Roslyn: Generic expression of a derived type cannot be handled by a pattern

[alexwiese]

Version Used:
Microsoft Visual Studio Professional 2017 RC
Version 15.0.26014.0 D15REL
Microsoft .NET Framework
Version 4.6.01586
Visual C# Compiler version 2.0.0.61213

Steps to Reproduce:

public class Packet
{
}

public class KeepalivePacket : Packet
{
}

public void Send<T>(T packet)
    where T : Packet
{
    if (packet is KeepalivePacket keepalive)
    {
        // Do stuff with keepalive
    }

    switch (packet)
    {
        case KeepalivePacket keepalivePacket:
            // Do stuff with keepalivePacket
            break;
    }
}

I get a compilation error for both the if statement and case statement.

CS8121: An expression of type T cannot be handled by a pattern of type KeepalivePacket

The code compiles successfully if the type is _not_ a derived type of Packet (ie. packet is Packet keepalive or packet is object keepalive works fine).

The code compiles without error if I first cast the parameter to object.

if ((object)packet is KeepalivePacket keepalive)
{
    // This works
}

I couldn't find any mention of unsupported pattern matching with generic parameters/variables in the design notes/blog posts.

Is this expected to fail? Does this compilation error need a more informative message?

Roslyn is recommending pattern matching for the following code with IDE0019. Applying the code fix to the following code results in the compilation error above.

var keepalive = packet as KeepalivePacket;
if (keepalive != null)
{
    // Do stuff with keepalive
}

Does IDE0019 need to be updated to not apply when the variable/parameter is generic?

[HaloFour]

That's interesting. If I had to guess type-switch pattern matching might be disabled on generic variables due to having to emit IL that would support both value and reference types. However, in this case at least, T is constrained to a reference type. If intended I do hope that this is a limitation that could be removed in the future as I see your use case as one that wouldn't be all that uncommon.

[alrz]

Oddly enough, it works with constrained generic types,

public class C
{
    public void Send<T, U>(T packet)
        /* where U : class */
        where T : /* class, */ U
    {
        if (packet is U keepalive)
        {
            // Do stuff with keepalive
        }

        switch (packet)
        {
            case U keepalivePacket:
                // Do stuff with keepalivePacket
                break;
        }
    }
}

Uncommenting class constraint on either generic types only causes a different code to be generated, but it compiles anyways.

[gafter]

Looks like a bug.

[gafter]

The reason it doesn’t work is that there is no conversion (explicit or implicit) defined from T to KeepalivePacket. Pattern matching requires such a conversion to exist, as it is defined in terms of the cast operator, which requires a conversion exist. The language specification and compiler agree that no conversion exists

        var keepalive = (KeepalivePacket)packet; // error: Cannot convert type ‘T’ to ‘KeepalivePacket’

However, the compiler does allow the following to compile (because there is no requirement that a conversion exists), and in fact it can return true:

        var isKeepAlive = packet is KeepalivePacket;

It seems strange to me that the language specification is defined such that no (explicit) conversion exists here. We'll look at what we can do about that.

[gafter]

We're not going to do anything about this in C# 7. You'll have to add a cast to your code to work around it.

    if ((Packet)packet is KeepalivePacket keepalive)
    {
        // Do stuff with keepalive
    }

    switch ((Packet)packet)
    {
        case KeepalivePacket keepalivePacket:
            // Do stuff with keepalivePacket
            break;
    }

Once we have recursive patterns, this may be more difficult to work around. Moreover, the awkward language rule that underlies this issue (i.e. that there is no conversion from T to KeepalivePacket) doesn't make a lot of sense.

[gafter]

The "as type" operator has special dispensation for open types. Applying that same dispensation to the "is pattern" operator may make this work.

7.10.11 The as operator

The as operator is used to explicitly convert a value to a given reference type or nullable type. Unlike a cast expression (§7.7.7), the as operator never throws an exception. Instead, if the indicated conversion is not possible, the resulting value is null.

In an operation of the form E as T, E must be an expression and T must be a reference type, a type parameter known to be a reference type, or a nullable type. Furthermore, at least one of the following must be true, or otherwise a compile-time error occurs:

• An identity (§6.1.1), implicit nullable (§6.1.4), implicit reference (§6.1.6), boxing (§6.1.7), explicit nullable (§6.2.3), explicit reference (§6.2.4), or unboxing (§6.2.5) conversion exists from E to T.
• The type of E or T is an open type.
• E is the null literal.

[simonbuchan]

It seems strange to me that the language specification is defined such that no (explicit) conversion exists here. We'll look at what we can do about that.

@gafter Did you find a design note or something about this? It's quite surprising that (T)e errors in the case where the type of e and T are known to share a base, but I suppose it doesn't come up often enough to warrant the spec + compiler special-casing it.

[MichaelPuckett2]

Having the same problem, different code. Visual Studio tells me it can be simplified and simplifies it for me, but I get the error as well.

[fanoI]

@gafter so this fix will be in C# 7.1? But how the update of the compiler work? We will get an update of VS2017?

[gafter]

@fanol Yes, we're hoping to include this feature with C# 7.1, which we expect to ship as an update of VS2017.

[nvmkpk]

If it takes longer to fix this, please update the analyzer to not suggest using pattern matching in this case.