Announcing .NET 8 Preview 2

Alexandra Barry

March 14th, 202312 11

We’re excited to share what’s new in .NET 8 Preview 2. This release is a quick follow-up to the larger Preview 1 release. You’ll continue to see many more features show up with these monthly releases. .NET 6 and 7 users will want to follow this release closely. We’ve focused on making it a straightforward upgrade path.

You can download .NET 8 Preview 2 for Linux, macOS, and Windows.

Check out what’s new in ASP.NET Core and EF Core in the Preview 2 release. Stay current with what’s new and coming in What’s New in .NET 8. It will be kept updated throughout the release.

.NET 8 has been tested with 17.6 Preview 2. We recommend that you use the preview channel builds if you want to try .NET 8 with the Visual Studio family of products. Visual Studio for Mac support for .NET 8 isn’t yet available.

Let’s take a look at some new features.

Download .NET 8 Preview 2

Libraries

The new features in Preview 2 are in the libraries.

System.ComponentModel.DataAnnotations Extensions

We’ve introduced extensions to the built-in validation attributes in System.ComponentModel.DataAnnotations.

RequiredAttribute.DisallowAllDefaultValues

The RequiredAttribute now allows validating that structs do not equal their default values. For example:

[Required(DisallowAllDefaultValues = true)]
public Guid MyGuidValue { get; set; }

This example will fail validation if its value equals Guid.Empty.

RangeAttribute exclusive bounds

Users can now specify exclusive bounds in their range validation:

[Range(0d, 1d, MinimumIsExclusive = true, MaximumIsExclusive = true)]
public double Sample { get; set; }

This attribute accepts any values in the open interval but rejects the boundary values 0 and 1.

LengthAttribute

The LengthAttribute can now be used to set both lower and upper bounds for strings or collections:

[Length(10, 20)] // Require at least 10 elements and at most 20 elements.
public ICollection<int> Values { get; set; }

AllowedValuesAttribute and DeniedValuesAttribute

These attributes can be used to specify allow lists and deny lists for validating a property:

[AllowedValues("apple", "banana", "mango")]
public string Fruit { get; set; }

[DeniedValues("pineapple", "anchovy", "broccoli")]
public string PizzaTopping { get; set; }

Base64StringAttribute

As the name suggests, this attribute validates that a given string is a valid Base64 representation.

System.Reflection: introspection support for function pointers

Function pointers were added as part of .NET 5 and C# 9, however, we didn’t add a matching experience for the feature in Reflection. This new feature adds the capability to obtain function pointer metadata via Reflection, including parameter types, the return type and the calling conventions. Previously, the IntPtr type was used for a function pointer type such as with typeof(delegate*<void>()) or when obtaining a function pointer type through reflection such as with FieldInfo.FieldType.

A function pointer instance, which is a physical address to a function, continues to be represented as an IntPtr; only the reflection type was changed with this feature.This new functionality is currently implemented in the CoreCLR runtime and in MetadataLoadContext. Support for the Mono and NativeAOT runtimes are expected later.

An example using reflection:

FieldInfo fieldInfo = typeof(MyClass).GetField(nameof(MyClass._fp));

// Obtain the function pointer type from a field. This used to be the 'IntPtr' type, now it's 'Type':
Type fpType = fieldInfo.FieldType;

// New methods to determine if a type is a function pointer:
Console.WriteLine(fpType.IsFunctionPointer); // True
Console.WriteLine(fpType.IsUnmanagedFunctionPointer); // True

// New methods to obtain the return and parameter types:
Console.WriteLine($"Return type: {fpType.GetFunctionPointerReturnType()}"); // System.Void

foreach (Type parameterType in fpType.GetFunctionPointerParameterTypes())
{
    Console.WriteLine($"Parameter type: {parameterType}"); // System.Int32&
}

// Access to custom modifiers and calling conventions requires a "modified type":
Type modifiedType = fieldInfo.GetModifiedFieldType();

// A modified type forwards most members to its underlying type which can be obtained with Type.UnderlyingSystemType:
Type normalType = modifiedType.UnderlyingSystemType;

// New methods to obtain the calling conventions:
foreach (Type callConv in modifiedType.GetFunctionPointerCallingConventions())
{
    Console.WriteLine($"Calling convention: {callConv}");
    // System.Runtime.CompilerServices.CallConvSuppressGCTransition
    // System.Runtime.CompilerServices.CallConvCdecl
}

// New methods to obtain the custom modifiers:
foreach (Type modreq in modifiedType.GetFunctionPointerParameterTypes()[0].GetRequiredCustomModifiers())
{
    Console.WriteLine($"Required modifier for first parameter: {modreq }");
    // System.Runtime.InteropServices.InAttribute
}

// Sample class that contains a function pointer field:
public unsafe class MyClass
{
    public delegate* unmanaged[Cdecl, SuppressGCTransition]<in int, void> _fp;
}

Parameterized types including generics, pointers, and arrays such as an array of function pointers (e.g. delegate*<void>[]) are supported. Thus, the Type.ElementType property and the Type.GetGenericArguments() method can be used to obtain further types which ultimately may be a function pointer. In addition, a function pointer parameter type is allowed to be another function pointer type.

Closing

.NET 8 Preview 2 offers a short, but exciting array of theme updates, new features, and improvements.

We want to extend our sincere thanks to everyone who has contributed to .NET 8 so far, whether it was through code contributions, bug reports, or providing feedback. Your contributions have been instrumental in the making .NET 8 Previews, and we look forward to continuing to work together to build a brighter future for .NET and the entire technology community.

What are you waiting for? Go see for yourself what’s next in .NET!

Alexandra Barry Product Manager, .NET Team/NuGet