How do you make a tuple or keyvalue pair json serialize like a dictionary in C#

One way to solve this problem is via the usage of ILookup<,> and a custom JsonConverter.

• You can think of the ILookup<T1, T2> as a Dictionary<T1, IEnumerable<T2>>
• So, it is a Bag data structure.

var dataSource = new List<KeyValuePair<string, string>> { new KeyValuePair<string, string>("test", "value1"), new KeyValuePair<string, string>("test", "value2"), new KeyValuePair<string, string>("test", "value3"), new KeyValuePair<string, string>("test2", "somevalue"), }; var toBeSerializedData = dataSource.ToLookup(pair => pair.Key, pair => pair.Value); var serializedData =JsonConvert.SerializeObject(toBeSerializedData); 

This will generate the following json:

[ [ "value1", "value2", "value3" ], [ "somevalue" ] ] 

• As you see the values are grouped by the Keys.
• But the keys are omitted and the values are in arrays.

In order to overcome of these we can define a custom JsonConverter:

public class LookupSerializer : JsonConverter { public override bool CanConvert(Type objectType) => objectType.GetInterfaces() .Any(a => a.IsGenericType && a.GetGenericTypeDefinition() == typeof(ILookup<,>)); public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer) { writer.WriteStartObject(); foreach (object values in (IEnumerable)value) { var keyProp = values.GetType().GetProperty("Key"); var keyValue = keyProp.GetValue(values, null); foreach (var val in (IEnumerable)values) { writer.WritePropertyName(keyValue.ToString()); writer.WriteValue(val.ToString()); } } writer.WriteEndObject(); } public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer) => throw new NotImplementedException(); } 

• CanConvert: restricts the usage only for ILookup.
• WriteJson: iterates through the groups with the outer foreach and iterates through the values via the inner foreach.
• Here we can't use JObject (and its Add method) to create the output because it would fail with an ArgumentException:

Can not add property test to Newtonsoft.Json.Linq.JObject.
Property with the same name already exists on object.

• So, we have to use lower level JsonWriter to construct the output.

If you pass an instance of the LookupSerialzer to the SerializeObject (or register the converter application-wide):

var serializedData =JsonConvert.SerializeObject(toBeSerializedData, new LookupSerializer()); 

then the output will be the one as desired:

{ "test" : "value1", "test" : "value2", "test" : "value3", "test2" : "somevalue" }