Issue 1671: Add MemberNotNullWhen attribute for Content property in IApiResponse<T> (#1672)

* Issue 1671: Add MemberNotNullWhen attribute for Content property in IApiResponse<T> interface

* Fix missing IApiResponse implementation for ApiResponse<T> class

* Update APIResponse

Update APIResponse as MemberNotNullWhen needs to be in same interface as members

---------

Co-authored-by: Chris Pulman <chris.pulman@yahoo.com>

Author: sguryev

Refit/ApiResponse.cs

@@ -29,7 +29,7 @@ internal static T Create<T, TBody>(
     /// Implementation of <see cref="IApiResponse{T}"/> that provides additional functionalities.
     /// </summary>
     /// <typeparam name="T"></typeparam>
-    public sealed class ApiResponse<T> : IApiResponse<T>
+    public sealed class ApiResponse<T> : IApiResponse<T>, IApiResponse
     {
         readonly HttpResponseMessage response;
         bool disposed;
@@ -60,6 +60,8 @@ public ApiResponse(
         /// </summary>
         public T? Content { get; }
 
+        object? IApiResponse.Content => Content;
+
         /// <summary>
         /// Refit settings used to send the request.
         /// </summary>
@@ -163,14 +165,29 @@ public interface IApiResponse<out T> : IApiResponse
         /// <summary>
         /// Deserialized request content as <typeparamref name="T"/>.
         /// </summary>
-        T? Content { get; }
+        new T? Content { get; }
     }
 
     /// <summary>
     /// Base interface used to represent an API response.
     /// </summary>
     public interface IApiResponse : IDisposable
     {
+        /// <summary>
+        /// Indicates whether the request was successful.
+        /// </summary>
+#if NET6_0_OR_GREATER
+        [MemberNotNullWhen(true, nameof(ContentHeaders))]
+        [MemberNotNullWhen(false, nameof(Error))]
+        [MemberNotNullWhen(true, nameof(Content))]
+#endif
+        bool IsSuccessStatusCode { get; }
+
+        /// <summary>
+        /// Deserialized request content as an object.
+        /// </summary>
+        object? Content { get; }
+
         /// <summary>
         /// HTTP response headers.
         /// </summary>
@@ -181,15 +198,6 @@ public interface IApiResponse : IDisposable
         /// </summary>
         HttpContentHeaders? ContentHeaders { get; }
 
-        /// <summary>
-        /// Indicates whether the request was successful.
-        /// </summary>
-#if NET6_0_OR_GREATER
-        [MemberNotNullWhen(true, nameof(ContentHeaders))]
-        [MemberNotNullWhen(false, nameof(Error))]
-#endif
-        bool IsSuccessStatusCode { get; }
-
         /// <summary>
         /// HTTP response status code.
         /// </summary>