Compiler Error CS0039

无法通过引用转换、装箱转换、取消装箱转换、包装转换或 Null 类型转换将类型“type1”转换为“type2”

继承、引用转换和装箱转换允许使用 as（C# 参考）运算符的转换。有关更多信息，请参见转换运算符（C# 编程指南）。

下面的示例生成 CS0039。

// CS0039.cs
using System;
class A
{
}
class B: A
{
}
class C: A
{
}
class M
{
    static void Main()
    {
        A a = new C();
        B b = new B();
        C c;

        // This is valid; there is a built-in reference
        // conversion from A to C.
        c = a as C;  

        //The following generates CS0039; there is no
        // built-in reference conversion from B to C.
        c = b as C;  // CS0039
    }
}