Using default as a default value for a nullable value type parameter does not assign null

Question

The post default value expressions (C# programming guide) by microsoft explains the usage and behaviour of default(T) . The behaviour was (and still is) as follows:

var x = default(uint?);
Assert.IsNull(x); // ok

uint? y = default;
Assert.IsNull(y); // ok

The post then continues and explains the usage and behaviour of default ( default literal and type inference ) which was introduced in C# 7.1. Before C# 7.1 we did:

void BeforeCsharp7_1(uint? z = default(uint?))
{
   Assert.IsNull(z); // ok
}

With C# 7.1 we can do:

void WithCsharp7_1(uint? z = default)
{
   Assert.IsNull(z); // Fail, expected null but was 0
}

You might be surprised to read that the last snipped will not assert because the parameter z equals 0 . After upgrading my project to C# 7.1 Visual Studio kept telling me that default(uint?) could be simplified to default which then results in this very different behaviour. This must be a bug in Visual Studio.

Is this also a bug in the C# Specification / Roslyn or is this by design?

Answer 1

This was a bug in Roslyn which has now been fixed in VS15.5 / .NET Core SDK 2.1.200 and above. Yes, the default literal should indeed return null for nullable value types.