眾所周知,C# 只支援對 基類/介面/class/struct/new() 以及一些 IDE 魔法的約束,比如這樣
public static string Test<T>(T value) where T : ITest
{
return value.Test();
}
public interface ITest
{
string Test();
}
但是如果我們想要隨心所欲的約束就不行了
public static string Test<T>(T value) where T : { string Test(); }
{
return value.Test();
}
最近無聊亂折騰 MSIL,弄出來好多不能跑的魔法,雖然不能跑但是反編譯出的 C# 看著很神奇,其中正好就有想看看能不能弄個神奇的泛型出來,於是我胡寫了一段程式碼
.assembly _
{
}
.class public Test
{
.method public void .ctor()
{
ldarg.0
call instance void object::.ctor()
ret
}
.method public static void Main()
{
.entrypoint
newobj instance void Test::.ctor()
call string Test::Test<class Test>(!!0)
call void [mscorlib]System.Console::WriteLine(string)
ret
}
.method public static string Test<T>(!!T t)
{
ldarg.s t
callvirt instance string !!T::Test()
ret
}
.method public string Test()
{
ldstr "Call instance string Test::Test()"
ret
}
}
反編譯出來是這樣的
public class Test
{
public static void Main()
{
Console.WriteLine(Test(new Test()));
}
public unsafe static string Test<T>(T t)
{
return ((T*)t)->Test();
}
public string Test()
{
return "Call instance string Test::Test()";
}
}
這段程式碼是無法執行的,在 .NET Framework 會直接無返回,而在 Mono 會報錯
[ERROR] FATAL UNHANDLED EXCEPTION: System.MissingMethodException: Method not found: string .T_REF.Test()at Test.Main () [0x00005] in <ddf64a5d94ef4722be4197eb692d9478>:0
於是我就當這是 .NET 泛型的侷限性了,後來有群友提醒我說約束會影響執行時,於是我就嘗試加上約束
.method public static string Test<(Test) T>(!!T t)
{
ldarg.s t
callvirt instance string !!T::Test()
ret
}
發現真的能跑了(Framework 依然無返回。。。),於是我就看看能不能同時約束兩個型別
.method public static string Test<(Test, Test2) T>(!!T t)
{
ldarg.s t
callvirt instance string !!T::Test()
ret
}
Mono 成功輸出
Call instance string Test::Test()
Call instance string Test2::Test()
而 Framework 直接執行時約束了。。。
未經處理的異常: System.Security.VerificationException: 方法 Test.Test: 型別引數“Test”與型別引數“T”的約束衝突。
在 Test.Main()
很明顯 Mono 給泛型開了洞
隨後測試發現,只要約束的類有相關成員就可以正常呼叫,於是我就利用抽象類做介面
.assembly _
{
}
.class public Test
{
.field string Test
.method public void .ctor()
{
ldarg.0
ldstr "This is Test"
stfld string Test::Test
ldarg.0
call instance void object::.ctor()
ret
}
.method public static void Main()
{
.entrypoint
.locals init (
class Test test,
class Test2 test2
)
newobj instance void Test::.ctor()
stloc.s test
ldloc.s test
call void Test::Test<class Test>(!!0)
ldloc.s test
call string Test::Test<class Test>(!!0)
call void [mscorlib]System.Console::WriteLine(string)
newobj instance void Test2::.ctor()
stloc.s test2
ldloc.s test2
call void Test::Test<class Test2>(!!0)
ldloc.s test2
call string Test::Test<class Test2>(!!0)
call void [mscorlib]System.Console::WriteLine(string)
ret
}
.method public static void Test<(WithTest) T>(!!T t)
{
ldarg.s t
ldfld string !!T::Test
call void [mscorlib]System.Console::WriteLine(string)
ret
}
.method public static string Test<(WithTest) T>(!!T t)
{
ldarg.s t
callvirt instance string !!T::Test()
ret
}
.method public string Test()
{
ldstr "Call instance string Test::Test()"
ret
}
}
.class public Test2
{
.field string Test
.method public void .ctor()
{
ldarg.0
ldstr "This is Test2"
stfld string Test2::Test
ldarg.0
call instance void object::.ctor()
ret
}
.method public newslot virtual string Test()
{
ldstr "Call instance string Test2::Test()"
ret
}
}
.class public abstract WithTest
{
.field string Test
.method public newslot abstract virtual string Test()
{
}
}
正確輸出
This is Test
Call instance string Test::Test()
This is Test2
Call instance string Test2::Test()
Framework 自然還是炸的
最後反編譯出來長這樣
public class Test
{
private string m_Test = "This is Test";
public static void Main()
{
Test t = new Test();
global::Test.Test<Test>(t);
Console.WriteLine(global::Test.Test<Test>(t));
Test2 t2 = new Test2();
global::Test.Test<Test2>(t2);
Console.WriteLine(global::Test.Test<Test2>(t2));
}
public unsafe static void Test<T>(T t) where T : WithTest
{
Console.WriteLine(((T*)t)->Test);
}
public unsafe static string Test<T>(T t) where T : WithTest
{
return ((T*)t)->Test();
}
public string Test()
{
return "Call instance string Test::Test()";
}
}
public class Test2
{
private string m_Test = "This is Test2";
public virtual string Test()
{
return "Call instance string Test2::Test()";
}
}
public abstract class WithTest
{
private string m_Test;
public abstract string Test();
}
當然,這種操作僅限娛樂,經測試 .NET Framework 和 .NET Core App 都會卡在約束,所以 .NET 是別想有隨意的約束了,不過 C# 題案 "Roles and extensions" 倒是給出了曲線實現方案