AnyOf<> generic class to handle polymorphic parameters

src/Stripe.net/Infrastructure/FormEncoding/FormEncoder.cs

@@ -100,6 +100,10 @@ private static List<KeyValuePair<string, object>> FlattenParamsValue(object valu
                     flatParams = SingleParam(keyPrefix, string.Empty);
                     break;
 
+                case IAnyOf anyOf:
+                    flatParams = FlattenParamsValue(anyOf.GetValue(), keyPrefix);
+                    break;
+
                 case INestedOptions options:
                     flatParams = FlattenParamsOptions(options, keyPrefix);
                     break;

src/Stripe.net/Infrastructure/JsonConverters/AnyOfConverter.cs

@@ -0,0 +1,106 @@
+namespace Stripe.Infrastructure
+{
+    using System;
+    using System.Linq;
+    using System.Reflection;
+    using Newtonsoft.Json;
+    using Newtonsoft.Json.Linq;
+
+    /// <summary>
+    /// Converts a <see cref="IAnyOf"/> to and from JSON.
+    /// </summary>
+    public class AnyOfConverter : JsonConverter
+    {
+        /// <summary>
+        /// Gets a value indicating whether this <see cref="JsonConverter"/> can write JSON.
+        /// </summary>
+        /// <value>
+        ///     <c>true</c> if this <see cref="JsonConverter"/> can write JSON; otherwise, <c>false</c>.
+        /// </value>
+        public override bool CanWrite => true;
+
+        /// <summary>
+        /// Writes the JSON representation of the object.
+        /// </summary>
+        /// <param name="writer">The <see cref="JsonWriter"/> to write to.</param>
+        /// <param name="value">The value.</param>
+        /// <param name="serializer">The calling serializer.</param>
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            switch (value)
+            {
+                case null:
+                    serializer.Serialize(writer, null);
+                    break;
+
+                case IAnyOf anyOf:
+                    serializer.Serialize(writer, anyOf.GetValue());
+                    break;
+
+                default:
+                    throw new JsonSerializationException(string.Format(
+                        "Unexpected value when converting AnyOf. Expected IAnyOf, got {0}.",
+                        value.GetType()));
+            }
+        }
+
+        /// <summary>
+        /// Reads the JSON representation of the object.
+        /// </summary>
+        /// <param name="reader">The <see cref="JsonReader"/> to read from.</param>
+        /// <param name="objectType">Type of the object.</param>
+        /// <param name="existingValue">The existing value of object being read.</param>
+        /// <param name="serializer">The calling serializer.</param>
+        /// <returns>The object value.</returns>
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            if (reader.TokenType == JsonToken.Null)
+            {
+                return null;
+            }
+
+            object o = null;
+
+            // Try to deserialize with each possible type
+            var jToken = JToken.Load(reader);
+            foreach (var type in objectType.GenericTypeArguments)
+            {
+                try
+                {
+                    using (var subReader = jToken.CreateReader())
+                    {
+                        o = serializer.Deserialize(subReader, type);
+                    }
+
+                    // If deserialization succeeds, stop
+                    break;
+                }
+                catch (JsonException)
+                {
+                    // Do nothing, just try the next type
+                }
+            }
+
+            if (o == null)
+            {
+                throw new JsonSerializationException(string.Format(
+                    "Cannot deserialize the current JSON object into any of the expected types ({0}).",
+                    string.Join(", ", objectType.GenericTypeArguments.Select(t => t.FullName))));
+            }
+
+            return Activator.CreateInstance(objectType, o);
+        }
+
+        /// <summary>
+        /// Determines whether this instance can convert the specified object type.
+        /// </summary>
+        /// <param name="objectType">Type of the object.</param>
+        /// <returns>
+        ///     <c>true</c> if this instance can convert the specified object type; otherwise, <c>false</c>.
+        /// </returns>
+        public override bool CanConvert(Type objectType)
+        {
+            return typeof(IAnyOf).GetTypeInfo().IsAssignableFrom(objectType.GetTypeInfo());
+        }
+    }
+}

src/Stripe.net/Services/Account/AccountSharedOptions.cs

@@ -1,6 +1,5 @@
 namespace Stripe
 {
-    using System;
     using System.Collections.Generic;
     using Newtonsoft.Json;
     using Stripe.Infrastructure;
@@ -25,14 +24,22 @@ public abstract class AccountSharedOptions : BaseOptions
         [JsonProperty("email")]
         public string Email { get; set; }
 
+        /// <summary>
+        /// <para>
+        /// A card or bank account to attach to the account. You can provide either a token, like
+        /// the ones returned by <a href="https://stripe.com/docs/stripe.js">Stripe.js</a>, or a
+        /// <see cref="AccountBankAccountOptions"/> or <see cref="AccountCardOptions"/> instance.
+        /// </para>
+        /// <para>
+        /// By default, providing an external account sets it as the new default external account
+        /// for its currency, and deletes the old default if one exists. To add additional external
+        /// accounts without replacing the existing default for the currency, use the bank account
+        /// or card creation API.
+        /// </para>
+        /// </summary>
         [JsonProperty("external_account")]
-        public string ExternalAccountId { get; set; }
-
-        [JsonProperty("external_account")]
-        public AccountCardOptions ExternalCardAccount { get; set; }
-
-        [JsonProperty("external_account")]
-        public AccountBankAccountOptions ExternalBankAccount { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, AccountBankAccountOptions, AccountCardOptions> ExternalAccount { get; set; }
 
         [JsonProperty("metadata")]
         public Dictionary<string, string> Metadata { get; set; }

src/Stripe.net/Services/BalanceTransactions/BalanceTransactionListOptions.cs

@@ -2,14 +2,17 @@ namespace Stripe
 {
     using System;
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class BalanceTransactionListOptions : ListOptionsWithCreated
     {
+        /// <summary>
+        /// A filter on the list based on the object available_on field. The value can be a
+        /// <see cref="DateTime"/> or a <see cref="DateRangeOptions"/>.
+        /// </summary>
         [JsonProperty("available_on")]
-        public DateTime? AvailableOn { get; set; }
-
-        [JsonProperty("available_on")]
-        public DateRangeOptions AvailableOnRange { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<DateTime?, DateRangeOptions> AvailableOn { get; set; }
 
         [JsonProperty("currency")]
         public string Currency { get; set; }

src/Stripe.net/Services/BankAccounts/BankAccountCreateOptions.cs

@@ -1,13 +1,18 @@
 namespace Stripe
 {
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class BankAccountCreateOptions : BaseOptions
     {
+        /// <summary>
+        /// REQUIRED. Either a token, like the ones returned by
+        /// <a href="https://stripe.com/docs/stripe.js">Stripe.js</a>, or a
+        /// <see cref="SourceBankAccount"/> instance containing a user’s bank account
+        /// details.
+        /// </summary>
         [JsonProperty("source")]
-        public string SourceToken { get; set; }
-
-        [JsonProperty("source")]
-        public SourceBankAccount SourceBankAccount { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, SourceBankAccount> Source { get; set; }
     }
 }

src/Stripe.net/Services/Cards/CardCreateOptions.cs

@@ -1,8 +1,8 @@
 namespace Stripe
 {
-    using System;
     using System.Collections.Generic;
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class CardCreateOptions : BaseOptions
     {
@@ -12,11 +12,15 @@ public class CardCreateOptions : BaseOptions
         [JsonProperty("metadata")]
         public Dictionary<string, string> Metadata { get; set; }
 
+        /// <summary>
+        /// REQUIRED. Either a token, like the ones returned by
+        /// <a href="https://stripe.com/docs/stripe.js">Stripe.js</a>, or a
+        /// <see cref="CardCreateNestedOptions"/> instance containing a user’s card
+        /// details.
+        /// </summary>
         [JsonProperty("source")]
-        public string SourceToken { get; set; }
-
-        [JsonProperty("source")]
-        public CardCreateNestedOptions SourceCard { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, CardCreateNestedOptions> Source { get; set; }
 
         [JsonProperty("validate")]
         public bool? Validate { get; set; }

src/Stripe.net/Services/Charges/ChargeCreateOptions.cs

@@ -3,6 +3,7 @@ namespace Stripe
     using System;
     using System.Collections.Generic;
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class ChargeCreateOptions : BaseOptions
     {
@@ -92,11 +93,15 @@ public class ChargeCreateOptions : BaseOptions
         [JsonProperty("customer")]
         public string CustomerId { get; set; }
 
+        /// <summary>
+        /// A payment source to be charged. This can be the ID of a card (i.e., credit or debit
+        /// card), a bank account, a source, a token, or a connected account. For certain
+        /// sources—namely, cards, bank accounts, and attached sources—you must also pass the ID of
+        /// the associated customer.
+        /// </summary>
         [JsonProperty("source")]
-        public string SourceId { get; set; }
-
-        [JsonProperty("source")]
-        public CardCreateNestedOptions SourceCard { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, CardCreateNestedOptions> Source { get; set; }
 
         /// <summary>
         /// An arbitrary string to be displayed on your customer's credit card statement. This may

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

@@ -37,11 +37,23 @@ public class CustomerCreateOptions : BaseOptions
         [JsonProperty("shipping")]
         public ShippingOptions Shipping { get; set; }
 
+        /// <summary>
+        /// The source can either be a Token or a Source, as returned by
+        /// <a href="https://stripe.com/docs/elements">Elements</a>, or a
+        /// <see cref="CardCreateNestedOptions"/> containing a user’s credit card details. You must
+        /// provide a source if the customer does not already have a valid source attached, and you
+        /// are subscribing the customer to be charged automatically for a plan that is not free.
+        /// Passing <c>source</c> will create a new source object, make it the customer default
+        /// source, and delete the old customer default if one exists. If you want to add an
+        /// additional source, instead use
+        /// <see cref="CardService.Create(string, CardCreateOptions, RequestOptions)"/> to add the
+        /// card and then <see cref="CustomerService.Update(string, CustomerUpdateOptions, RequestOptions)"/>
+        /// to set it as the default. Whenever you attach a card to a customer, Stripe will
+        /// automatically validate the card.
+        /// </summary>
         [JsonProperty("source")]
-        public string SourceToken { get; set; }
-
-        [JsonProperty("source")]
-        public CardCreateNestedOptions SourceCard { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, CardCreateNestedOptions> Source { get; set; }
 
         [JsonProperty("tax_info")]
         public CustomerTaxInfoOptions TaxInfo { get; set; }

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

@@ -1,8 +1,8 @@
 namespace Stripe
 {
-    using System;
     using System.Collections.Generic;
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class CustomerUpdateOptions : BaseOptions
     {
@@ -33,11 +33,18 @@ public class CustomerUpdateOptions : BaseOptions
         [JsonProperty("shipping")]
         public ShippingOptions Shipping { get; set; }
 
+        /// <summary>
+        /// A Token’s or a Source’s ID, as returned by
+        /// <a href="https://stripe.com/docs/elements">Elements</a>. Passing <c>source</c> will
+        /// create a new source object, make it the new customer default source, and delete the old\
+        /// customer default if one exists. If you want to add additional sources instead of
+        /// replacing the existing default, use
+        /// <see cref="CardService.Create(string, CardCreateOptions, RequestOptions)"/>. Whenever
+        /// you attach a card to a customer, Stripe will automatically validate the card.
+        /// </summary>
         [JsonProperty("source")]
-        public string SourceToken { get; set; }
-
-        [JsonProperty("source")]
-        public CardCreateNestedOptions SourceCard { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, CardCreateNestedOptions> Source { get; set; }
 
         [JsonProperty("tax_info")]
         public CustomerTaxInfoOptions TaxInfo { get; set; }

src/Stripe.net/Services/ExternalAccounts/ExternalAccountCreateOptions.cs

@@ -2,19 +2,33 @@ namespace Stripe
 {
     using System.Collections.Generic;
     using Newtonsoft.Json;
+    using Stripe.Infrastructure;
 
     public class ExternalAccountCreateOptions : BaseOptions
     {
+        /// <summary>
+        /// REQUIRED. Either a token, like the ones returned by
+        /// <a href="https://stripe.com/docs/stripe.js">Stripe.js</a>, or a
+        /// <see cref="AccountBankAccountOptions"/> instance containing a user’s bank account
+        /// details.
+        /// </summary>
         [JsonProperty("external_account")]
-        public string ExternalAccountTokenId { get; set; }
-
-        [JsonProperty("external_account")]
-        public AccountBankAccountOptions ExternalAccountBankAccount { get; set; }
-
-        [JsonProperty("metadata")]
-        public Dictionary<string, string> Metadata { get; set; }
+        [JsonConverter(typeof(AnyOfConverter))]
+        public AnyOf<string, AccountBankAccountOptions> ExternalAccount { get; set; }
 
+        /// <summary>
+        /// When set to <c>true</c>, or if this is the first external account added in this
+        /// currency, this account becomes the default external account for its currency.
+        /// </summary>
         [JsonProperty("default_for_currency")]
         public bool? DefaultForCurrency { get; set; }
-    }
+
+        /// <summary>
+        /// A set of key-value pairs that you can attach to an external account object. It can be
+        /// useful for storing additional information about the external account in a structured
+        /// format.
+        /// </summary>
+         [JsonProperty("metadata")]
+        public Dictionary<string, string> Metadata { get; set; }
+   }
 }