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Compiler Error CS0039

Cannot convert type 'type1' to 'type2' via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion

A conversion with the as (C# Reference) operator is allowed by inheritance, reference conversions, and boxing conversions. For more information, see Conversion Operators (C# Programming Guide).

The following example generates 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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