[Generator] Move CustomAttributeDataExtensions to its own file and enable nullability. (#17526)

Co-authored-by: GitHub Actions Autoformatter <github-actions-autoformatter@xamarin.com>

src/bgen/AttributeManager.cs

@@ -857,26 +857,3 @@ public static class EnumerableExtensions {
 		yield return item;
 	}
 }
-
-public static class CustomAttributeDataExtensions {
-#if !NET
-	static Type roCustomAttributeDataType;
-	static PropertyInfo attributeTypeProperty;
-
-	static CustomAttributeDataExtensions ()
-	{
-		roCustomAttributeDataType = typeof (MetadataLoadContext).Assembly.GetType ("System.Reflection.TypeLoading.RoCustomAttributeData");
-		attributeTypeProperty = roCustomAttributeDataType.GetProperty ("AttributeType");
-	}
-
-	public static Type GetAttributeType (this CustomAttributeData data)
-	{
-		// Workaround for CustomAttributeData.AttributeType not being declared as virtual in Mono
-		if (data.GetType ().IsSubclassOf (roCustomAttributeDataType))
-			return (Type) attributeTypeProperty.GetValue (data);
-		return data.AttributeType;
-	}
-#else
-	public static Type GetAttributeType (this CustomAttributeData data) => data.AttributeType;
-#endif
-}

src/bgen/CustomAttributeDataExtensions.cs

@@ -0,0 +1,28 @@
+using System;
+using System.Reflection;
+
+#nullable enable
+
+public static class CustomAttributeDataExtensions {
+#if !NET
+	static readonly Type roCustomAttributeDataType;
+	static readonly PropertyInfo attributeTypeProperty;
+
+	static CustomAttributeDataExtensions ()
+	{
+		roCustomAttributeDataType = typeof (MetadataLoadContext).Assembly.GetType ("System.Reflection.TypeLoading.RoCustomAttributeData");
+		attributeTypeProperty = roCustomAttributeDataType.GetProperty ("AttributeType")
+								?? throw new InvalidOperationException ("Could not retrieve property 'AttributeType'.");
+	}
+
+	public static Type GetAttributeType (this CustomAttributeData data)
+	{
+		// Workaround for CustomAttributeData.AttributeType not being declared as virtual in Mono
+		if (data.GetType ().IsSubclassOf (roCustomAttributeDataType))
+			return (Type) attributeTypeProperty.GetValue (data);
+		return data.AttributeType;
+	}
+#else
+	public static Type GetAttributeType (this CustomAttributeData data) => data.AttributeType;
+#endif
+}

src/generator.csproj

@@ -97,6 +97,7 @@
     <Compile Include="..\src\bgen\AttributeManager.cs" />
     <Compile Include="..\src\bgen\Attributes.cs" />
     <Compile Include="..\src\bgen\BindingTouch.cs" />
+    <Compile Include="..\src\bgen\CustomAttributeDataExtensions.cs" />
     <Compile Include="..\src\bgen\Enums.cs" />
     <Compile Include="..\src\bgen\ExtensionMethods.cs" />
     <Compile Include="..\src\bgen\Frameworks.cs" />