How to programmatically assign reviewers to an AzureDevOps PullRequest?

Note: To get the following code working, you need to first execute the following command in NuGet console Install-Package Microsoft.TeamFoundationServer.ExtendedClient, to install the required libraries.

In this solution, the pull request is created first, and then reviewers are added. The following code shows all the steps needed to create the pull request and then add the reviewer. The code can be easily modified to add multiple reviewers.

    using System;
    using System.Threading;
    using Microsoft.TeamFoundation.SourceControl.WebApi;
    using System.Threading.Tasks;
    using Microsoft.VisualStudio.Services.Identity;
    using Microsoft.VisualStudio.Services.Identity.Client;
    using Microsoft.VisualStudio.Services.Common;

namespace AddingReviewersToVstsPullRequestProgramatically
{
    public class PullRequestReviewerAdder
    {

            /// <summary>
            /// Creates a pull request, and then adds a reviewer to it.
            /// </summary>
            /// <param name="gitHttpClient"> GitHttpClient that is created for accessing vsts</param>
            /// <param name="gitPullRequest"> the pull request to be created</param>
            /// <param name="repositoryId"> the unique identifier of the repository</param>
            /// <param name="reviewerAlias"> reviewer's alias in vsts</param>
            /// <param name="vstsAccountUrl">vsts account's url</param>
            /// <param name="personalToken"> personal access token to access the vsts account. </param>
            public static async void CreatePullRequestAndAddReviewer(
                GitHttpClient gitHttpClient,
                GitPullRequest gitPullRequest,
                string repositoryId,
                string reviewerAlias,
                Uri vstsAccountUrl,
                string personalToken)
            {
                // 1- Create the pull request.
                GitPullRequest pullRequest = gitHttpClient.CreatePullRequestAsync(gitPullRequest, repositoryId, cancellationToken: CancellationToken.None).Result;

                // 2- Create an Identity Client to get the reviewer's vsts id
                IdentityHttpClient identityHttpClient = CreateIdentityClient(vstsAccountUrl, personalToken);

                // 3- Find the reviewer's vsts identity.
                Identity identity = SearchForReviewerVstsIdentity(identityHttpClient, reviewerAlias).Result;

                // 4- Create a IdentityRefWithVote for the reviewer
                IdentityRefWithVote identityRefWithVote = new IdentityRefWithVote
                {
                    Id = identity.Id.ToString(),
                    IsRequired = true // false otherwise.
                };

                // 5- Finally add the reviewer to the pull request.
                await AddReviewerToPullRequest(gitHttpClient, pullRequest, identityRefWithVote);
            }

            /// <summary>
            /// Creates an identity client. This is needed for fetching a reviewer's vsts identity.
            /// </summary>
            /// <param name="vstsAccountUrl">vsts account's url</param>
            /// <param name="personalToken"> personal access token to access the vsts account. </param>
            /// <returns>an IdentityHttpClient to use for retrieving identities from vsts. </returns>
            public static IdentityHttpClient CreateIdentityClient(Uri vstsAccountUrl, string personalToken)
            {
                var vstsCredential = new VssBasicCredential(string.Empty, personalToken);
                IdentityHttpClient identityHttpClient = new IdentityHttpClient(vstsAccountUrl, vstsCredential);
                return identityHttpClient;
            }

            /// <summary>
            /// Given an alias on vsts, searches for its vsts identity.
            /// </summary>
            /// <param name="identityHttpClient"> is the vsts identity client.</param>
            /// <param name="alias">is the alias for which the identity is being searched for.</param>
            public static async Task<Identity> SearchForReviewerVstsIdentity(IdentityHttpClient identityHttpClient, string alias)
            {
                try
                {
                    // Notice : you can also search based on factors other than alias.
                    IdentitiesCollection identitiesPerAlias = await identityHttpClient
                        .ReadIdentitiesAsync(IdentitySearchFilter.Alias, alias).ConfigureAwait(false);
                    if (identitiesPerAlias.Count == 1) // Found one identity-- the ideal case
                    {
                        return identitiesPerAlias[0];
                    }

                    Console.WriteLine($"Encountered a problem finding vsts identity foralias {alias}.");
                }
                catch (Exception ex)
                {
                    Console.WriteLine($"Exception when looking for vsts identity for alias {alias}. {ex}");
                }

                // Notice : watch out for null case...
                return null;
            }

            /// <summary>
            /// Adds a reviewer to a an already created pull request.
            /// </summary>
            /// <param name="gitHttpClient">GitHttpClient that is created for accessing vsts</param>
            /// <param name="pullRequest"> pull request that is already created.</param>
            /// <param name="identity">identity of the reviewer that we want to add to the pull request.</param>
            public static async Task AddReviewerToPullRequest(GitHttpClient gitHttpClient, GitPullRequest pullRequest, IdentityRefWithVote identity)
            {
                identity = await gitHttpClient.CreatePullRequestReviewerAsync(
                    identity,
                    pullRequest.Repository.Id,
                    pullRequest.PullRequestId,
                    identity.Id).ConfigureAwait(false);
            }
   }