How to fix: Avoid non-readonly static fields

Question

I am using NDepend and in the following code, it detects this code smell.

But if I add readonly, then it wont compile.

namespace todo
{
    using System;
    using System.Collections.Generic;
    using System.Configuration;
    using System.Linq;
    using System.Linq.Expressions;
    using System.Threading.Tasks;
    using Microsoft.Azure.Documents;
    using Microsoft.Azure.Documents.Client;
    using Microsoft.Azure.Documents.Linq;

    public static class DocumentDBRepository<T> where T : class
    {
        private static readonly string DatabaseId = ConfigurationManager.AppSettings["database"];
        private static readonly string CollectionId = ConfigurationManager.AppSettings["collection"];
        private static DocumentClient client;

        public static async Task<T> GetItemAsync(string id)
        {
            try
            {
                Document document = await client.ReadDocumentAsync(UriFactory.CreateDocumentUri(DatabaseId, CollectionId, id));
                return (T)(dynamic)document;
            }
            catch (DocumentClientException e)
            {
                if (e.StatusCode == System.Net.HttpStatusCode.NotFound)
                {
                    return null;
                }
                else
                {
                    throw;
                }
            }
        }

        public static async Task<IEnumerable<T>> GetItemsAsync(Expression<Func<T, bool>> predicate)
        {
            IDocumentQuery<T> query = client.CreateDocumentQuery<T>(
                UriFactory.CreateDocumentCollectionUri(DatabaseId, CollectionId),
                new FeedOptions { MaxItemCount = -1 })
                .Where(predicate)
                .AsDocumentQuery();

            List<T> results = new List<T>();
            while (query.HasMoreResults)
            {
                results.AddRange(await query.ExecuteNextAsync<T>());
            }

            return results;
        }

        public static async Task<Document> CreateItemAsync(T item)
        {
            return await client.CreateDocumentAsync(UriFactory.CreateDocumentCollectionUri(DatabaseId, CollectionId), item);
        }

        public static async Task<Document> UpdateItemAsync(string id, T item)
        {
            return await client.ReplaceDocumentAsync(UriFactory.CreateDocumentUri(DatabaseId, CollectionId, id), item);
        }

        public static async Task DeleteItemAsync(string id)
        {
            await client.DeleteDocumentAsync(UriFactory.CreateDocumentUri(DatabaseId, CollectionId, id));
        }

        public static void Initialize()
        {
            client = new DocumentClient(new Uri(ConfigurationManager.AppSettings["endpoint"]), ConfigurationManager.AppSettings["authKey"]);
            CreateDatabaseIfNotExistsAsync().Wait();
            CreateCollectionIfNotExistsAsync().Wait();
        }

        private static async Task CreateDatabaseIfNotExistsAsync()
        {
            try
            {
                await client.ReadDatabaseAsync(UriFactory.CreateDatabaseUri(DatabaseId));
            }
            catch (DocumentClientException e)
            {
                if (e.StatusCode == System.Net.HttpStatusCode.NotFound)
                {
                    await client.CreateDatabaseAsync(new Database { Id = DatabaseId });
                }
                else
                {
                    throw;
                }
            }
        }

        private static async Task CreateCollectionIfNotExistsAsync()
        {
            try
            {
                await client.ReadDocumentCollectionAsync(UriFactory.CreateDocumentCollectionUri(DatabaseId, CollectionId));
            }
            catch (DocumentClientException e)
            {
                if (e.StatusCode == System.Net.HttpStatusCode.NotFound)
                {
                    await client.CreateDocumentCollectionAsync(
                        UriFactory.CreateDatabaseUri(DatabaseId),
                        new DocumentCollection { Id = CollectionId },
                        new RequestOptions { OfferThroughput = 1000 });
                }
                else
                {
                    throw;
                }
            }
        }
    }
}

The warning line is:

private static DocumentClient client;

How would you recommend to fix this NDepend warning?

Rule Description:

This rule warns about static fields that are not declared as read-only.

In Object-Oriented-Programming the natural artifact to hold states that can be modified is instance fields. Such mutable static fields create confusion about the expected state at runtime and impairs the code testability since the same mutable state is re-used for each test.

More discussion on the topic can be found here: http://codebetter.com/patricksmacchia/2011/05/04/back-to-basics-usage-of-static-members/

Answer 1

Try moving the initialization to the declaration:

private static readonly DocumentClient client = new DocumentClient( . . . . );