Add Unsafe.Unbox (#31133)

src/System.Runtime.CompilerServices.Unsafe/ref/System.Runtime.CompilerServices.Unsafe.cs

@@ -40,6 +40,7 @@ public unsafe static void InitBlockUnaligned(void* startAddress, byte value, uin
         public static ref T Subtract<T>(ref T source, int elementOffset) { throw null; }
         public unsafe static void* Subtract<T>(void* source, int elementOffset) { throw null; }
         public static ref T Subtract<T>(ref T source, System.IntPtr elementOffset) { throw null; }
+        public static ref T Unbox<T>(object box) where T : struct { throw null; }
         public unsafe static void Write<T>(void* destination, T value) { }
         public unsafe static void WriteUnaligned<T>(void* destination, T value) { }
         public static void WriteUnaligned<T>(ref byte destination, T value) { }

src/System.Runtime.CompilerServices.Unsafe/src/System.Runtime.CompilerServices.Unsafe.il

@@ -303,6 +303,15 @@
         ret
   } // end of method Unsafe::As
 
+  .method public hidebysig static !!T& Unbox<valuetype .ctor ([CORE_ASSEMBLY]System.ValueType) T> (object 'box') cil managed aggressiveinlining
+  {
+        .custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = ( 01 00 00 00 )
+        .maxstack 1
+        ldarg.0
+        unbox !!T
+        ret
+  } // end of method Unsafe::Unbox
+
   .method public hidebysig static !!T& Add<T>(!!T& source, int32 elementOffset) cil managed aggressiveinlining
   {
         .custom instance void System.Runtime.Versioning.NonVersionableAttribute::.ctor() = ( 01 00 00 00 )

src/System.Runtime.CompilerServices.Unsafe/tests/UnsafeTests.cs

@@ -793,6 +793,42 @@ public static unsafe void WriteUnaligned_Ptr_Struct()
             Assert.Equal(123456789, actual.Int32);
             Assert.Equal(3.42, actual.Double);
         }
+
+        [Fact]
+        public static void Unbox_Int32()
+        {
+            object box = 42;
+
+            Assert.True(Unsafe.AreSame(ref Unsafe.Unbox<int>(box), ref Unsafe.Unbox<int>(box)));
+
+            Assert.Equal(42, (int)box);
+            Assert.Equal(42, Unsafe.Unbox<int>(box));
+
+            ref int value = ref Unsafe.Unbox<int>(box);
+            value = 84;
+            Assert.Equal(84, (int)box);
+            Assert.Equal(84, Unsafe.Unbox<int>(box));
+
+            Assert.Throws<InvalidCastException>(() => Unsafe.Unbox<Byte4>(box));
+        }
+
+        [Fact]
+        public static void Unbox_CustomValueType()
+        {
+            object box = new Int32Double();
+
+            Assert.Equal(0, ((Int32Double)box).Double);
+            Assert.Equal(0, ((Int32Double)box).Int32);
+
+            ref Int32Double value = ref Unsafe.Unbox<Int32Double>(box);
+            value.Double = 42;
+            value.Int32 = 84;
+
+            Assert.Equal(42, ((Int32Double)box).Double);
+            Assert.Equal(84, ((Int32Double)box).Int32);
+
+            Assert.Throws<InvalidCastException>(() => Unsafe.Unbox<bool>(box));
+        }
     }
 
     [StructLayout(LayoutKind.Explicit)]