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warning SYSLIB1037: The type '' defines init-only properties, deserialization of which is currently not supported in source generation mode.

2 days ago I introduced the System.Text.Json source generator and showed you how to use it in ASP.NET Core. What I didn’t told you that it didn’t go so smoothly.

If you had a good look at the generated code I shared, you maybe noticed the following lines:

Getter = static (obj) => ((global::eShopExample.Application.Products.Dto.ProductDto)obj).ProductId,
Setter = static (obj, value) => throw new global::System.InvalidOperationException("Deserialization of init-only properties is currently not supported in source generation mode."),
view raw JsonContext.g.cs hosted with ❤ by GitHub

The generated setter on the property will not work and instead throw an InvalidOperationException.

This also appears as a warning during the build process:

CSC : warning SYSLIB1037: The type 'OrderDto' defines init-only properties, deserialization of which is currently not supported in source generation mode.

More information about this warning: https://docs.microsoft.com/en-us/dotnet/fundamentals/syslib-diagnostics/syslib1037

The reason is that my OrderDto is a record type using init-only properties:

public record OrderDto(int Id, DateTime OrderDate, DateTime? ShippedDate);
view raw OrderDto.cs hosted with ❤ by GitHub
public record ProductDto
{
public ProductDto()
{
}
public int ProductId { get; init; }
public string ProductName { get; init; }
}
view raw ProductDto.cs hosted with ❤ by GitHub

There is not much we can do to fix this else then switching to mutable properties:

public record OrderDto{
public int Id {get;set}= default!;
public DateTime OrderDate {get;set;}= default!;
public DateTime? ShippedDate {get;set;}=default!;
}
view raw OrderDto.cs hosted with ❤ by GitHub

I searched around further on the web about this issue and found the following related Github issues:

The good news that Microsoft is looking into this issue and that maybe a solution will be provided in .NET 7.

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