为什么在使用具有 new() 约束的泛型类型调用 new 时，c# 编译器会发出 Activator.CreateInstance? [英] Why does the c# compiler emit Activator.CreateInstance when calling new in with a generic type with a new() constraint?

问题描述

当您有如下代码时:

static T GenericConstruct<T>() where T : new()
{
    return new T();
}

C# 编译器坚持发出对 Activator.CreateInstance 的调用，这比本地构造函数慢得多.

The C# compiler insists on emitting a call to Activator.CreateInstance, which is considerably slower than a native constructor.

我有以下解决方法:

public static class ParameterlessConstructor<T>
    where T : new()
{
    public static T Create()
    {
        return _func();
    }

    private static Func<T> CreateFunc()
    {
        return Expression.Lambda<Func<T>>( Expression.New( typeof( T ) ) ).Compile();
    }

    private static Func<T> _func = CreateFunc();
}

// Example:
// Foo foo = ParameterlessConstructor<Foo>.Create();

但我不明白为什么需要这种解决方法.

But it doesn't make sense to me why this workaround should be necessary.

推荐答案

我怀疑这是一个 JITting 问题.目前，JIT 为所有引用类型参数重用相同的生成代码 - 因此 List 的 vtable 指向与 List.如果每个 new T() 调用都必须在 JIT 代码中解析，那这将行不通.

I suspect it's a JITting problem. Currently, the JIT reuses the same generated code for all reference type arguments - so a List<string>'s vtable points to the same machine code as that of List<Stream>. That wouldn't work if each new T() call had to be resolved in the JITted code.

只是一个猜测，但它一定有一定的意义.

Just a guess, but it makes a certain amount of sense.

一个有趣的小点:在两种情况下都不会调用值类型的无参数构造函数，如果有的话(这种情况非常罕见).请参阅我最近的博文详情.不知道有没有什么方法可以强制到表达式树中.

One interesting little point: in neither case does the parameterless constructor of a value type get called, if there is one (which is vanishingly rare). See my recent blog post for details. I don't know whether there's any way of forcing it in expression trees.

这篇关于为什么在使用具有 new() 约束的泛型类型调用 new 时，c# 编译器会发出 Activator.CreateInstance?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！