diff --git a/generator/plugins/dotnet/custom/CustomArrayConverter.cs b/generator/plugins/dotnet/custom/CustomArrayConverter.cs
new file mode 100644
index 0000000..d34eb62
--- /dev/null
+++ b/generator/plugins/dotnet/custom/CustomArrayConverter.cs
@@ -0,0 +1,42 @@
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+using System.Collections.Immutable;
+
+public class CustomArrayConverter<T> : JsonConverter<ImmutableArray<T>>
+{
+    public override ImmutableArray<T> ReadJson(JsonReader reader, Type objectType, ImmutableArray<T> existingValue, bool hasExistingValue, JsonSerializer serializer)
+    {
+        if (reader.TokenType == JsonToken.Null)
+        {
+            return default(ImmutableArray<T>);
+        }
+
+        JArray array = JArray.Load(reader);
+        ImmutableArray<T>.Builder builder = ImmutableArray.CreateBuilder<T>();
+
+        for (int i = 0; i < array.Count; i++)
+        {
+            builder.Add((T)array[i].ToObject(typeof(T))!);
+        }
+
+        return builder.ToImmutable();
+
+    }
+
+    public override void WriteJson(JsonWriter writer, ImmutableArray<T> value, JsonSerializer serializer)
+    {
+        if (value.IsDefault)
+        {
+            writer.WriteNull();
+        }
+        else
+        {
+            writer.WriteStartArray();
+            foreach (var item in value)
+            {
+                serializer.Serialize(writer, item);
+            }
+            writer.WriteEndArray();
+        }
+    }
+}
\ No newline at end of file
diff --git a/generator/plugins/dotnet/custom/CustomObjectConverter.cs b/generator/plugins/dotnet/custom/CustomObjectConverter.cs
index 4245c1a..e03b588 100644
--- a/generator/plugins/dotnet/custom/CustomObjectConverter.cs
+++ b/generator/plugins/dotnet/custom/CustomObjectConverter.cs
@@ -9,15 +9,15 @@ public override T ReadJson(JsonReader reader, Type objectType, T? existingValue,
     {
         if (reader.TokenType == JsonToken.Null)
         {
-            return default(T);
+            return default(T)!;
         }
 
         Dictionary<string, object?>? o = serializer.Deserialize<Dictionary<string, object?>>(reader);
         if (o == null)
         {
-            return default(T);
+            return default(T)!;
         }
-        return (T)Activator.CreateInstance(typeof(T), o) ?? default(T);
+        return (T)Activator.CreateInstance(typeof(T), o)! ?? default(T)!;
     }
 
     public override void WriteJson(JsonWriter writer, T? value, JsonSerializer serializer)
diff --git a/generator/plugins/dotnet/custom/DocumentSelectorConverter.cs b/generator/plugins/dotnet/custom/DocumentSelectorConverter.cs
index 55ac9cf..3124416 100644
--- a/generator/plugins/dotnet/custom/DocumentSelectorConverter.cs
+++ b/generator/plugins/dotnet/custom/DocumentSelectorConverter.cs
@@ -20,7 +20,7 @@ public override DocumentSelector ReadJson(JsonReader reader, Type objectType, Do
     {
         if (reader.TokenType == JsonToken.Null)
         {
-            return null;
+            return null!;
         }
         var token = JToken.Load(reader);
         if (token.Type == JTokenType.Array)
diff --git a/generator/plugins/dotnet/custom/OrType.cs b/generator/plugins/dotnet/custom/OrType.cs
index e80a336..2ddf320 100644
--- a/generator/plugins/dotnet/custom/OrType.cs
+++ b/generator/plugins/dotnet/custom/OrType.cs
@@ -23,12 +23,12 @@ public OrType(U u)
         return obj.Value is T x ? x : default;
     }
 
-    public static explicit operator OrType<T, U>(U obj) => obj is null ? null : new OrType<T, U>(obj);
-    public static explicit operator OrType<T, U>(T obj) => obj is null ? null : new OrType<T, U>(obj);
+    public static explicit operator OrType<T, U>(U obj) => obj is null ? null! : new OrType<T, U>(obj);
+    public static explicit operator OrType<T, U>(T obj) => obj is null ? null! : new OrType<T, U>(obj);
 
     public override string ToString()
     {
-        return Value?.ToString();
+        return Value?.ToString()!;
     }
 }
 
@@ -66,15 +66,15 @@ public OrType(V v)
         return obj.Value is V x ? x : default;
     }
 
-    public static explicit operator OrType<T, U, V>(U obj) => obj is null ? null : new OrType<T, U, V>(obj);
+    public static explicit operator OrType<T, U, V>(U obj) => obj is null ? null! : new OrType<T, U, V>(obj);
 
-    public static explicit operator OrType<T, U, V>(T obj) => obj is null ? null : new OrType<T, U, V>(obj);
+    public static explicit operator OrType<T, U, V>(T obj) => obj is null ? null! : new OrType<T, U, V>(obj);
 
-    public static explicit operator OrType<T, U, V>(V obj) => obj is null ? null : new OrType<T, U, V>(obj);
+    public static explicit operator OrType<T, U, V>(V obj) => obj is null ? null! : new OrType<T, U, V>(obj);
 
     public override string ToString()
     {
-        return Value?.ToString();
+        return Value?.ToString()!;
     }
 }
 
@@ -123,16 +123,16 @@ public OrType(W w)
         return obj.Value is W x ? x : default;
     }
 
-    public static explicit operator OrType<T, U, V, W>(U obj) => obj is null ? null : new OrType<T, U, V, W>(obj);
+    public static explicit operator OrType<T, U, V, W>(U obj) => obj is null ? null! : new OrType<T, U, V, W>(obj);
 
-    public static explicit operator OrType<T, U, V, W>(T obj) => obj is null ? null : new OrType<T, U, V, W>(obj);
+    public static explicit operator OrType<T, U, V, W>(T obj) => obj is null ? null! : new OrType<T, U, V, W>(obj);
 
-    public static explicit operator OrType<T, U, V, W>(V obj) => obj is null ? null : new OrType<T, U, V, W>(obj);
+    public static explicit operator OrType<T, U, V, W>(V obj) => obj is null ? null! : new OrType<T, U, V, W>(obj);
 
-    public static explicit operator OrType<T, U, V, W>(W obj) => obj is null ? null : new OrType<T, U, V, W>(obj);
+    public static explicit operator OrType<T, U, V, W>(W obj) => obj is null ? null! : new OrType<T, U, V, W>(obj);
 
     public override string ToString()
     {
-        return Value?.ToString();
+        return Value?.ToString()!;
     }
 }
diff --git a/generator/plugins/dotnet/custom/OrTypeArrayConverter.cs b/generator/plugins/dotnet/custom/OrTypeArrayConverter.cs
index 65bb36d..f2dbf23 100644
--- a/generator/plugins/dotnet/custom/OrTypeArrayConverter.cs
+++ b/generator/plugins/dotnet/custom/OrTypeArrayConverter.cs
@@ -1,9 +1,9 @@
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 using System;
+using System.Collections.Immutable;
 
-
-public class OrTypeArrayConverter<T, U> : JsonConverter<OrType<T, U>[]>
+public class OrTypeArrayConverter<T, U> : JsonConverter<ImmutableArray<OrType<T, U>>>
 {
     private OrTypeConverter<T, U> _converter;
 
@@ -12,27 +12,27 @@ public OrTypeArrayConverter()
         _converter = new OrTypeConverter<T, U>();
     }
 
-    public override OrType<T, U>[] ReadJson(JsonReader reader, Type objectType, OrType<T, U>[]? existingValue, bool hasExistingValue, JsonSerializer serializer)
+    public override ImmutableArray<OrType<T, U>> ReadJson(JsonReader reader, Type objectType, ImmutableArray<OrType<T, U>> existingValue, bool hasExistingValue, JsonSerializer serializer)
     {
         if (reader.TokenType == JsonToken.Null)
         {
-            return null;
+            return default(ImmutableArray<OrType<T, U>>);
         }
 
         JArray array = JArray.Load(reader);
-        var result = new OrType<T, U>[array.Count];
+        ImmutableArray<OrType<T, U>>.Builder builder = ImmutableArray.CreateBuilder<OrType<T, U>>();
 
         for (int i = 0; i < array.Count; i++)
         {
-            result[i] = (OrType<T, U>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U>), null, serializer);
+            builder.Add((OrType<T, U>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U>), null, serializer)!);
         }
 
-        return result;
+        return builder.ToImmutable();
     }
 
-    public override void WriteJson(JsonWriter writer, OrType<T, U>[]? value, JsonSerializer serializer)
+    public override void WriteJson(JsonWriter writer, ImmutableArray<OrType<T, U>> value, JsonSerializer serializer)
     {
-        if (value is null)
+        if (value.IsDefault)
         {
             writer.WriteNull();
         }
@@ -49,7 +49,7 @@ public override void WriteJson(JsonWriter writer, OrType<T, U>[]? value, JsonSer
         }
     }
 }
-public class OrTypeArrayConverter<T, U, V> : JsonConverter<OrType<T, U, V>[]>
+public class OrTypeArrayConverter<T, U, V> : JsonConverter<ImmutableArray<OrType<T, U, V>>>
 {
     private OrTypeConverter<T, U, V> _converter;
 
@@ -58,27 +58,27 @@ public OrTypeArrayConverter()
         _converter = new OrTypeConverter<T, U, V>();
     }
 
-    public override OrType<T, U, V>[] ReadJson(JsonReader reader, Type objectType, OrType<T, U, V>[]? existingValue, bool hasExistingValue, JsonSerializer serializer)
+    public override ImmutableArray<OrType<T, U, V>> ReadJson(JsonReader reader, Type objectType, ImmutableArray<OrType<T, U, V>> existingValue, bool hasExistingValue, JsonSerializer serializer)
     {
         if (reader.TokenType == JsonToken.Null)
         {
-            return null;
+            return default(ImmutableArray<OrType<T, U, V>>);
         }
 
         JArray array = JArray.Load(reader);
-        var result = new OrType<T, U, V>[array.Count];
+        ImmutableArray<OrType<T, U, V>>.Builder builder = ImmutableArray.CreateBuilder<OrType<T, U, V>>();
 
         for (int i = 0; i < array.Count; i++)
         {
-            result[i] = (OrType<T, U, V>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U, V>), null, serializer);
+            builder.Add((OrType<T, U, V>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U, V>), null, serializer)!);
         }
 
-        return result;
+        return builder.ToImmutable();
     }
 
-    public override void WriteJson(JsonWriter writer, OrType<T, U, V>[]? value, JsonSerializer serializer)
+    public override void WriteJson(JsonWriter writer, ImmutableArray<OrType<T, U, V>> value, JsonSerializer serializer)
     {
-        if (value is null)
+        if (value.IsDefault)
         {
             writer.WriteNull();
         }
@@ -97,7 +97,7 @@ public override void WriteJson(JsonWriter writer, OrType<T, U, V>[]? value, Json
 }
 
 
-public class OrTypeArrayConverter<T, U, V, W> : JsonConverter<OrType<T, U, V, W>[]>
+public class OrTypeArrayConverter<T, U, V, W> : JsonConverter<ImmutableArray<OrType<T, U, V, W>>>
 {
     private OrTypeConverter<T, U, V, W> _converter;
 
@@ -106,27 +106,27 @@ public OrTypeArrayConverter()
         _converter = new OrTypeConverter<T, U, V, W>();
     }
 
-    public override OrType<T, U, V, W>[] ReadJson(JsonReader reader, Type objectType, OrType<T, U, V, W>[]? existingValue, bool hasExistingValue, JsonSerializer serializer)
+    public override ImmutableArray<OrType<T, U, V, W>> ReadJson(JsonReader reader, Type objectType, ImmutableArray<OrType<T, U, V, W>> existingValue, bool hasExistingValue, JsonSerializer serializer)
     {
         if (reader.TokenType == JsonToken.Null)
         {
-            return null;
+            return default(ImmutableArray<OrType<T, U, V, W>>);
         }
 
         JArray array = JArray.Load(reader);
-        var result = new OrType<T, U, V, W>[array.Count];
+        ImmutableArray<OrType<T, U, V, W>>.Builder builder = ImmutableArray.CreateBuilder<OrType<T, U, V, W>>();
 
         for (int i = 0; i < array.Count; i++)
         {
-            result[i] = (OrType<T, U, V, W>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U, V, W>), null, serializer);
+            builder.Add((OrType<T, U, V, W>)_converter.ReadJson(array[i].CreateReader(), typeof(OrType<T, U, V, W>), null, serializer)!);
         }
 
-        return result;
+        return builder.ToImmutable();
     }
 
-    public override void WriteJson(JsonWriter writer, OrType<T, U, V, W>[]? value, JsonSerializer serializer)
+    public override void WriteJson(JsonWriter writer, ImmutableArray<OrType<T, U, V, W>> value, JsonSerializer serializer)
     {
-        if (value is null)
+        if (value.IsDefault)
         {
             writer.WriteNull();
         }
diff --git a/generator/plugins/dotnet/custom/OrTypeConverter.cs b/generator/plugins/dotnet/custom/OrTypeConverter.cs
index ae3c12a..f2aadbf 100644
--- a/generator/plugins/dotnet/custom/OrTypeConverter.cs
+++ b/generator/plugins/dotnet/custom/OrTypeConverter.cs
@@ -1,6 +1,28 @@
 using Newtonsoft.Json;
 using Newtonsoft.Json.Linq;
 using System;
+using System.Collections.Generic;
+using System.Linq;
+
+internal class OrTypeConverterHelpers
+{
+    public static Type[] SortTypesByHeuristic(Type[] types, JToken jToken)
+    {
+        var typePropertyScores = new Dictionary<Type, int>();
+
+        string[] jTokenPropertyNames = jToken.Children<JProperty>().Select(p => p.Name.ToUpper()).ToArray();
+
+        foreach (Type type in types)
+        {
+            string[] typePropertyNames = type.GetProperties().Select(p => p.Name.ToUpper()).ToArray();
+
+            int score = jTokenPropertyNames.Count(propertyName => typePropertyNames.Contains(propertyName));
+            typePropertyScores[type] = score;
+        }
+
+        return types.OrderByDescending(type => typePropertyScores[type]).ToArray();
+    }
+}
 
 public class OrTypeConverter<T, U> : JsonConverter<OrType<T, U>>
 {
@@ -32,7 +54,8 @@ public class OrTypeConverter<T, U> : JsonConverter<OrType<T, U>>
             return ReadStringToken(reader, serializer, types);
         }
 
-        return OrTypeConverter<T, U>.ReadObjectToken(JToken.Load(reader), serializer, types);
+        var token = JToken.Load(reader);
+        return OrTypeConverter<T, U>.ReadObjectToken(token, serializer, OrTypeConverterHelpers.SortTypesByHeuristic(types, token));
     }
 
     private static OrType<T, U> ReadIntegerToken(JsonReader reader, JsonSerializer serializer, Type[] types)
@@ -93,7 +116,7 @@ private static OrType<T, U> ReadBooleanToken(JsonReader reader, JsonSerializer s
 
     private static OrType<T, U> ReadStringToken(JsonReader reader, JsonSerializer serializer, Type[] types)
     {
-        string str = serializer.Deserialize<string>(reader);
+        string str = serializer.Deserialize<string>(reader)!;
         if (typeof(T) == typeof(string))
         {
             return new OrType<T, U>((T)(object)str);
@@ -198,7 +221,8 @@ public class OrTypeConverter<T, U, V> : JsonConverter<OrType<T, U, V>>
             return ReadStringToken(reader, serializer, types);
         }
 
-        return OrTypeConverter<T, U, V>.ReadObjectToken(JToken.Load(reader), serializer, types);
+        var token = JToken.Load(reader);
+        return OrTypeConverter<T, U, V>.ReadObjectToken(token, serializer, OrTypeConverterHelpers.SortTypesByHeuristic(types, token));
     }
 
     private static OrType<T, U, V> ReadIntegerToken(JsonReader reader, JsonSerializer serializer, Type[] types)
@@ -275,7 +299,7 @@ private static OrType<T, U, V> ReadBooleanToken(JsonReader reader, JsonSerialize
 
     private static OrType<T, U, V> ReadStringToken(JsonReader reader, JsonSerializer serializer, Type[] types)
     {
-        string str = serializer.Deserialize<string>(reader);
+        string str = serializer.Deserialize<string>(reader)!;
         if (typeof(T) == typeof(string))
         {
             return new OrType<T, U, V>((T)(object)str);
@@ -351,7 +375,7 @@ public override void WriteJson(JsonWriter writer, OrType<T, U, V>? value, JsonSe
         }
         else
         {
-            serializer.Serialize(writer, value.Value);
+            serializer.Serialize(writer, value?.Value);
         }
     }
 }
@@ -386,7 +410,8 @@ public class OrTypeConverter<T, U, V, W> : JsonConverter<OrType<T, U, V, W>>
             return ReadStringToken(reader, serializer, types);
         }
 
-        return OrTypeConverter<T, U, V, W>.ReadObjectToken(JToken.Load(reader), serializer, types);
+        var token = JToken.Load(reader);
+        return OrTypeConverter<T, U, V, W>.ReadObjectToken(token, serializer, OrTypeConverterHelpers.SortTypesByHeuristic(types, token));
     }
 
     private static OrType<T, U, V, W> ReadIntegerToken(JsonReader reader, JsonSerializer serializer, Type[] types)
@@ -479,7 +504,7 @@ private static OrType<T, U, V, W> ReadBooleanToken(JsonReader reader, JsonSerial
 
     private static OrType<T, U, V, W> ReadStringToken(JsonReader reader, JsonSerializer serializer, Type[] types)
     {
-        string str = serializer.Deserialize<string>(reader);
+        string str = serializer.Deserialize<string>(reader)!;
         if (typeof(T) == typeof(string))
         {
             return new OrType<T, U, V, W>((T)(object)str);
@@ -564,7 +589,7 @@ public override void WriteJson(JsonWriter writer, OrType<T, U, V, W>? value, Jso
         }
         else
         {
-            serializer.Serialize(writer, value.Value);
+            serializer.Serialize(writer, value?.Value);
         }
     }
-}
+}
\ No newline at end of file
diff --git a/generator/plugins/dotnet/dotnet_classes.py b/generator/plugins/dotnet/dotnet_classes.py
index fce8a9e..0a705cd 100644
--- a/generator/plugins/dotnet/dotnet_classes.py
+++ b/generator/plugins/dotnet/dotnet_classes.py
@@ -24,7 +24,8 @@
     to_upper_camel_case,
 )
 
-CONVERTER_RE = re.compile(r"OrType<(?P<parts>.*)>\[\]")
+ORTYPE_CONVERTER_RE = re.compile(r"OrType<(?P<parts>.*)>")
+IMMUTABLE_ARRAY_CONVERTER_RE = re.compile(r"ImmutableArray<(?P<elements>.*)>")
 
 
 def _get_enum(name: str, spec: model.LSPModel) -> Optional[model.Enum]:
@@ -69,6 +70,16 @@ def lsp_to_base_types(lsp_type: model.BaseType):
     raise ValueError(f"Unknown base type: {lsp_type.name}")
 
 
+def get_types_for_usings(code: List[str]) -> List[str]:
+    immutable = []
+    for line in code:
+        if "ImmutableArray<" in line:
+            immutable.append("ImmutableArray")
+        if "ImmutableDictionary<" in line:
+            immutable.append("ImmutableDictionary")
+    return list(set(immutable))
+
+
 def has_null_base_type(items: List[model.LSP_TYPE_SPEC]) -> bool:
     return any(item.kind == "base" and item.name == "null" for item in items)
 
@@ -96,7 +107,7 @@ def get_type_name(
         else:
             name = get_special_case_class_name(type_def.name)
     elif type_def.kind == "array":
-        name = f"{get_type_name(type_def.element, types, spec, name_context)}[]"
+        name = f"ImmutableArray<{get_type_name(type_def.element, types, spec, name_context)}>"
     elif type_def.kind == "map":
         name = generate_map_type(type_def, types, spec, name_context)
     elif type_def.kind == "base":
@@ -156,7 +167,7 @@ def generate_map_type(
 
     else:
         value_type = get_type_name(type_def.value, types, spec, name_context)
-    return f"Dictionary<{key_type}, {value_type}>"
+    return f"ImmutableDictionary<{key_type}, {value_type}>"
 
 
 def get_converter(type_def: model.LSP_TYPE_SPEC, type_name: str) -> Optional[str]:
@@ -169,7 +180,7 @@ def get_converter(type_def: model.LSP_TYPE_SPEC, type_name: str) -> Optional[str
         return f"[JsonConverter(typeof(CustomStringConverter<{type_def.name}>))]"
     elif type_def.kind == "reference" and type_def.name == "DocumentSelector":
         return "[JsonConverter(typeof(DocumentSelectorConverter))]"
-    elif type_def.kind == "or" and type_name.startswith("OrType<"):
+    elif type_def.kind == "or":
         subset = filter_null_base_type(type_def.items)
         if len(subset) == 1:
             return get_converter(subset[0], type_name)
@@ -177,10 +188,20 @@ def get_converter(type_def: model.LSP_TYPE_SPEC, type_name: str) -> Optional[str
             converter = type_name.replace("OrType<", "OrTypeConverter<")
             return f"[JsonConverter(typeof({converter}))]"
     elif type_def.kind == "array" and type_name.startswith("OrType<"):
-        matches = CONVERTER_RE.match(type_name).groupdict()
+        matches = ORTYPE_CONVERTER_RE.match(type_name).groupdict()
         if "parts" in matches:
             converter = f"OrTypeArrayConverter<{matches['parts']}>"
             return f"[JsonConverter(typeof({converter}))]"
+    elif type_def.kind == "array":
+        matches = IMMUTABLE_ARRAY_CONVERTER_RE.match(type_name).groupdict()
+        elements = matches["elements"]
+        if elements.startswith("OrType<"):
+            matches = ORTYPE_CONVERTER_RE.match(elements).groupdict()
+            converter = f"OrTypeArrayConverter<{matches['parts']}>"
+            return f"[JsonConverter(typeof({converter}))]"
+        else:
+            converter = f"CustomArrayConverter<{elements}>"
+            return f"[JsonConverter(typeof({converter}))]"
     return None
 
 
@@ -202,7 +223,15 @@ def generate_property(
     special_optional = prop_def.type.kind == "or" and has_null_base_type(
         prop_def.type.items
     )
-    optional = "?" if prop_def.optional or special_optional else ""
+    optional = (
+        "?"
+        if (prop_def.optional or special_optional)
+        and not (
+            type_name.startswith("ImmutableArray<")
+            or type_name.startswith("ImmutableDictionary<")
+        )
+        else ""
+    )
     lines = (
         get_doc(prop_def.documentation)
         + generate_extras(prop_def)
@@ -283,7 +312,7 @@ def generate_literal_type(
 
     lines = namespace_wrapper(
         NAMESPACE,
-        get_usings(usings),
+        get_usings(usings + get_types_for_usings(inner)),
         class_wrapper(literal, inner),
     )
     types.add_type_info(literal, literal.name, lines)
@@ -311,7 +340,12 @@ def generate_constructor(
             prop.type.items
         )
         if prop.optional or special_optional:
-            optional_args += [f"{prop_type}? {name} = null"]
+            if prop_type.startswith("ImmutableArray<") or prop_type.startswith(
+                "ImmutableDictionary<"
+            ):
+                optional_args += [f"{prop_type} {name} = default!"]
+            else:
+                optional_args += [f"{prop_type}? {name} = null"]
             ctor_data += [(prop_type, name, True)]
         elif prop.name == "jsonrpc":
             optional_args += [f'{prop_type} {name} = "2.0"']
@@ -347,6 +381,9 @@ def generate_class_from_struct(
     if types.get_by_name(struct.name) or struct.name.startswith("_"):
         return
 
+    if attributes is None:
+        attributes = []
+
     inner = []
     usings = ["DataContract", "JsonConstructor"]
 
@@ -362,7 +399,7 @@ def generate_class_from_struct(
 
     lines = namespace_wrapper(
         NAMESPACE,
-        get_usings(usings),
+        get_usings(usings + get_types_for_usings(inner + attributes)),
         class_wrapper(struct, inner, derived, attributes),
     )
     types.add_type_info(struct, struct.name, lines)
@@ -410,8 +447,9 @@ def generate_type_alias_constructor(
             for t in subset:
                 sub_type = get_type_name(t, types, spec, type_def.name)
                 arg = get_special_case_property_name(to_camel_case(sub_type))
-                if arg.endswith("[]"):
-                    arg = f"{arg[:-2]}s"
+                matches = re.match(r"ImmutableArray<(?P<arg>\w+)>", arg)
+                if matches:
+                    arg = f"{matches['arg']}s"
 
                 constructor += [
                     f"public {type_name}({sub_type} {arg}): base({arg}) {{}}",
@@ -490,7 +528,9 @@ def generate_type_alias_converter(
         "}",
     ]
 
-    code = namespace_wrapper(NAMESPACE, get_usings(["JsonConverter"]), code)
+    code = namespace_wrapper(
+        NAMESPACE, get_usings(["JsonConverter"] + get_types_for_usings(code)), code
+    )
 
     ref = model.Structure(**{"name": converter, "properties": []})
     types.add_type_info(ref, converter, code)
@@ -514,7 +554,7 @@ def generate_class_from_type_alias(
     inner = generate_type_alias_constructor(type_def, spec, types)
     lines = namespace_wrapper(
         NAMESPACE,
-        get_usings(usings),
+        get_usings(usings + get_types_for_usings(inner)),
         class_wrapper(type_def, inner, type_name, class_attributes),
     )
     types.add_type_info(type_def, type_def.name, lines)
@@ -576,9 +616,17 @@ def generate_code_for_variant_struct(
     conditions = []
     for prop, prop_type in zip(struct.properties, prop_types):
         name = get_special_case_property_name(to_camel_case(prop.name))
-        constructor_args += [f"{prop_type}? {name}"]
+        immutable = prop_type.startswith("ImmutableArray<") or prop_type.startswith(
+            "ImmutableDictionary<"
+        )
+        constructor_args += [
+            f"{prop_type} {name}" if immutable else f"{prop_type}? {name}"
+        ]
         ctor_data = [(prop_type)]
-        conditions += [f"({name} is null)"]
+        if immutable:
+            conditions += [f"({name}.IsDefault)"]
+        else:
+            conditions += [f"({name} is null)"]
 
     sig = ", ".join(constructor_args)
     types.add_ctor(struct.name, ctor_data)
@@ -611,7 +659,7 @@ def generate_code_for_variant_struct(
 
     return namespace_wrapper(
         NAMESPACE,
-        get_usings(usings),
+        get_usings(usings + get_types_for_usings(inner)),
         class_wrapper(struct, inner, None),
     )
 
@@ -738,7 +786,7 @@ def generate_request_notification_methods(spec: model.LSPModel, types: TypeData)
 
     lines = namespace_wrapper(
         NAMESPACE,
-        get_usings(["System"]),
+        get_usings(["System"] + get_types_for_usings(inner_lines)),
         ["public static class LSPMethods", "{", *indent_lines(inner_lines), "}"],
     )
     enum_type = model.Enum(
@@ -923,7 +971,7 @@ def generate_all_classes(spec: model.LSPModel, types: TypeData):
             (
                 f"IRequest<{get_type_name(request.params, types, spec)}>"
                 if request.params
-                else "IRequest<LSPAny>"
+                else "IRequest<LSPAny?>"
             ),
             [
                 f"[Direction(MessageDirection.{to_upper_camel_case(request.messageDirection)})]",
diff --git a/generator/plugins/dotnet/dotnet_helpers.py b/generator/plugins/dotnet/dotnet_helpers.py
index 9527142..2eb357a 100644
--- a/generator/plugins/dotnet/dotnet_helpers.py
+++ b/generator/plugins/dotnet/dotnet_helpers.py
@@ -208,4 +208,8 @@ def get_usings(types: List[str]) -> List[str]:
         if t in types:
             usings.append("using System.Collections.Generic;")
 
+    for t in ["ImmutableArray", "ImmutableDictionary"]:
+        if t in types:
+            usings.append("using System.Collections.Immutable;")
+
     return sorted(list(set(usings)))
diff --git a/tests/dotnet/lsprotocol_tests/LSPTests.cs b/tests/dotnet/lsprotocol_tests/LSPTests.cs
index d25c556..80a19ae 100644
--- a/tests/dotnet/lsprotocol_tests/LSPTests.cs
+++ b/tests/dotnet/lsprotocol_tests/LSPTests.cs
@@ -63,7 +63,7 @@ private static void RunTest(bool valid, string data, Type type)
                 JToken token2 = JToken.Parse(newJson);
                 RemoveNullProperties(token1);
                 RemoveNullProperties(token2);
-                Assert.True(JToken.DeepEquals(token1, token2), $"Failed for : {data}");
+                Assert.True(JToken.DeepEquals(token1, token2), $"JSON before and after serialization don't match:\r\nBEFORE:{data}\r\nAFTER:{newJson}");
             }
             catch (Exception e)
             {