这个问题在这里已经有了答案:
Type of conditional expression cannot be determined because there is no implicit conversion between 'string' and 'System.DBNull'
(1 个回答)
6年前关闭。
...毕竟,这实际上可能不是差异问题。当我编译我的代码时,Visual Studio 会给我以下错误:
Type of conditional expression cannot be determined because there is no implicit conversion between 'ClassA' and 'ClassB'
我在这里阅读了这个错误,听起来确实不可能将抽象类用作接口(interface)之类的契约,我可以在其中使用派生类代替它们的基类。为了使事情更简单,我编写了一些模拟实际代码关系的测试类。请考虑以下几点:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace AbstractClassImplicitConversion {
class Program {
static void Main(string[] args) {
StartClass temp = new StartClass();
}
}
public class StartClass
{
public AbstractClass _ac { get; private set; }
public StartClass()
{
bool which = true;
// this block will compile
/*
if( which)
_ac = new DerivedClass1();
else
_ac = new DerivedClass2();
*/
// this block will not compile
_ac = which ? new DerivedClass1() : new DerivedClass2();
}
}
public interface SomeInterface
{
void SiFoo();
}
public abstract class InnerAbstractClass
{
public abstract void IacFoo();
}
public abstract class AbstractClass : InnerAbstractClass, SomeInterface
{
public override void IacFoo() {
}
public void SiFoo() {
}
public abstract void OverrideMe();
public abstract void OverrideMeToo<T>();
}
public class DerivedClass1 : AbstractClass
{
public override void OverrideMe() {}
public override void OverrideMeToo<T>() {}
}
public class DerivedClass2 : AbstractClass
{
private int _blah;
public override void OverrideMe() {}
public override void OverrideMeToo<T>() {}
}
}
问题出在这条线上:
_ac = which ? new DerivedClass1() : new DerivedClass2();
如果我使用内联 if,我会得到那个可怕的编译器错误。但是,如果我改用这个:
if( which)
_ac = new DerivedClass1();
else
_ac = new DerivedClass2();
我的代码编译得很好。
谁能解释这是什么原因?我一直认为两者是等价的。谢谢!
最佳答案
这正是语言标准的定义方式:
Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other.
DerivedClass1
之间没有隐式转换和 DerivedClass2
- 它仅通过存在的基类。运算符的规范并没有说它会查看赋值的结果值类型,因此编译器正在执行它的设计目标。来源:https://msdn.microsoft.com/en-us/library/ty67wk28.aspx
关于c# - 寻找有关差异问题的解释,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/32404758/