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!


Comments are closed. Login to edit/delete your existing comments

  • Avatar
    Kyle Brown

    Can you expand on “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”?
    What parts of C# 8 are expected to work on .NET Framework and which aren’t?

    • Phillip Carter
      Phillip CarterMicrosoft employee

      Hey Kyle,

      There are a number of features that won’t work on .NET Framework. First is Async Streams, backed by IAsyncEnumerable, which is unavailable on .NET Framework. Next is ranges, which use the Range and Index types, which are also unavailable on .NET Framework. Most prominently is support for default implementations in interfaces, which is a brand-new runtime concept that is not supported on .NET Framework. And none of the nullable annotations for first-party .NET APIs are available for .NET Framework. Everything else can technically run on .NET Framework, but since that’s only a subset of the language (and missing all the big features), we’re making it clear where the lines are drawn to avoid confusion about why feature X would appear to work, but feature Y would not.

      • Avatar
        Jonas Nyrup

        What features of C#8 will not available on .net core 2.x / .netstandard 2.0?

        • Phillip Carter
          Phillip CarterMicrosoft employee

          Hey Jonas,

          The same as with .NET Framework. Async enumerables, Index and Range support, Default Implementations, and Nullable Annotations in first-party libraries.

  • Avatar
    Tony Henrique

    This is very cool, and also nice to adopt, specially new projects. Will bring much less runtime errors.

  • Avatar
    . .

    thank you for a great article!
    > 1. Signal that if IsNullOrEmpty returns true, then value is non-null
    I think the above sentense should be “Signal that if IsNullOrEmpty returns false, then value is non-null".

      • Avatar
        Viacheslav Ivanov

        Hi, one more type in “If `IsNullOrEmpty` is `true`, it’s safe to “dot” into value“. I think should be “If `IsNullOrEmpty` is `false`, it’s safe to “dot” into value“.

        • Avatar
          Nathan Zahra

          `If IsNullOrEmpty is true, it’s safe to “dot” into value`
          should be
          `If IsNullOrEmpty is false, it’s safe to “dot” into value`

  • Avatar

    Great article, however, what about tuples?
    public async Task<(string, T)> SomeMethodAsync(string x)

    How can I make only the second item in the tuple which is in the Task `MaybeNull`? (async method cannot use an `out` parameter)

  • Avatar
    Tomy Black

    Would it be possible to have not nullable string properties initialized as empty strings on classes, just like int is initialized to 0 by default? So you don’t have initialize every string in the constructor, especially in asp net classes? Or it could be forced in object initializer syntax.

    • Avatar
      Immo LandwerthMicrosoft employee

      You can file the feature request in However, I don’t think the language team would be eager in supporting this. Initializing fields/properties to the empty string requires very few characters and the compiler will warn you if you didn’t initialize non-null members from all the constructors.

  • Олександр Ляхевич
    Олександр Ляхевич

    Great article, thanks!
    I have a question about those attributes. Can I use them somehow, if I have library targeting multiple frameworks? I see that those are available only for dotnetcore 3.0. I hope I won’t need to put each of those to #if precompiler block?

    • Phillip Carter
      Phillip CarterMicrosoft employee


      It’s currently a bit tricky when multitargeting. We’ve updated the specification for .NET Standard 2.1 to include all of the more advanced attributes so that you can share that code across .NET Core and Xamarin. But the .NET Standard 2.0 spec won’t be updated, so if you use that then you won’t have access to these types. Similarly, since they aren’t on .NET Framework, any specific target for net4X will not see these types.

  • Avatar
    x x

    It’s going to be a nightmare to apply all these required attributes and generic constraints in every single class and every single method. The spaghetti code will be harder and harder to read and maintain. Additional 10-20% of time for the project to just fight NRE using language patches – NRE which we already know how to deal with. Also juniors will have difficult time to learn all this. Do not ever remove a way to disable current behavior. I was a big supporter of new features in C# but now thinking more and more about switching back to Java… All the mess with .NET Framework, .NET Core, .NET Standard, Portable .NET, .NET 5 and beyond, multiplatform feature imparity, contradicting ideas every year. You’ll not clean this in next 10 years, I’m worried you’ll only proliferate it further.

    • Avatar
      Immo LandwerthMicrosoft employee

      > It’s going to be a nightmare to apply all these required attributes and generic constraints in every single class and every single method.

      Based on our experience annotating .NET Core’s corlib, I don’t think this will be true. The vast majority of our APIs didn’t need any custom attributes as the types are either fully generic or not-null.

      > Additional 10-20% of time for the project to just fight NRE using language patches – NRE which we already know how to deal with.

      One can certainly make an argument that nullable reference types doesn’t free you from thinking about null references; however, the goal is to make null-states part of the design and thus part of the public API signature. The cost for doing this will certainly not be zero, but the hope is that it’s less time that you — and your consumers — spend today with debugging and fixing null reference exceptions.

  • Avatar
    Wil Wilder Apaza Bustamante

    This is definitely great move probably biggest since LINQ
    I think it’s time to make a revolutionary step after many years of evolution
    MS should develop new language without burden of legacy code compatibility and with full natural support of nullable types whether ref or val
    It should be just <Type> or <Type>? with compilation errors and no special attributes to remember and soft warnings
    Kotlin does it and new C# can do it
    It might be 2 diff languages or compiler settings: 1 for legacy and another for future

  • Avatar
    Ian Good

    Thanks for the writeup!I’m not sure if I missed it in this article, but are there options for enumerable results?For instance, `myList.OfType<Foo>()` will only yield non-null results;`myList.Cast<Foo>()` may yield null elements.

    • Avatar
      Immo LandwerthMicrosoft employee

      We haven’t annotated Linq yet. However, custom attributes cannot be applied to nested generics, so we can’t express that OfType() will never return null values, so it will inherit the nullability of the underlying enumerable.

  • Avatar

    Can you explain why with these changes, this method is still not possible?

    public T? Get<T>() where T : notnull{    return null;}

    • Avatar
      Immo LandwerthMicrosoft employee

      Because T could either be a value type or a referene type. But the representation between the two would be quote different: T? for a value type means that you’re effectively replacing T? with Nullable<T>. But for a reference type the underlying type would just be T.