Simplify decimal conversions

src/Microsoft.Windows.CsWin32/templates/DECIMAL.cs

@@ -1,39 +1,16 @@
 internal partial struct DECIMAL
 {
-	public DECIMAL(decimal value)
+	public unsafe DECIMAL(decimal value)
 	{
-		unchecked
-		{
-			const int SignMask = (int)0x80000000;
-#if NET5_0_OR_GREATER
-			Span<int> bits = stackalloc int[4];
-			decimal.GetBits(value, bits);
-#else
-			int[] bits = decimal.GetBits(value);
-#endif
-			uint lo32 = (uint)bits[0];
-			uint mid32 = (uint)bits[1];
-			uint hi32 = (uint)bits[2];
-			byte scale = (byte)(bits[3] >> 16);
-			byte sign = (bits[3] & SignMask) == SignMask ? (byte)0x80 : (byte)0x00;
-			this.Anonymous2 = new _Anonymous2_e__Union() { Anonymous = new _Anonymous2_e__Union._Anonymous_e__Struct() { Lo32 = lo32, Mid32 = mid32 } };
-			this.Hi32 = hi32;
-			this.Anonymous1 = new _Anonymous1_e__Union() { Anonymous = new _Anonymous1_e__Union._Anonymous_e__Struct() { scale = scale, sign = sign } };
-			this.wReserved = 0;
-		}
+		// DECIMAL is layout-compatible with decimal.
+		this = *(DECIMAL*)&value;
 	}
 
-	public static implicit operator decimal(DECIMAL value)
+	public static unsafe implicit operator decimal(DECIMAL value)
 	{
-		return new decimal(
-			(int)value.Anonymous2.Anonymous.Lo32,
-			(int)value.Anonymous2.Anonymous.Mid32,
-			(int)value.Hi32,
-			value.Anonymous1.Anonymous.sign == 0x80,
-			value.Anonymous1.Anonymous.scale);
+		// DECIMAL is layout-compatible with decimal.
+		return *(decimal*)&value;
 	}
 
-#if NET5_0_OR_GREATER
 	public static implicit operator DECIMAL(decimal value) => new DECIMAL(value);
-#endif
 }

test/GenerationSandbox.Tests/BasicTests.cs

@@ -153,11 +153,9 @@ public void DecimalConversion(decimal value)
         decimal valueRoundTripped = nativeDecimal;
         Assert.Equal(value, valueRoundTripped);
 
-#if NET5_0_OR_GREATER
         nativeDecimal = value;
         valueRoundTripped = nativeDecimal;
         Assert.Equal(value, valueRoundTripped);
-#endif
     }
 
     [Fact]

test/Microsoft.Windows.CsWin32.Tests/GeneratorTests.cs

@@ -82,6 +82,16 @@ public void SimplestMethod(string tfm)
         }
     }
 
+    [Fact]
+    public void DecimalWorksInNet35()
+    {
+        this.compilation = this.starterCompilations["net35"];
+        this.generator = this.CreateGenerator();
+        Assert.True(this.generator.TryGenerate("VarDecDiv", CancellationToken.None));
+        this.CollectGeneratedCode(this.generator);
+        this.AssertNoDiagnostics();
+    }
+
     [Fact]
     public void DbgHelpExternMethodsCanLoadAppLocal()
     {