHandling)
{
var rewriter = new CsSwiftRewriter() { currentFileName = fileName, withWin32ErrorHandling = isWithWin32ErrorHandling };
rewriter.methodsToResolve = rewriter.getMethodsToResolve(source, fileName);
var resolveIncludeRegex = new Regex(@"(\#ResolveInclude)\b");
source = resolveIncludeRegex.Replace(source, "//$1");
var tree = CSharpSyntaxTree.ParseText(source);
var syntaxRoot = tree.GetRoot();
var result = rewriter.Visit(tree.GetRoot());
return "#line 1 \"" + Path.GetFileName(fileName) + "\"\r\n" + result.ToFullString();
}
internal List<string> transformGenericArguments(List<string> arguments, GenericNameSyntax gName, TypeParameterListSyntax typeParameterList)
{
var res = new List<string>();
var typeParameters = typeParameterList.ChildNodes().ToList();
foreach (var argument in arguments)
{
var arg = argument;
for (int i = 0; i < gName.TypeArgumentList.Arguments.Count; i++)
{
var key = typeParameters[i];
var replacement = gName.TypeArgumentList.Arguments[i].ToString();
var regex = new System.Text.RegularExpressions.Regex($@"\b{key}\b");
arg = regex.Replace(arg, replacement);
}
res.Add(arg);
}
return res;
}
const string prefix = "";
internal List<string> extractExtraArguments(MethodDeclarationSyntax methodDeclaration)
{
var res = new List<String>();
foreach (var parm in methodDeclaration.ParameterList.Parameters)
{
foreach (var attrList in parm.AttributeLists)
{
foreach (var attr in attrList.Attributes)
{
if (attr.Name is IdentifierNameSyntax && string.Compare(((IdentifierNameSyntax)attr.Name).Identifier.Text, "ResolvedAs") == 0)
{
var programmCode = attr.ArgumentList.Arguments.First().ToString().Trim();
var trimmedProgrammCode = (programmCode.Length >= 2 && programmCode[0] == '"' && programmCode[programmCode.Length - 1] == '"') ? programmCode.Substring(1, programmCode.Length - 2) : programmCode;
res.Add(prefix + parm.Identifier.Text + ":" + trimmedProgrammCode);
}
}
}
}
return res;
}
internal List<string> extractExtraArguments(MethodDeclarationSyntax methodDeclaration, SimpleNameSyntax name)
{
var arguments = extractExtraArguments(methodDeclaration);
if (name != null && name is GenericNameSyntax)
{
var gName = name as GenericNameSyntax;
return transformGenericArguments(arguments, gName, methodDeclaration.TypeParameterList);
}
return arguments;
}
public override SyntaxNode VisitInvocationExpression(InvocationExpressionSyntax c_expressionStatement)
{
InvocationExpressionSyntax expressionStatement = (InvocationExpressionSyntax) base.VisitInvocationExpression(c_expressionStatement);
List<string> addedArguments = null;
switch (expressionStatement.Expression)
{
case MemberAccessExpressionSyntax exp:
if (methodsToResolve.ContainsKey(exp.Name?.Identifier.ValueText))
{
addedArguments = extractExtraArguments(methodsToResolve[exp.Name.Identifier.ValueText], exp.Name);
}
break;
case GenericNameSyntax gName:
if (methodsToResolve.ContainsKey(gName.Identifier.ValueText))
{
addedArguments = extractExtraArguments(methodsToResolve[gName.Identifier.ValueText], gName);
}
break;
default:
var name = (from el in expressionStatement.ChildNodes()
where el is GenericNameSyntax
select (el as GenericNameSyntax)).FirstOrDefault();
if (name != default(GenericNameSyntax))
{
if (methodsToResolve.ContainsKey(name.Identifier.ValueText))
{
addedArguments = extractExtraArguments(methodsToResolve[name.Identifier.ValueText], name);
}
}
break;
}
if (addedArguments?.Count > 0)
{
var addedArgumentsString = string.Join(",", addedArguments);
var args = expressionStatement.ArgumentList.ToFullString();
var paras = $"({(expressionStatement.ArgumentList.Arguments.Count > 0 ? string.Join(",", args.Substring(1,args.Length - 2), addedArgumentsString) : addedArgumentsString)})" ;
var argList = SyntaxFactory.ParseArgumentList(paras);
return expressionStatement.WithArgumentList(argList);
}
return expressionStatement;
}
}
可以使用 T4 腳本調用預編譯器,可選擇在編譯時重新生成源代碼。
通過執行以下操作,仍然可以實現高性能:
//
public List<R> GetAllItems<R>() where R : IBaseRO, new() {
var list = new List<R>();
using ( var wl = new ReaderLock<T>( this ) ) {
foreach ( var bo in this.items ) {
T t = bo.Value.Data as T;
R r = new R();
r.Initialize( t );
list.Add( r );
}
}
return list;
}
和
//
///<summary>Base class for read-only objects</summary>
public partial interface IBaseRO {
void Initialize( IDTO dto );
void Initialize( object value );
}
然后相關的類必須從這個接口派生並相應地初始化。 請注意,就我而言,此代碼是周圍類的一部分,該類已經將 <T> 作為泛型參數。 R,就我而言,也是一個只讀類。 IMO,Initialize() 函數的公開可用性對不變性沒有負面影響。 此類的用戶可以放入另一個對象,但這不會修改底層集合。
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