When you create an attribute class, by default, C will allow you to use that attribute on any of the possible attribute targets.
If you want to restrict your attribute to certain targets, you can do so by using the AttributeUsageAttribute on your attribute class. That's right, an attribute on an attribute! If you attempt to put the above attribute on something that's not a class or a struct, you will get a compiler error like Attribute 'MyAttributeForClassAndStructOnly' is not valid on this declaration type. It is only valid on 'class, struct' declarations.
To find and act on attributes, Reflection is generally needed. I won't cover Reflection in-depth in this tutorial, but the basic idea is that Reflection allows you to write code in C that examines other code. For instance, you can use Reflection to get information about a class add using System. Reflection; at the head of your code :. This will return a collection of Attribute objects. You can also use GetCustomAttribute and specify an Attribute type.
Try adding other attributes to MyClass. It's important to note that these Attribute objects are instantiated lazily. They are also instantiated each time. Attributes are used by many tools and frameworks. PostSharp uses the attribute syntax to allow aspect-oriented programming in C. This one was used in the above examples, and it lives in the System namespace. It is useful to provide declarative documentation about a changing code base. A message can be provided in the form of a string, and another boolean parameter can be used to escalate from a compiler warning to a compiler error.
This attribute is in the System. Diagnostics namespace. This attribute can be applied to methods or attribute classes. You must pass a string to the constructor. Learn more. FromBody attribute Ask Question. Asked 3 years, 5 months ago.
Active 2 years, 1 month ago. Viewed 18k times. Kiran Shahi 6, 6 6 gold badges 33 33 silver badges 65 65 bronze badges. You're telling the model binder where to find the User object. It's not necessarily in the body — 3dd. Add a comment. Active Oldest Votes. But post by default send the data in the body of the request so why using [FormBody]? If you wanted to read this then the model would be bound using the FromForm attribute see docs. If you do not want to add [FromBody] , you could try Form Values binding and send request from Postman like below: [FromBody] will override default data source and specify the model binder's data source from the request body.
CodeAnalysis to use this and other attributes discussed in this article. The attribute is applied to the property, not the set accessor. The AllowNull attribute specifies pre-conditions , and only applies to arguments. The get accessor has a return value, but no parameters. Therefore, the AllowNull attribute only applies to the set accessor. The preceding example demonstrates what to look for when adding the AllowNull attribute on an argument:.
Most often you'll need this attribute for properties, or in , out , and ref arguments. The AllowNull attribute is the best choice when a variable is typically non-null, but you need to allow null as a precondition. 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 :.
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