Better string enums

src/Stripe.net/Services/Customers/CustomerCreateOptions.cs

@@ -114,7 +114,7 @@ public class CustomerCreateOptions : BaseOptions
         public decimal? TaxPercent { get; set; }
 
         [JsonProperty("trial_end")]
-        public AnyOf<DateTime?, SubscriptionTrialEnd?> TrialEnd { get; set; }
+        public AnyOf<DateTime?, SubscriptionTrialEnd> TrialEnd { get; set; }
 
         [JsonProperty("validate")]
         public bool? Validate { get; set; }

src/Stripe.net/Services/Invoices/UpcomingInvoiceListLineItemsOptions.cs

@@ -38,7 +38,7 @@ public class UpcomingInvoiceListLineItemsOptions : ListOptions
         /// <see cref="Stripe.SubscriptionBillingCycleAnchor"/>.
         /// </summary>
         [JsonProperty("subscription_billing_cycle_anchor")]
-        public AnyOf<DateTime?, SubscriptionBillingCycleAnchor?> SubscriptionBillingCycleAnchor { get; set; }
+        public AnyOf<DateTime?, SubscriptionBillingCycleAnchor> SubscriptionBillingCycleAnchor { get; set; }
 
         /// <summary>
         /// Boolean indicating whether this subscription should cancel at the end of the current

src/Stripe.net/Services/Invoices/UpcomingInvoiceOptions.cs

@@ -38,7 +38,7 @@ public class UpcomingInvoiceOptions : BaseOptions
         /// <see cref="Stripe.SubscriptionBillingCycleAnchor"/>.
         /// </summary>
         [JsonProperty("subscription_billing_cycle_anchor")]
-        public AnyOf<DateTime?, SubscriptionBillingCycleAnchor?> SubscriptionBillingCycleAnchor { get; set; }
+        public AnyOf<DateTime?, SubscriptionBillingCycleAnchor> SubscriptionBillingCycleAnchor { get; set; }
 
         /// <summary>
         /// Boolean indicating whether this subscription should cancel at the end of the current

src/Stripe.net/Services/Plans/PlanTierOptions.cs

@@ -23,6 +23,6 @@ public class PlanTierOptions : INestedOptions
         /// tier.
         /// </summary>
         [JsonProperty("up_to")]
-        public AnyOf<long?, PlanTierUpTo?> UpTo { get; set; }
+        public AnyOf<long?, PlanTierUpTo> UpTo { get; set; }
     }
 }

src/Stripe.net/Services/Plans/PlanTierUpTo.cs

@@ -1,14 +1,13 @@
 namespace Stripe
 {
-    using System.Runtime.Serialization;
-    using Newtonsoft.Json;
-    using Newtonsoft.Json.Converters;
-
-    [JsonConverter(typeof(StringEnumConverter))]
-    public enum PlanTierUpTo
+    public class PlanTierUpTo : StringEnum
     {
         /// <summary>Use this value to define a fallback tier.</summary>
-        [EnumMember(Value = "inf")]
-        Inf,
+        public static readonly PlanTierUpTo Inf = new PlanTierUpTo("inf");
+
+        private PlanTierUpTo(string value)
+            : base(value)
+        {
+        }
     }
 }

src/Stripe.net/Services/Subscriptions/SubscriptionBillingCycleAnchor.cs

@@ -1,18 +1,18 @@
 namespace Stripe
 {
-    using System.Runtime.Serialization;
-    using Newtonsoft.Json;
-    using Newtonsoft.Json.Converters;
-
-    [JsonConverter(typeof(StringEnumConverter))]
-    public enum SubscriptionBillingCycleAnchor
+    public class SubscriptionBillingCycleAnchor : StringEnum
     {
         /// <summary>Resets the subscription's billing cycle anchor to the current time.</summary>
-        [EnumMember(Value = "now")]
-        Now,
+        public static readonly SubscriptionBillingCycleAnchor Now
+            = new SubscriptionBillingCycleAnchor("now");
 
         /// <summary>Leaves the subscription's billing cycle anchor unchanged.</summary>
-        [EnumMember(Value = "unchanged")]
-        Unchanged,
+        public static readonly SubscriptionBillingCycleAnchor Unchanged
+            = new SubscriptionBillingCycleAnchor("unchanged");
+
+        private SubscriptionBillingCycleAnchor(string value)
+            : base(value)
+        {
+        }
     }
 }

src/Stripe.net/Services/Subscriptions/SubscriptionSharedOptions.cs

@@ -106,7 +106,7 @@ public abstract class SubscriptionSharedOptions : BaseOptions
         /// immediately.
         /// </summary>
         [JsonProperty("trial_end")]
-        public AnyOf<DateTime?, SubscriptionTrialEnd?> TrialEnd { get; set; }
+        public AnyOf<DateTime?, SubscriptionTrialEnd> TrialEnd { get; set; }
 
         /// <summary>
         /// Boolean. Decide whether to use the default trial on the plan when creating a subscription.

src/Stripe.net/Services/Subscriptions/SubscriptionTrialEnd.cs

@@ -1,14 +1,13 @@
 namespace Stripe
 {
-    using System.Runtime.Serialization;
-    using Newtonsoft.Json;
-    using Newtonsoft.Json.Converters;
-
-    [JsonConverter(typeof(StringEnumConverter))]
-    public enum SubscriptionTrialEnd
+    public class SubscriptionTrialEnd : StringEnum
     {
         /// <summary>Special value used to end a customer's trial immediately.</summary>
-        [EnumMember(Value = "now")]
-        Now,
+        public static readonly SubscriptionTrialEnd Now = new SubscriptionTrialEnd("now");
+
+        private SubscriptionTrialEnd(string value)
+            : base(value)
+        {
+        }
     }
 }

src/Stripe.net/Services/Subscriptions/SubscriptionUpdateOptions.cs

@@ -13,7 +13,7 @@ public class SubscriptionUpdateOptions : SubscriptionSharedOptions
         /// <a href="https://stripe.com/docs/billing/subscriptions/billing-cycle">documentation</a>.
         /// </summary>
         [JsonProperty("billing_cycle_anchor")]
-        public SubscriptionBillingCycleAnchor? BillingCycleAnchor { get; set; }
+        public SubscriptionBillingCycleAnchor BillingCycleAnchor { get; set; }
 
         /// <summary>
         /// List of subscription items, each with an attached plan.

src/Stripe.net/Services/_base/StringEnum.cs

@@ -0,0 +1,46 @@
+namespace Stripe
+{
+    /// <summary>
+    /// Abstract base class for string enum parameters.
+    /// </summary>
+    /// <remarks>
+    /// This class is used to define request parameters that can only take certain string values.
+    /// We use it instead of defining a regular <c>enum</c> with
+    /// <see cref="System.Runtime.Serialization.EnumMemberAttribute"/> annotations for
+    /// serialization because <c>enum</c> is really an integer under the hood. This can be
+    /// problematic in some cases, like when a parameter can be an integer OR a string enum (e.g.
+    /// <see cref="PlanTierOptions.UpTo"/>).
+    /// </remarks>
+    /// <example>
+    /// This sample shows how to define a new string enum type.
+    /// <code>
+    /// public class FooBar : StringEnum
+    /// {
+    ///     public static readonly FooBar Foo = new FooBar("foo");
+    ///     public static readonly FooBar Bar = new FooBar("bar");
+    ///
+    ///     private FooBar(string value) : base(value) {}
+    /// }
+    /// </code>
+    /// </example>
+    public abstract class StringEnum
+    {
+        /// <summary>Initializes a new instance of the <see cref="StringEnum"/> class.</summary>
+        /// <param name="value">The serialized value for the instance.</param>
+        protected StringEnum(string value)
+        {
+            this.Value = value;
+        }
+
+        /// <summary>Gets or sets the serialized value.</summary>
+        /// <returns>The serialized value.</returns>
+        public string Value { get; protected set; }
+
+        /// <summary>Returns the serialized value.</summary>
+        /// <returns>The serialized value.</returns>
+        public override string ToString()
+        {
+            return this.Value;
+        }
+    }
+}

src/StripeTests/Infrastructure/FormEncoding/FormEncoderTest.cs

@@ -353,6 +353,24 @@ public void CreateQueryString()
                     },
                     want = "string="
                 },
+
+                // StringEnum
+                new
+                {
+                    data = new TestOptions
+                    {
+                        StringEnum = TestOptions.TestStringEnum.Foo,
+                    },
+                    want = "string_enum=foo"
+                },
+                new
+                {
+                    data = new TestOptions
+                    {
+                        StringEnum = null,
+                    },
+                    want = string.Empty,
+                },
             };
 
             foreach (var testCase in testCases)

src/StripeTests/Infrastructure/TestData/TestOptions.cs

@@ -57,5 +57,18 @@ public enum TestEnum
 
         [JsonProperty("string")]
         public string String { get; set; }
+
+        [JsonProperty("string_enum")]
+        public TestStringEnum StringEnum { get; set; }
+
+        public class TestStringEnum : StringEnum
+        {
+            public static readonly TestStringEnum Foo = new TestStringEnum("foo");
+
+            private TestStringEnum(string value)
+                : base(value)
+            {
+            }
+        }
     }
 }