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                # Compiler Error CS0039 無法通過引用轉換、裝箱轉換、取消裝箱轉換、包裝轉換或 Null 類型轉換將類型“type1”轉換為“type2” 繼承、引用轉換和裝箱轉換允許使用 [as(C# 參考)](https://msdn.microsoft.com/zh-cn/library/cscsdfbt.aspx) 運算符的轉換。有關更多信息,請參見[轉換運算符(C# 編程指南)](https://msdn.microsoft.com/zh-cn/library/09479473.aspx)。 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 } } ```
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