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.NET 9 upgrade–notnull constraint

With the Christmas Holidays after us, I finally got some time to start upgrading some older .NET Core applications to .NET 9. First thing I noticed after upgrading is that I got some new warnings related to the ‘notnull’ constraint.

The ‘notnull’ constraint was introduced as part of the nullable reference types feature in C#.  The goal of nullable reference types was to solve the ‘million dollar mistake’ and help you to prevent common issue of null reference exceptions. They allow  to explicitly state whether a reference type variable can be null or not, helping to catch potential null dereferences at compile time.

The feature is in fact a combination of multiple elements all with the goal of avoiding unexpected null reference exceptions:

  • Nullable and Non-Nullable Reference Types:

    • Non-Nullable: If you declare a reference type without the ? suffix (e.g., string name), it is considered non-nullable. The compiler issues warnings if you try to assign a null value to it.

    • Nullable: If you declare a reference type with the ? suffix (e.g., string? name), it can hold a null value. The compiler warns you when you might dereference it without a null check.

  • Null-State Analysis: The compiler tracks the null-state of each variable, which can be either not-null or maybe-null. It issues warnings if there's a possibility of a null dereference.

  • Annotations and Attributes: You can use various attributes (like [NotNullWhen(false)]) and annotations to give the compiler more information about the nullability of parameters and return values in methods.

    • As part of this feature a new generic constraint was introduced, the ‘notnull’ constraint. This guarantees that the provided type is a non-nullable type. This can either be a non-nullable reference type or a non-nullable value type.

    Remark: As this feature was introduced already in C# 8.0, I think it is just a coincidence I noticed it only after upgrading to .NET 9. One extra reason to enable the ‘Treat warnings as errors’ flag…

    I got 2 places where I got this warning:

    ModuleExtensions.cs(83,137,83,144): warning CS8714: The type 'T' cannot be used as type parameter 'TKey' in the generic type or method 'Dictionary<TKey, TValue>'. Nullability of type argument 'T' doesn't match 'notnull' constraint.

    NoOpLogger.cs(15,24,15,34): warning CS8633: Nullability in constraints for type parameter 'TState' of method 'NoOpLogger.BeginScope<TState>(TState)' doesn't match the constraints for type parameter 'TState' of interface method 'ILogger.BeginScope<TState>(TState)'. Consider using an explicit interface implementation instead.

    Here is each time the original code and the fix:

    Original code:

    Fix:

    Original code:

    Fix:

    More information

    Nullable reference types - C# | Microsoft Learn

    The history of C# | Microsoft Learn

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