.NET: How to Configure JsonSerializerOptions Correctly in a Builder ServiceCollection

Recently saw an issue in one of our applications that Configures JsonSerializerOptions as part of the builders ServiceCollection (FunctionsApplicationBuilder, WebApplicationBuilder etc). 

To give a minimal example, say we want to configure JsonSerializerOptions.WriteIndented to true (in configuration it's false by default).

FunctionsApplicationBuilder builder = FunctionsApplication.CreateBuilder(args);
 
builder.Services.Configure<JsonSerializerOptions>(options =>
{
    options = new JsonSerializerOptions
    {
        WriteIndented = true,
    };
});

Above seems correct and could be easily unnoticeable in reviewing a PR.

But above is actually incorrect. If you resolve the JsonSerializerOptions and inspect the value of WriteIndented, it would still be false.

JsonSerializerOptions configuredJsonSerializerOptions = builder.Services.BuildServiceProvider().GetService<IOptions<JsonSerializerOptions>>().Value;
 
// This will be false.
bool isWriteIndented = configuredJsonSerializerOptions.WriteIndented;

The correct way is,

builder.Services.Configure<JsonSerializerOptions>(options =>
{
    options.WriteIndented = true;
});

That is rather than initializing a new JsonSerializerOptions with required configurations, update the existing JsonSerializerOptions that was supplied in the delegate.

Now JsonSerializerOptions will have the correct value.

JsonSerializerOptions configuredJsonSerializerOptions = builder.Services.BuildServiceProvider().GetService<IOptions<JsonSerializerOptions>>().Value;
 
// This will be true.
bool isWriteIndented = configuredJsonSerializerOptions.WriteIndented;

This is a simple mistake that could potentially cause a lot of issues, so hope this helps.

Happy Coding.

Regards,

Jaliya