Use NullabilityInfoContext to determine dictionary nullability (#3041)

Use `NullabilityInfoContext` to determine dictionary type nullability correctly.

Author: patrikwlund

src/Swashbuckle.AspNetCore.SwaggerGen/SchemaGenerator/MemberInfoExtensions.cs

@@ -61,6 +61,23 @@ public static bool IsNonNullableReferenceType(this MemberInfo memberInfo)
 
         public static bool IsDictionaryValueNonNullable(this MemberInfo memberInfo)
         {
+#if NET6_0_OR_GREATER
+            var context = new NullabilityInfoContext();
+            var nullableInfo = memberInfo.MemberType == MemberTypes.Field
+                ? context.Create((FieldInfo)memberInfo)
+                : context.Create((PropertyInfo)memberInfo);
+
+            if (nullableInfo.GenericTypeArguments.Length != 2)
+            {
+                var length = nullableInfo.GenericTypeArguments.Length;
+                var type = nullableInfo.Type.FullName;
+                var container = memberInfo.DeclaringType.FullName;
+                var member = memberInfo.Name;
+                throw new InvalidOperationException($"Expected Dictionary to have two generic type arguments but it had {length}. Member: {container}.{member} Type: {type}.");
+            }
+
+            return nullableInfo.GenericTypeArguments[1].ReadState == NullabilityState.NotNull;
+#else
             var memberType = memberInfo.MemberType == MemberTypes.Field
                 ? ((FieldInfo)memberInfo).FieldType
                 : ((PropertyInfo)memberInfo).PropertyType;
@@ -104,6 +121,7 @@ public static bool IsDictionaryValueNonNullable(this MemberInfo memberInfo)
             }
 
             return false;
+#endif
         }
 
         private static object GetNullableAttribute(this MemberInfo memberInfo)