如果BaseFruit有一个构造函数接受int权重，我可以实例化一个水果在一个泛型方法像这样?
public void AddFruit<T>()where T: BaseFruit{
BaseFruit fruit = new T(weight); /*new Apple(150);*/
fruit.Enlist(fruitManager);
}
注释后面添加了一个示例。似乎只有给BaseFruit一个无参数的构造函数，然后通过成员变量填充所有内容，才能做到这一点。在我的实际代码中(不是关于水果)，这是相当不切实际的。
- update - 所以它似乎不能用任何约束条件来解决。从答案中可以得出三个备选方案:
工厂模式 反射 激活剂
我倾向于认为反射是最不干净的一种，但我无法在另外两种之间做出决定。
当前回答
你可以使用反射:
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);
}
编辑:增加了构造函数== null检查。
编辑:一个更快的变种使用缓存:
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) });
}
其他回答
另外还有一个简单的例子:
return (T)Activator.CreateInstance(typeof(T), new object[] { weight });
注意，在T上使用new()约束只是为了让编译器在编译时检查公共无参数构造函数，用于创建类型的实际代码是Activator类。
你需要确保自己了解特定的构造函数存在，这种需求可能是一种代码气味(或者更确切地说，在c#的当前版本中应该尽量避免的东西)。
是的,改变你的位置:
where T:BaseFruit, new()
但是，这只适用于无参数构造函数。您必须使用其他方法来设置属性(设置属性本身或类似的东西)。
你不能使用任何参数化构造函数。如果有“where T: new()”约束，则可以使用无参数构造函数。
这很痛苦，但生活就是这样。
这是我想用“静态接口”解决的问题之一。然后可以约束T以包含静态方法、操作符和构造函数，然后调用它们。
如果你愿意使用c#预编译器，你可以解决这个问题，这样它就有编译时间限制:
// 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>();
...
你的预编译器会将Fruit用户源转换为:
...
fruitManager.AddFruit<Apple>((int p) => new Apple(p));
...
使用Roslyn，你的预编译器看起来像这样(这里有改进的空间):
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脚本调用预编译器，在编译时可选地重新生成源。
作为user1471935建议的补充:
要使用带有一个或多个参数的构造函数实例化泛型类，现在可以使用Activator类。
T instance = Activator.CreateInstance(typeof(T), new object[] {...})
对象列表是您想要提供的参数。微软表示:
调用CreateInstance(…]使用最匹配指定参数的构造函数创建指定类型的实例。
还有一个CreateInstance的通用版本(CreateInstance<T>())，但该版本也不允许提供构造函数参数。
