Contrast that with scenarios for using DisallowNull : You use this attribute to specify that an argument of a nullable reference type shouldn't be null. Consider a property where null is the default value, but clients can only set it to a non-null value.
Consider the following code:. The preceding code is the best way to express your design that the ReviewComment could be null , but can't be set to null. Once this code is nullable aware, you can express this concept more clearly to callers using the System. DisallowNullAttribute :. In a nullable context, the ReviewComment get accessor could return the default value of null.
The compiler warns that it must be checked before access. Furthermore, it warns callers that, even though it could be null , callers shouldn't explicitly set it to null. The DisallowNull attribute also specifies a pre-condition , it doesn't affect the get accessor. You use the DisallowNull attribute when you observe these characteristics about:.
These situations are common in code that was originally null oblivious. It may be that object properties are set in two distinct initialization operations. It may be that some properties are set only after some asynchronous work has completed.
The AllowNull and DisallowNull attributes enable you to specify that preconditions on variables may not match the nullable annotations on those variables.
These provide more detail about the characteristics of your API. This additional information helps callers use your API correctly. Remember you specify preconditions using the following attributes:. You've likely written a method like this to return null when the name sought wasn't found. The null clearly indicates that the record wasn't found. In this example, you'd likely change the return type from Customer to Customer? Declaring the return value as a nullable reference type specifies the intent of this API clearly:.
For reasons covered under Generics nullability that technique may not produce the static analysis that matches your API. You may have a generic method that follows a similar pattern:. The method returns null when the sought item isn't found. You can clarify that the method returns null when an item isn't found by adding the MaybeNull annotation to the method return:. The preceding code informs callers that the return value may actually be null.
It also informs the compiler that the method may return a null expression even though the type is non-nullable. When you have a generic method that returns an instance of its type parameter, T , you can express that it never returns null by using the NotNull attribute. You can also specify that a return value or an argument isn't null even though the type is a nullable reference type.
The following method is a helper method that throws if its first argument is null :. After enabling null reference types, you want to ensure that the preceding code compiles without warnings. When the method returns, the value parameter is guaranteed to be not null. However, it's acceptable to call ThrowWhenNull with a null reference. You can make value a nullable reference type, and add the NotNull post-condition to the parameter declaration:.
The preceding code expresses the existing contract clearly: Callers can pass a variable with the null value, but the argument is guaranteed to never be null if the method returns without throwing an exception. You're likely familiar with the string method String. IsNullOrEmpty String. This method returns true when the argument is null or an empty string. Feedback will be sent to Microsoft: By pressing the submit button, your feedback will be used to improve Microsoft products and services.
Privacy policy. You declare a variable to be a nullable reference type by appending a? For example, string? You can use these new types to more clearly express your design intent: some variables must always have a value , others may be missing a value. You'll need to set up your machine to run. NET Core, including the C 8. The C 8. NET Core 3. This tutorial assumes you're familiar with C and. NET, including either Visual Studio or the. In this tutorial, you'll build a library that models running a survey.
The code uses both nullable reference types and non-nullable reference types to represent the real-world concepts. The survey questions can never be null. A respondent might prefer not to answer a question. The responses might be null in this case. The code you'll write for this sample expresses that intent, and the compiler enforces that intent. Create a new console application either in Visual Studio or from the command line using dotnet new console.
Name the application NullableIntroduction. Once you've created the application, you'll need to specify that the entire project compiles in an enabled nullable annotation context.
Open the. Set its value to enable. You must opt into the nullable reference types feature, even in C 8. That's because once the feature is turned on, existing reference variable declarations become non-nullable reference types. Value gets the value of an underlying type if HasValue is true. The following example uses the HasValue property to test whether the variable contains a value before displaying it:. You can also compare a variable of a nullable value type with null instead of using the HasValue property, as the following example shows:.
If you want to assign a value of a nullable value type to a non-nullable value type variable, you might need to specify the value to be assigned in place of null. Use the null-coalescing operator?? GetValueOrDefault T method for the same purpose :. GetValueOrDefault method. You can also explicitly cast a nullable value type to a non-nullable type, as the following example shows:. At run time, if the value of a nullable value type is null , the explicit cast throws an InvalidOperationException.
A non-nullable value type T is implicitly convertible to the corresponding nullable value type T? The predefined unary and binary operators or any overloaded operators that are supported by a value type T are also supported by the corresponding nullable value type T?
These operators, also known as lifted operators , produce null if one or both operands are null ; otherwise, the operator uses the contained values of its operands to calculate the result. For the bool? For more information, see the Nullable Boolean logical operators section of the Boolean logical operators article. The following example shows that 10 is.
For the inequality operator! In this blog post, I will show you how to use this new language feature and how to achieve similar benefits if you still cannot use.
If our project is targeting. However, when we add? CS The annotation for nullable reference types should only be used in code within a ' nullable' annotations context. Our code will compile but all the rules related to the non-nullable references will be ignored our code will be interpreted as it was before C 8.
Because this feature is a breaking change in the language, it should be explicitly enabled. To test it in a single file, add nullable enable directive. If you are serious about using non-nullable references, I would recommend enabling it for the whole solution by adding Directory. Since now all the references to the reference types are treated as non-nullable unless they are explicitly marked as nullable with?
These new rules are verified by the Roslyn analyzers from Microsoft. CSharp package which is added implicitly to all C projects. There are over 40 rules related to the nullability and the complete list is presented below:. My advice is to go through this list carefully and increase the severity to ERROR level for most of them. Do not forget to add this file to your source control system in order to enforce all these rules across your teammates and CI.
Now you can start configuring severity level for individual rules. After double clicking on your ruleset file, Visual Studio should open it in Ruleset editor. CSharp branch. You can find them all by writing a null keyword in the search field.
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