Add NullableResponse<T> that Response<T> inherits from

[tg-msft]

Response<T> requires that T Value is not null. When we want to work around that and return a Response<T> with no Value (like conditional requests that return 3XX, batching requests that haven't been sent to the service yet, DPG errors that were classified as success, ...) we usually throw when accessed. See NoBodyResponse and ErrorResponse as examples.

While that technically meets the contract for Response<T>, it doesn't match the spirit of it. We'd like to introduce a NullableResponse<T> that we can start using for this case. To avoid a bifurcated response hierarchy, we'd like to make the existing Response<T> inherit from it. Here's a rough example of what the API might look like:

public abstract class NullableResponse<T> {
    public abstract bool HasValue { get; }
    public abstract T Value { get;  }
    public abstract Response GetRawResponse() {}
}
 
public abstract class Response<T> : NullableResponse<T> {
     [EditorBrowsable(EditorBrowsableState.Never)]
     public override bool HasValue => true;
     public static implicit operator T(Response<T> response)
}

We also want to evaluate whether DPG protocol methods (and anything accepting RequestContext) should return this NullableResponse<T> instead of regular Response<T>.

[annelo-msft]

We also want to evaluate whether DPG protocol methods (and anything accepting RequestContext) should return this NullableResponse<T> instead of regular Response<T>.

DPG protocol methods only return Response, not Response<T>. It is the responsibility of the protocol method caller if they pass a response classifier saying errors are acceptable to check Response for errors in order to return a value that correctly matches the Convenience API return type without throwing an exception.

[annelo-msft]

Per offline conversation, this isn't required for DPG GA. We'll address it once we start adding the Get<Resource>IfExists methods to the management plane (after v1 GA) or to DPG.

[christothes]

Note: This could affect any extensions to Response
guidelines

• use nullableResponse when mainline scenario may not return a value
• ETag scenarios : it depends