https://github.com/JetBrains/YouTrackSharp/issues/40

src/YouTrackSharp/ConnectionExtensions.cs

@@ -59,5 +59,15 @@ public static TimeTrackingManagementService CreateTimeTrackingManagementService(
         {
             return new TimeTrackingManagementService(connection);
         }
+
+        /// <summary>
+        /// Creates a <see cref="ProjectCustomFieldsService" />.
+        /// </summary>
+        /// <param name="connection">The <see cref="Connection" /> to create a service with.</param>
+        /// <returns><see cref="ProjectCustomFieldsService" /> for accessing custom project fields.</returns>
+        public static ProjectCustomFieldsService ProjectCustomFieldsService(this Connection connection)
+        {
+            return new ProjectCustomFieldsService(connection);
+        }
     }
 }

src/YouTrackSharp/Projects/CustomField.cs

@@ -0,0 +1,52 @@
+using Newtonsoft.Json;
+
+namespace YouTrackSharp.Projects
+{
+    /// <summary>
+    /// Custom field for a project
+    /// </summary>
+    public class CustomField
+    {
+        /// <summary>
+        /// Creates an instance of the <see cref="CustomField" /> class.
+        /// </summary>
+        public CustomField()
+        {
+            Name = string.Empty;
+            Url = string.Empty;
+            Type = string.Empty;
+            CanBeEmpty = false;
+            EmptyText = string.Empty;
+        }
+
+        /// <summary>
+        ///  Name of project custom field.
+        /// </summary>
+        [JsonProperty("name")]
+        public string Name { get; set; }
+
+        /// <summary>
+        /// The Url of the custom field.
+        /// </summary>
+        [JsonProperty("url")]
+        public string Url { get; set; }
+
+        /// <summary>
+        /// Type of this custom field.
+        /// </summary>
+        [JsonProperty("type")]
+        public string Type { get; set; }
+
+        /// <summary>
+        ///  Mandatory binary parameter defining if the field can have empty value or not.
+        /// </summary>
+        [JsonProperty("canBeEmpty")]
+        public bool CanBeEmpty { get; set; }
+
+        /// <summary>
+        ///  Text that is shown when the custom field has no value.
+        /// </summary>
+        [JsonProperty("emptyText")]
+        public string EmptyText { get; set; }
+    }
+}

src/YouTrackSharp/Projects/ProjectCustomFieldsService.cs

@@ -0,0 +1,159 @@
+using System;
+using System.Collections.Generic;
+using System.Net;
+using System.Net.Http;
+using System.Net.Http.Headers;
+using System.Threading.Tasks;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using YouTrackSharp.Management;
+
+namespace YouTrackSharp.Projects
+{
+    /// <summary>
+    /// A class that represents a REST API client for <a href="https://www.jetbrains.com/help/youtrack/standalone/Project-Custom-Fields.html"> methods related to operations with custom fields of a project</a>.
+    /// It uses a <see cref="Connection" /> implementation to connect to the remote YouTrack server instance.
+    /// </summary>
+    public class ProjectCustomFieldsService
+    {
+        private readonly Connection _connection;
+
+        /// <summary>
+        /// Creates an instance of the <see cref="ProjectCustomFieldsService" /> class.
+        /// </summary>
+        /// <param name="connection">A <see cref="Connection" /> instance that provides a connection to the remote YouTrack server instance.</param>
+        public ProjectCustomFieldsService(Connection connection)
+        {
+            _connection = connection ?? throw new ArgumentNullException(nameof(connection));
+        }
+
+        /// <summary>
+        /// Get custom fields used in a project.
+        /// </summary>
+        /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/GET-Project-Custom-Fields.html">Get Project Custom Fields</a>.</remarks>
+        /// <param name="projectId">Id of the project to get the custom fields for.</param>
+        /// <returns>A <see cref="T:System.Collections.Generic.ICollection`1" /> of <see cref="CustomField" /> that are accessible for currently logged in user.</returns>
+        /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>
+        public async Task<ICollection<CustomField>> GetProjectCustomFields(string projectId)
+        {
+            var client = await _connection.GetAuthenticatedHttpClient();
+            var response = await client.GetAsync($"rest/admin/project/{projectId}/customfield");
+
+            response.EnsureSuccessStatusCode();
+
+            return JsonConvert.DeserializeObject<ICollection<CustomField>>(await response.Content.ReadAsStringAsync());
+        }
+
+        /// <summary>
+        /// Get a project's custom field by its name.
+        /// </summary>
+        /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/GET-Project-Custom-Field.html">Get Project Custom Field</a>.</remarks>
+        /// <param name="projectId">Id of the project to get the custom field for.</param>
+        /// <param name="customFieldName">Name of the custom field to get.</param>
+        /// <returns><see cref="CustomField" />.</returns>
+        /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>
+        public async Task<CustomField> GetProjectCustomField(string projectId, string customFieldName)
+        {
+            var client = await _connection.GetAuthenticatedHttpClient();
+            var response = await client.GetAsync($"rest/admin/project/{projectId}/customfield/{customFieldName}");
+
+            response.EnsureSuccessStatusCode();
+
+            return JsonConvert.DeserializeObject<CustomField>(await response.Content.ReadAsStringAsync());
+        }
+
+        /// <summary>
+        /// Remove specified custom field from a project.
+        /// </summary>
+        /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/DELETE-Project-Custom-Field.html">Delete a project custom field</a>.</remarks>
+        /// <param name="projectId">Id of the project to delete the custom field for.</param>
+        /// <param name="customFieldName">Name of the custom field to delete.</param>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="projectId"/> is null or empty.</exception>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="customFieldName"/> is null or empty.</exception>
+        /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>
+        public async Task DeleteProjectCustomField(string projectId, string customFieldName)
+        {
+            if (string.IsNullOrEmpty(projectId))
+            {
+                throw new ArgumentNullException(nameof(projectId));
+            }
+            if (string.IsNullOrEmpty(customFieldName))
+            {
+                throw new ArgumentNullException(nameof(customFieldName));
+            }
+
+            var client = await _connection.GetAuthenticatedHttpClient();
+            var response = await client.DeleteAsync($"rest/admin/project/{projectId}/customfield/{customFieldName}");
+
+            if (response.StatusCode == HttpStatusCode.NotFound)
+            {
+                return;
+            }
+
+            response.EnsureSuccessStatusCode();
+        }
+
+        /// <summary>
+        /// Adds an existing custom field to a specific project.
+        /// </summary>
+        /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/PUT-Project-Custom-Field.html">Create a project custom field</a>.</remarks>
+        /// <param name="projectId">Id of the project to add a custom field.</param>
+        /// <param name="customField"><see cref="CustomField" /> to add to the project.</param>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="projectId"/> is null or empty.</exception>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="customField"/> is null or empty.</exception>
+        /// <exception cref="T:YouTrackErrorException">When the call to the remote YouTrack server instance failed and YouTrack reported an error message.</exception>
+        /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>
+        public async Task CreateProjectCustomField(string projectId, CustomField customField)
+        {
+            if (string.IsNullOrEmpty(customField?.Name))
+            {
+                throw new ArgumentNullException(nameof(customField));
+            }
+
+            string query = string.Empty;
+            if (!string.IsNullOrEmpty(customField.EmptyText))
+            {
+                query = $"?emptyFieldText={WebUtility.UrlEncode(customField.EmptyText)}";
+            }
+
+            var client = await _connection.GetAuthenticatedHttpClient();
+            var response = await client.PutAsync($"rest/admin/project/{projectId}/customfield/{customField.Name}{query}", new MultipartContent());
+
+            response.EnsureSuccessStatusCode();
+        }
+
+        /// <summary>
+        /// Updates a custom field to a specific project.
+        /// </summary>
+        /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/POST-Project-Custom-Field.html">Updates a project custom field</a>.</remarks>
+        /// <param name="projectId">Id of the project.</param>
+        /// <param name="customField"><see cref="CustomField" /> to update in project.</param>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="projectId"/> is null or empty.</exception>
+        /// <exception cref="T:System.ArgumentNullException">When the <paramref name="customField"/> is null or empty.</exception>
+        /// <exception cref="T:YouTrackErrorException">When the call to the remote YouTrack server instance failed and YouTrack reported an error message.</exception>
+        /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>
+        public async Task UpdateProjectCustomField(string projectId, CustomField customField)
+        {
+            if (string.IsNullOrEmpty(projectId))
+            {
+                throw new ArgumentNullException(nameof(projectId));
+            }
+
+            if (string.IsNullOrEmpty(customField?.Name))
+            {
+                throw new ArgumentNullException(nameof(customField));
+            }
+
+            string query = string.Empty;
+            if (!string.IsNullOrEmpty(customField.EmptyText))
+            {
+                query = $"?emptyFieldText={WebUtility.UrlEncode(customField.EmptyText)}";
+            }
+
+            var client = await _connection.GetAuthenticatedHttpClient();
+            var response = await client.PostAsync($"rest/admin/project/{projectId}/customfield/{customField.Name}{query}", new MultipartContent());
+
+            response.EnsureSuccessStatusCode();
+        }
+    }
+}

src/YouTrackSharp/TimeTracking/TimeTrackingService.cs

@@ -30,7 +30,7 @@ public TimeTrackingService(Connection connection)
         /// Get work types for a specific project from the server.
         /// </summary>
         /// <remarks>Uses the REST API <a href="https://www.jetbrains.com/help/youtrack/standalone/GET-Work-Types-for-a-Project.html">GET Work Types for a Project</a>.</remarks>
-        /// <param name="projectId">Id of the issue to get work items for.</param>
+        /// <param name="projectId">Id of the project to get work items for.</param>
         /// <returns>An <see cref="T:System.Collections.Generic.IEnumerable`1" /> of <see cref="WorkType" /> for the requested project <paramref name="projectId"/>.</returns>
         /// <exception cref="T:System.ArgumentNullException">When the <paramref name="projectId"/> is null or empty.</exception>
         /// <exception cref="T:System.Net.HttpRequestException">When the call to the remote YouTrack server instance failed.</exception>

tests/YouTrackSharp.Tests/Integration/Projects/CreateProjectCustomField.cs

@@ -0,0 +1,74 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Xunit;
+using YouTrackSharp.Projects;
+using YouTrackSharp.Tests.Infrastructure;
+
+namespace YouTrackSharp.Tests.Integration.Projects
+{
+    public partial class ProjectCustomFieldsServiceTests
+    {
+        public class CreateProjectCustomField
+        {
+            [Fact]
+            public async Task Valid_Connection_Creates_CustomField_For_Project()
+            {
+                // Arrange
+                var connection = Connections.Demo1Password;
+                var service = connection.ProjectCustomFieldsService();
+                var customField = new CustomField {Name = "TestField"};
+                var projectId = "DP1";
+
+                // Act
+                await service.CreateProjectCustomField(projectId, customField);
+
+                var created = await service.GetProjectCustomField(projectId, customField.Name);
+
+                // Assert
+                Assert.NotNull(created);
+
+                Assert.Equal(customField.Name, created.Name);
+                Assert.Equal(customField.EmptyText, string.Empty);
+
+                // cleanup
+                await service.DeleteProjectCustomField(projectId, customField.Name);
+            }
+
+            [Fact]
+            public async Task Valid_Connection_Creates_CustomField_With_EmptyText_For_Project()
+            {
+                // Arrange
+                var connection = Connections.Demo1Password;
+                var service = connection.ProjectCustomFieldsService();
+                var customField = new CustomField { Name = "TestField", EmptyText = "empty" };
+                var projectId = "DP1";
+
+                // Act
+                await service.CreateProjectCustomField(projectId, customField);
+
+                var created = await service.GetProjectCustomField(projectId, customField.Name);
+
+                // Assert
+                Assert.NotNull(created);
+
+                Assert.Equal(customField.Name, created.Name);
+                Assert.Equal(customField.EmptyText, created.EmptyText);
+
+                // cleanup
+                await service.DeleteProjectCustomField(projectId, customField.Name);
+            }
+
+            [Fact]
+            public async Task Invalid_Connection_Throws_UnauthorizedConnectionException()
+            {
+                // Arrange
+                var service = Connections.UnauthorizedConnection.ProjectCustomFieldsService();
+
+                // Act & Assert
+                await Assert.ThrowsAsync<UnauthorizedConnectionException>(
+                    async () => await service.GetProjectCustomField("DP1", "TestField"));
+            }
+        }
+    }
+}

tests/YouTrackSharp.Tests/Integration/Projects/DeleteProjectCustomField.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+using Xunit;
+using YouTrackSharp.Tests.Infrastructure;
+
+namespace YouTrackSharp.Tests.Integration.Projects
+{
+    public partial class ProjectCustomFieldsServiceTests
+    {
+        public class DeleteProjectCustomField
+        {
+            [Fact]
+            public async Task Valid_Connection_Deletes_CustomField_For_Project()
+            {
+                // Arrange
+                var connection = Connections.Demo1Token;
+                var service = connection.ProjectCustomFieldsService();
+
+                // Act & Assert
+                var acted = false;
+                await service.DeleteProjectCustomField("DP1", " TestField");
+                acted = true;
+
+                Assert.True(acted);
+            }
+
+            [Fact]
+            public async Task Invalid_Connection_Throws_UnauthorizedConnectionException()
+            {
+                // Arrange
+                var service = Connections.UnauthorizedConnection.ProjectCustomFieldsService();
+
+                // Act & Assert
+                await Assert.ThrowsAsync<UnauthorizedConnectionException>(
+                    async () => await service.DeleteProjectCustomField("DP1", "TestField"));
+            }
+        }
+    }
+}