Move field type resolving logic to separate class

src/Protobuf.System.Text.Json/FieldTypeResolver.cs

@@ -0,0 +1,64 @@
+using Google.Protobuf.Reflection;
+using Google.Protobuf.WellKnownTypes;
+using Type = System.Type;
+
+namespace Protobuf.System.Text.Json;
+
+internal static class FieldTypeResolver
+{
+    public static Type ResolverFieldType(FieldDescriptor fieldDescriptor, Dictionary<string, Type> propertyTypeLookup)
+    {
+        switch (fieldDescriptor.FieldType)
+        {
+            case FieldType.Double:
+                return typeof(double);
+            case FieldType.Float:
+                return typeof(float);
+            case FieldType.Int32:
+            case FieldType.SInt32:
+            case FieldType.SFixed32:
+                return typeof(int);
+            case FieldType.Int64:
+            case FieldType.SInt64:
+            case FieldType.SFixed64:
+                return typeof(long);
+            case FieldType.UInt32:
+            case FieldType.Fixed32:
+                return typeof(uint);
+            case FieldType.UInt64:
+            case FieldType.Fixed64:
+                return typeof(ulong);
+            case FieldType.Bool:
+                return typeof(bool);
+            case FieldType.String:
+                return typeof(string);
+            case FieldType.Message when fieldDescriptor.MessageType.ClrType is { } clrType:
+                if (clrType == typeof(DoubleValue))
+                    return typeof(double?);
+                if (clrType == typeof(FloatValue))
+                    return typeof(float?);
+                if (clrType == typeof(Int64Value))
+                    return typeof(long?);
+                if (clrType == typeof(UInt64Value))
+                    return typeof(ulong?);
+                if (clrType == typeof(Int32Value))
+                    return typeof(int?);
+                if (clrType == typeof(UInt32Value))
+                    return typeof(uint?);
+                if (clrType == typeof(BoolValue))
+                    return typeof(bool?);
+                if (clrType == typeof(StringValue))
+                    return typeof(string);
+                return clrType;
+            case FieldType.Enum when fieldDescriptor.IsRepeated:
+                var fieldType = propertyTypeLookup[fieldDescriptor.PropertyName];
+                return fieldType.GenericTypeArguments[0];
+            case FieldType.Enum:
+            case FieldType.Message:
+                return propertyTypeLookup[fieldDescriptor.PropertyName];
+            default:
+                throw new ArgumentOutOfRangeException(nameof(fieldDescriptor),
+                    $"FieldType: '{fieldDescriptor}' is not supported.");
+        }
+    }
+}

src/Protobuf.System.Text.Json/ProtobufConverter.cs

@@ -3,7 +3,6 @@
 using System.Text.Json.Serialization;
 using Google.Protobuf;
 using Google.Protobuf.Reflection;
-using Google.Protobuf.WellKnownTypes;
 using Protobuf.System.Text.Json.InternalConverters;
 using Type = System.Type;
 
@@ -33,7 +32,7 @@ public ProtobufConverter(JsonSerializerOptions jsonSerializerOptions, JsonProtob
             Accessor = fieldDescriptor.Accessor,
             IsRepeated = fieldDescriptor.IsRepeated,
             IsMap = fieldDescriptor.IsMap,
-            FieldType = GetFieldType(fieldDescriptor, propertyTypeLookup),
+            FieldType = FieldTypeResolver.ResolverFieldType(fieldDescriptor, propertyTypeLookup),
             JsonName = convertNameFunc(fieldDescriptor),
             IsOneOf = fieldDescriptor.ContainingOneof != null
         }).ToArray();
@@ -142,59 +141,4 @@ public override void Write(Utf8JsonWriter writer, T? value, JsonSerializerOption
 
         writer.WriteEndObject();
     }
-
-    private Type GetFieldType(FieldDescriptor fieldDescriptor, Dictionary<string, Type> propertyTypeLookup)
-    {
-        switch (fieldDescriptor.FieldType)
-        {
-            case FieldType.Double:
-                return typeof(double);
-            case FieldType.Float:
-                return typeof(float);
-            case FieldType.Int32:
-            case FieldType.SInt32:
-            case FieldType.SFixed32:
-                return typeof(int);
-            case FieldType.Int64:
-            case FieldType.SInt64:
-            case FieldType.SFixed64:
-                return typeof(long);
-            case FieldType.UInt32:
-            case FieldType.Fixed32:
-                return typeof(uint);
-            case FieldType.UInt64:
-            case FieldType.Fixed64:
-                return typeof(ulong);
-            case FieldType.Bool:
-                return typeof(bool);
-            case FieldType.String:
-                return typeof(string);
-            case FieldType.Message when fieldDescriptor.MessageType.ClrType is { } clrType:
-                if (clrType == typeof(DoubleValue))
-                    return typeof(double?);
-                if (clrType == typeof(FloatValue))
-                    return typeof(float?);
-                if (clrType == typeof(Int64Value))
-                    return typeof(long?);
-                if (clrType == typeof(UInt64Value))
-                    return typeof(ulong?);
-                if (clrType == typeof(Int32Value))
-                    return typeof(int?);
-                if (clrType == typeof(UInt32Value))
-                    return typeof(uint?);
-                if (clrType == typeof(BoolValue))
-                    return typeof(bool?);
-                if (clrType == typeof(StringValue))
-                    return typeof(string);
-                return clrType;
-            case FieldType.Enum when fieldDescriptor.IsRepeated:
-                var fieldType = propertyTypeLookup[fieldDescriptor.PropertyName];
-                return fieldType.GenericTypeArguments[0];
-            case FieldType.Enum:
-            case FieldType.Message:
-                return propertyTypeLookup[fieldDescriptor.PropertyName];
-            default:
-                throw new ArgumentOutOfRangeException(nameof(fieldDescriptor), $"FieldType: '{fieldDescriptor}' is not supported.");
-        }
-    }
 }