C#通用方法，无法隐式转换

问题描述

我有以下代码:

public static T GetCar<T>() where T : ICar
{
    T objCar = default(T);

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar("");
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar("");
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar("");
    }

    return objCar;
}

这是我得到的错误:Cannot implicitly convert type 'Test.Cars' to 'T'

我在这里想念什么?所有类型的汽车都实现ICar接口.

What Am I missing here? All car types implement the ICar interface.

谢谢

推荐答案

由于T在编译时未知，因此您无法转换为T.如果要使代码正常工作，可以将返回类型更改为ICar并删除通用的T返回类型.

You cannot convert to T because of the fact that T isn't known at compile time.If you want to get your code to work you can change the return type to ICar and remove the generic T return type.

您还可以强制转换为T.这也可以.如果仅使用默认构造函数，则还可以包含new()并使用new T()来使代码正常工作.

You also can cast to T. This would work too.If you only using the default constructor you can also constain on new() and use new T() to get your code to work.

public ICar GetCar<T>()
    where T : ICar
{
    ICar objCar = null;

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar();
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar();
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar();
    }

    return objCar;
}

发布:

public T GetCar<T>()
    where T : ICar
{
    Object objCar = null;

    if (typeof(T) == typeof(SmallCar)) {
        objCar = new SmallCar();
    } else if (typeof(T) == typeof(MediumCar)) {
        objCar = new MediumCar();
    } else if (typeof(T) == typeof(BigCar)) {
        objCar = new BigCar();
    }

    return (T)objCar;
}

新约束:

public T GetCar<T>()
    where T : ICar, new()
{
    return new T();
}

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