The C# compiler has a /langversion switch that is rarely used; most people do not know what it is for, and if asked to guess, most guess wrong. So let me disabuse you of your wrong guess right away: the purpose of the langversion flag is not to decide what C# language version to use. It is not a "go into compatibility mode" switch or a "use a previous version of the compiler" switch. The only effect of the langversion switch is to put the compiler in a mode in which use of features from a version of the language higher than the given version cause the compiler to emit an error.
Let me illustrate with a hilariously contrived example:
class C
{
public static bool operator < (C c1, C c2) { return true; }
public static bool operator > (C c1, C c2) { return true; }
public static bool operator < (bool b1, C c2) { return true; }
public static bool operator > (bool b1, C c2) { return true; }
static C H = new C();
static C I = new C();
static C J = new C();
static void G(bool b) { }
static void Main()
{
G ( H < I > ( J ) );
}
}
This ridiculous program compiles successfully in C# 1.0; it invokes the < operator on H and I, and then invokes the > operator on the resulting bool and J.
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