Try out Nullable Reference Types

Phillip Carter

Try out Nullable Reference Types

With the release of .NET Core 3.0 Preview 7, C# 8.0 is considered "feature complete". That means that the biggest feature of them all, Nullable Reference Types, is also locked down behavior-wise for the .NET Core release. It will continue to improve after C# 8.0, but it is now considered stable with the rest of C# 8.0.

At this time, our aim is to collect as much feedback about the process of adopting nullability as possible, catch any issues, and collect feedback on further improvements to the feature that we can do after .NET Core 3.0. This is one of the largest features ever built for C#, and although we’ve done our best to get things right, we need your help!

It is at this junction that we especially call upon .NET library authors to try out the feature and begin annotating your libraries. We’d love to hear your feedback and help resolve any issues you come across.

Familiarize yourself with the feature

We recommend reading some of the Nullable Reference Types documentation before getting started with the feature. It covers essentials like:

  • A conceptual overview
  • How to specify a nullable reference type
  • How to control compiler analysis or override compiler analysis

If you’re unfamiliar with these concepts, please give the documentation a quick read before proceeding.

Turn on Nullable Reference Types

The first step in adopting nullability for your library is to turn it on. Here’s how:

Make sure you’re using C# 8.0

If your library explicitly targets netcoreapp3.0, you’ll get C# 8.0 by default. When we ship Preview 8, you’ll get C# 8.0 by default if you target netstandard2.1 too.

.NET Standard itself doesn’t have any nullable annotations yet. If you’re targeting .NET Standard, then you can use multi-targeting for .NET Standard and netcoreapp3.0, even if you don’t need .NET Core specific APIs. The benefit is that the compiler will use the nullable annotations from CoreFX to help you get your own annotations right.

If you cannot update your TFM for some reason, you can set the LangVersion explicitly:


Note that C# 8.0 is not meant for older targets, such as .NET Core 2.x or .NET Framework 4.x. So some additional language features may not work unless you are targeting .NET Core 3.0 or .NET Standard 2.1

From here, we recommend two general approaches to adopting nullability.

Opt in a project, opt out files

This approach is best for projects where you’ll be adding new files over time. The process is straightforward:

  1. Apply the following property to your project file:

  2. Disable nullability in every file for that project by adding this to the top of every existing file in the project:

    #nullable disable
  3. Pick a file, remove the #nullable disable directive, and fix the warnings. Repeat until all #nullable disable directives are gone.

This approach requires a bit more up front work, but it means that you can continue working in your library while you’re porting and ensure that any new files are automatically opted-in to nullability. This is the approach we generally recommend, and we are currently using it in some of our own codebases.

Note that you can also apply the Nullable property to a file if that fits your workflow better.

Opt in files one at a time

This approach is the inverse of the previous one.

  1. Enable nullability in a file for a project by adding this to the top of the file:

    #nullable enable
  2. Continue adding this to files until all files are annotated and all nullability warnings are addressed.

  3. Apply the following property to your project file:

  4. Remove all #nullable enable directives in source.

This approach requires more work at the end, but it allows you to start fixing nullability warnings immediately.

Note that you can also apply the Nullable property to a file if that fits your workflow better.

What’s new in Nullable Reference Types for Preview 7

The most critical additions to the feature are tools for working with generics and more advanced API usage scenarios. These were derived from our experience in beginning to annotate .NET Core.

The notnull generic constraint

It is quite common to intend that a generic type is specifically not allowed to be nullable. For example, given the following interface:

It may be desirable to only allow non-nullable reference and value types. So substituting with string or int should be fine, but substituting with string? or int? should not.

This can be accomplished with the notnull constraint:

This will then generate a warning if any implementing class does not also apply the same notnull constraints:

To fix it, we need to apply the same constraints:

And when creating an instance of that class, if you substitute it with a nullable reference type, a warning will also be generated:

It also works for value types:

This constraint is useful for generic code where you want to ensure that only non-nullable reference types can be used. One prominent example is Dictionary<TKey, TValue, where TKey is now constrained to be notnull, which disallows using null as a key:

However, not all nullability problems with generics can be solved in this way. This is where we’ve added some new attributes to allow you to influence nullable analysis in the compiler.

The issue with T?

So you have have wondered: why not "just" allow T? when specifying a generic type that could be substituted with a nullable reference or value type? The answer is, unfortunately, complicated.

A natural definition of T? would mean, "any nullable type". However, this would imply that T would mean "any non-nullable type", and that is not true! It is possible to substitute a T with a nullable value type today (such as bool?). This is because T is already an unconstrained generic type. This change in semantics would likely be unexpected and cause some grief for the vast amount of existing code that uses T as an unconstrained generic type.

Next, it’s important to note that a nullable reference type is not the same thing as a nullable value type. Nullable value types map to a concrete class type in .NET. So int? is actually Nullable<int>. But for string?, it’s actually the same string but with a compiler-generated attribute annotating it. This is done for backwards compatibility. In other words, string? is kind of a "fake type", whereas int? is not.

This distinction between nullable value types and nullable reference types comes up in a pattern such as this:

void M<T>(T? t) where T: notnull

This would mean that the parameter is the nullable version of T, and T is constrained to be notnull. If T were a string, then the actual signature of M would be M<string>([NullableAttribute] T t), but if T were an int, then M would be M<int>(Nullable<int> t). These two signatures are fundamentally different, and this difference is not reconcilable.

Because of this issue between the concrete representations of nullable reference types and nullable value types, any use of T? must also require you to constrain the T to be either class or struct.

Finally, the existence of a T? that worked for both nullable reference types and nullable value types does not address every issue with generics. You may want to allow for nullable types in a single direction (i.e., as only an input or only an output) and that is not expressible with either notnull nor a T and T? split unless you artificially add separate generic types for inputs and outputs.

Nullable preconditions: AllowNull and DisallowNull

Consider the following example:

This might have been an API that we supported prior to C# 8.0. However, the meaning of string now means non-nullable string! We may wish to actually still allow null values, but always give back some string value with the get. Here’s where AllowNull can come in and let you get fancy:

Since we always make sure that we get no null value with the getter, I’d like the type to remain string. But we want to still accept null values for backwards compatibility. The AllowNull attribute lets you specify that the setter accepts null values. Callers are then affected as you’d expect:

Note: there is currently a bug where assignment of null conflicts with nullable analysis. This will be addressed in a future update of the compiler.

Consider another API:

In this case, MyHandle refers to some handle to a resource. Typical use for this API is that we have a non-null instance that we pass by reference, but when it is cleared, the reference is null. We can get fancy and represent this with DisallowNull:

This will affect any caller by emitting a warning if they pass null, but will warn if you attempt to "dot" into the handle after the method is called:

These two attributes allow us single-direction nullability or non-nullability for those cases where we need them.

More formally:

The AllowNull attribute allows callers to pass null even if the type doesn’t allow it. The DisallowNull attribute disallows callers to pass null even if the type allows it. They can be specified on anything that takes input:

  • Value parameters
  • in parameters
  • ref parameters
  • fields
  • properties
  • indexers

Important: These attributes only affect nullable analysis for the callers of methods that are annotated with them. The bodies of annotated methods and things like interface implementation do not respect these attributes. We may add support for that in the future.

Nullable postconditions: MaybeNull and NotNull

Consider the following example API:

Here we have another problem. We’d like Find to give back default if nothing is found, which is null for reference types. We’d like Resize to accept a possibly null input, but we want to ensure that after Resize is called, the array value passed by reference is always non-null. Again, applying the notnull constraint doesn’t solve this. Uh-oh!

Enter [MaybeNull] and [NotNull]. Now we can get fancy with the nullability of the outputs! We can modify the example as such:

And these can now affect call sites:

The first method specifies that the T that is returned could be a null value. This means that callers of this method must check for null when using its result.

The second method has a trickier signature: [NotNull] ref T[]? array. This means that array could be null as an input, but when Resize is called, array will not be null. This means that if you "dot" into array after calling Resize, you will not get a warning. But after Resize is called, array will no longer be null.

More formally:

The MaybeNull attribute allows for a return type to be null, even if its type doesn’t allow it. The NotNull attribute disallows null results even if the type allows it. They can be specified on anything that produces output:

  • Method returns
  • out parameters (after a method is called)
  • ref parameters (after a method is called)
  • fields
  • properties
  • indexers

Important: These attributes only affect nullable analysis for the callers of methods that are annotated with them. The bodies of annotated methods and things like interface implementation do not respect these attributes. We may add support for that in the future.

Conditional postconditions: MaybeNullWhen(bool) and NotNullWhen(bool)

Consider the following example:

Methods like this are everywhere in .NET, where the return value of true or false corresponds to the nullability (or possible nullability) of a parameter. The MyQueue case is also a bit special, since it’s generic. TryDequeue should give a null for result if the result is false, but only if T is a reference type. If T is a struct, then it won’t be null.

So, we want to do three things:

  1. Signal that if IsNullOrEmpty returns false, then value is non-null
  2. Signal that if TryParse returns true, then version is non-null
  3. Signal that if TryDequeue returns false, then result could be null, provided it’s a reference type

Unfortunately, the C# compiler does not associate the return value of a method with the nullability of one of its parameters! Uh-oh!

Enter NotNullWhen(bool) and MaybeNullWhen(bool). Now we can get even fancier with parameters:

And these can now affect call sites:

This enables callers to work with APIs using the same patterns that they’ve used before, without any spurious warnings from the compiler:

  • If IsNullOrEmpty is true, it’s safe to "dot" into value
  • If TryParse is true, then version was parsed and is safe to "dot" into
  • If TryDequeue is false, then result might be null and a check is needed (example: returning false when the type is a struct is non-null, but false for a reference type means it could be null)

More formally:

The NotNullWhen(bool) signifies that a parameter is not null even if the type allows it, conditional on the bool returned value of the method. The MaybeNullWhen(bool) signifies that a parameter could be null even if the type disallows it, conditional on the bool returned value of the method. They can be specified on any parameter type.

Nullness dependence between inputs and outputs: NotNullIfNotNull(string)

Consider the following example:

In this case, we’d like to return a possibly null string, and we should also be able to accept a null value as input. So the signature accomplishes what I’d like to express.

However, if path is not null, we’d like to ensure that we always give back a string. That is, we want the return value of GetFileName to be non-null, conditional on the nullness of path. There’s no way to express this as-is. Uh-oh!

Enter NotNullIfNotNull(string). This attribute can make your code the fanciest, so use it with care! Here’s how we’ll use it in my API:

And this can now affect call sites:

More formally:

The NotNullIfNotNull(string) attribute signifies that any output value is non-null conditional on the nullability of a given parameter whose name is specified. They can be specified on the following constructs:

  • Method returns
  • ref parameters

Flow attributes: DoesNotReturn and DoesNotReturnIf(bool)

You may work with multiple methods that affect control flow of your program. For example, an exception helper method that will throw an exception if called, or an assertion method that will throw an exception if an input is true or false.

You may wish to do something like assert that a value is non-null, and we think you’d also like it if the compiler could understand that.

Enter DoesNotReturn and DoesNotReturnIf(bool). Here’s an example of how you could use either:

When ThrowArgumentNullException is called in a method, it throws an exception. The DoesNotReturn it is annotated with will signal to the compiler that no nullable analysis needs to happen after that point, since that code would be unreachable.

When MyAssert is called and the condition passed to it is false, it throws an exception. The DoesNotReturnIf(false) that annotates the condition parameter lets the compiler know that program flow will not continue if that condition is false. This is helpful if you want to assert the nullability of a value. In the code path following MyAssert(value != null); the compiler can assume value is not null.

DoesNotReturn can be used on methods. DoesNotReturnIf(bool) can be used on input parameters.

Evolving your annotations

Once you annotate a public API, you’ll want to consider the fact that updating an API can have downstream effects:

  • Adding nullable annotations where there weren’t any may introduce warnings to user code
  • Removing nullable annotations can also introduce warnings (e.g., interface implementation)

Nullable annotations are an integral part of your public API. Adding or removing annotations introduce new warnings. We recommend starting with a preview release where you solicit feedback, with aims to not change any annotations after a full release. This isn’t always going to be possible, but we recommend it nonetheless.

Current status of Microsoft frameworks and libraries

Because Nullable Reference Types are so new, the large majority of Microsoft-authored C# frameworks and libraries have not yet been appropriately annotated.

That said, the "Core Lib" part of .NET Core, which represents about ~20% of the .NET Core shared framework, has been fully updated. It includes namespaces like System, System.IO, and System.Collections.Generic. We’re looking for feedback on our decisions so that we can make appropriate tweaks as soon as possible, and before their usage becomes widespread.

Although there is still ~80% CoreFX to still annotate, the most-used APIs are fully annotated.

Roadmap for Nullable Reference Types

Currently, we view the full Nullable Reference Types experience as being in preview. It’s stable, but the feature involves spreading nullable annotations throughout our own technologies and the greater .NET ecosystem. This will take some time to complete.

That said, we’re encouraging library authors to start annotating their libraries now. The feature will only get better as more libraries adopt nullability, helping .NET become a more null-safe place.

Over the coming year or so, we’re going to continue to improve the feature and spread its use throughout Microsoft frameworks and libraries.

For the language, especially compiler analysis, we’ll be making numerous enhancements so that we can minimize your need to do things like use the null-forgiveness (!) operator. Many of these enhancements are already tracked on the Roslyn repo.

For CoreFX, we’ll be annotating the remaining ~80% of APIs and making appropriate tweaks based on feedback.

For ASP.NET Core and Entity Framework, we’ll be annotating public APIs once some new additions to CoreFX and the compiler are added.

We haven’t yet planned how to annotate WinForms and WPF APIs, but we’d love to hear your feedback on what kinds of things matter!

Finally, we’re going to continue enhancing C# tooling in Visual Studio. We have multiple ideas for features to help using the feature, but we’d love your input as well!

Next steps

If you’re still reading and haven’t tried out the feature in your code, especially your library code, give it a try and please give us feedback on anything you feel ought to be different. The journey to make unanticipated NullReferenceExceptions in .NET go away will be lengthy, but we hope that in the long run, developers simply won’t have to worry about getting bitten by implicit null values anymore. You can help us. Try out the feature and begin annotating your libraries. Feedback on your experience will help shorten that journey.

Cheers, and happy hacking!


Discussion is closed. Login to edit/delete existing comments.

  • MgSam 0

    What has always been missing for me from the inception of doing this in C# has been usage statistics of the non-null features that TypeScript added. In my anecdotal experience, the feature seems sparsely used outside Microsoft. Though that could be completely wrong.  Either way, that data should have been collected to make an informed decision about whether this monumental undertaking was worth the cost. 

  • paul louth 0

    Just installed to continue the migration of language-ext to C#8 nullable refs and had to stop after a minute.  The performance is appalling.  Every key press is taking at least .25 of a second sometimes longer, it’s totally unusable.  A real dissappointment. 

    • Phillip CarterMicrosoft employee 0

      Hey Paul,

      Sorry that you’re seeing such awful perf problems here. Could you file a bug with some diagnostic info over on the Roslyn repo? VS version, compiler version, etc.

      We haven’t seen any perf regressions like you mention in our own codebases (example: .NET project system), but there could be an issue we just haven’t caught yet. Theoretically it shouldn’t have a noticable impact on editor performance since autocompletion is an asynchronous operation, but if there’s an unaccounted for case that your codebase is exercising we’ll get that fixed ASAP.


  • John Landheer 0

    It should now be possible for the runtime to throw an exception upon assignment of NULL to a non-nullable type. Is that an option or would that be too costly?

    • Phillip CarterMicrosoft employee 0

      This would just shift the problem. For backwards compat reasons, we’d still need to allow null assignment, but instead of getting an NRE when you ‘.’ into a type, you’d get a different exception that wasn’t guarded effectively at compile time.

  • Aaron Kunz 0

    Great post with lots of details, the latter maybe being partially the source of some of the worries brought up in the replies. I can understand the thoughts that this feature makes the language a bit more complex and thus less approachable, as not all C# users work with it daily (IT ops, inhouse corp. devs etc.) and keeping all those corner cases with its appropriate “special” attribute in mind can feel overwhelming at first. IMHO “special” attributes (e.g. CAS in the past) always feel like a weird/magic interplay of language/framework/runtime which should present themselves more prominently (e.g. as language keyword, like co/variance did) than hiding as a “normal” attribute in disguise – but that’s obviously just my feeling.All in all I think this is a sensible approach considering such an effort at this point of the ecosystem and widespread usage of the language. I guess most of the time only library devs should be concerned with those corner cases and its attributes and thus the impact for “part time” C# users is hopefully restricted of just thinking more carefully about their nulls (with the compiler’s help) and expressing it with appropriate types.It is a bit like with TypeScript – you can leverage it without knowing all the details of its type system (which IMHO is way more complex than what we are talking about here).

  • Tore Nestenius 0

    When is the support for the generic notnull constraint available? I can’t seem to get it to be accepted in the 16.3.0 preview 2.0 interface IDoStuff<TIn, TOut> where TIn : notnull where TOut : notnull

  • endofunk 0

    Why was the value constraint on Nullable not removed, i.e. to “simply” support both value and reference types?  The issue is that the “fake” nullable boxing and separate need to type constrain value & reference types makes it rather difficult to conform nullable to FP algebras like: monoid, functor, applicative functor, monad, traversable, etc. 
    A good example of this increasing complexity with generic extension methods is a monadic lifter function like LiftM (function application with monadic arguments) with multiple arities (1 to 9, or more).
    public static Maybe<R> LiftM<A, B, C, D, E, F, G, H, I, J, R>(this Func<A, B, C, D, E, F, G, H, I, J, R> @this, Maybe<A> a, Maybe<B> b, Maybe<C> c, Maybe<D> d, Maybe<E> e, Maybe<F> f, Maybe<G> g, Maybe<H> h, Maybe<I> i, Maybe<J> j) => a.FlatMap(xa => b.FlatMap(xb => c.FlatMap(xc => d.FlatMap(xd => e.FlatMap(xe => f.FlatMap(xf => g.FlatMap(xg => h.FlatMap(xh => i.FlatMap(xi => j.FlatMap(xj => Just(@this(xa, xb, xc, xd, xe, xf, xg, xh, xi, xj))))))))))));
    To implement the same functionality for Nullable generic types that can either be constrained to value or reference makes this akwardly type constrained complex to implement.
    The same complexity similarly plays out with implementing extensions for the alternative using the SelectMany function i.e. monadic application using Linq query syntax e.g.
    public static Maybe<R> SelectMany<A, B, R>(this Maybe<A> @this, Func<A, Maybe<B>> fn, Func<A, B, R> select) => @this.FlatMap(a => fn(a).FlatMap(b => select(a, b).ToMaybe()));

    from _1 in …
    from _2 in …

    select f(_1, _2, …)

  • Shawn Clabough 0

    For the “Opt in files one at a time” option, I think you meant for the setting to be <Nullable>disable</Nullable> then use #nullable enable at the top of the files you want to be checked.

    • Phillip CarterMicrosoft employee 0

      Hey Shawn,

      That wouldn’t quite be right, since the preceding step is to address nullability issues in source files before making the project-level change.

      • Shawn Clabough 0

        You have both “Opt in a project, opt out files” and “Opt in files one at a time” specified as <Nullable>enable</Nullable> which for me starts showing warnings for every file. After setting <LangVersion>8.0</LangVersion>, I had to use <Nullable>disable</Nullable> and #nullable enable at the top of files to get it to work one file at a time.

        • Phillip CarterMicrosoft employee 0

          Hey Shawn,

          Not quite. In the post, I mention starting with #nullable enable, then setting the project file once issues are addressed, then removing the file directives. The post doesn’t say to do both at the same time.

          • Shawn Clabough 0

            Ah, thanks. My <Nullable>disable</Nullable> with #nullable enable is accomplishing the same thing as not having the <Nullable> setting in the project. 

  • Viktor Nikolaev 0

    Consider this code:

    public IEnumerable Test(IEnumerable strs)
    return strs.Where(x => !string.IsNullOrEmpty(x));

    After compiling I get such error: [CS8619] Nullability of reference types in value of type ‘IEnumerable<string?>’ doesn’t match target type ‘IEnumerable<string>’

    How can I fix this?

  • bit bonk 0

    Because of the feature, the confidence that something “can never be null here” will rise.
    So developers will be tempted to remove or not add a null check (if (thing != null), ArgumentNullException etc.).

    1. Library authors might be tempted to remove all null checks for non-nullable types in internal code and just guard agains null at the public API level.

    2. Application developers might be tempted to not guard against null because an API clearly states that its output is never intended to be null.

    So what would be your guidance on how to deal with null guards in this new world?

    I think it would be helpful to add some guidance about this to the documentation. I have raised an issue here:

    • Phillip CarterMicrosoft employee 0

      Generally I don’t think we’re quite in the business of developing strong guidance just yet, especially since the feature is still opt-in and has not percolated throughout the .NET ecosystem. We have developed guidance for CoreFX developers and I believe that much of this will also hold for many .NET library authors. However, the way we consider development in CoreFX is generally far different than app developers and some library developers. I imagine we’ll have more written guidance after the feature has been more broadly adopted outside of Microsoft and we’ve gotten enough feedback about how it “feels” from people to start collaborating on official guidance.

  • Francesco Pretto 0

    Why such important feature as Nullable Reference Types has no GUI support in the IDE yet? It gives the feeling that you don’t want strong adoption, for now.

    • Phillip CarterMicrosoft employee 0

      Hey Francesco,

      There has been a lot of work done in the VS 16.3 release to show annotations in tooling, and it’s continually improving. Is that what you’re referring to, or are you looking for something else when you say GUI support?

      • Francesco Pretto 0

        Hey Philip,

        thank you for the prompt answer. I have to admit that I haven’t tried recent VS 2019 builds, so I can’t say now for sure if my concern is still valid. In this article you still recommend to edit the project file that to enable per project Nullable Reference Types. This would probably translate to a regular checkbox in the Project Preferences UI, which wasn’t present back then when I first tried VS2019. Have this UI support been added or people are still supposed to edit the file manually to enable the feature?

        • Phillip CarterMicrosoft employee 0

          Currently you should edit your project file to enable this. There may be support in the UI in the future.

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