How it works
A serializer must decide how JSON property names map to .NET member names. The ordinary JsonSerializer options used in the first call perform case-sensitive matching. The lowercase count does not match Count, so this small model retains the integer property's default value of zero.
The second call explicitly enables case-insensitive matching and reads three. Both calls parse valid JSON; the difference lies in the mapping policy. A valid syntax tree does not guarantee that the intended model members were populated, which is why boundary validation should inspect the resulting domain values.
Naming policy and case-insensitive reading are different options. A camel-case naming policy defines the expected or emitted wire spelling, while case-insensitive matching accepts casing variations during reading. Framework-provided defaults can bundle several options, so inspect the actual configuration instead of assuming every JsonSerializer call uses web-style defaults.
Tolerating case variations may improve compatibility with an established producer, but it can also conceal inconsistent data contracts. If two producers use different names for the same concept, decide whether that is a harmless spelling variation or a schema problem that should be corrected. Compatibility choices should be documented and tested.
Practice with count, Count, COUNT, and an unrelated spelling such as total. Predict which inputs populate Count under each configuration. Add a validation rule requiring a positive count and demonstrate how a missing or unmatched member can be detected after deserialization.
Finally, place the serializer options at a clear application boundary and reuse a consistent configuration rather than creating subtly different policies at many call sites. Tests should include both accepted naming variations and deliberately rejected shapes. A precise wire contract is more valuable than a permissive parser whose defaults silently change the meaning of absent data.
A working example
using System;
using System.Linq;
using System.Collections.Generic;
using System.Globalization;
using System.Text;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Threading;
using System.Threading.Tasks;
using System.Runtime.CompilerServices;
const string json = """{"count":3}""";
Console.WriteLine(JsonSerializer.Deserialize<Counter>(json)!.Count);
var options = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
Console.WriteLine(JsonSerializer.Deserialize<Counter>(json, options)!.Count);
class Counter { public int Count { get; set; } }
Expected output
0 3
Changing case matching does not add domain validation for the resulting value.