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Hello.

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In this video, we're going to talk about Question 17.

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What are attributes, attributes of metadata to a type?

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In other words, there are a way to add information about a type or method to the existing metadata,

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which describes that type or method, which we can read from the type object.

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First, let's understand what metadata is, generally speaking.

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Metadata is data providing information about other data.

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For example, when working with databases, the data stored inside the database is the actual data.

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While the structure of tables and relations between them is metadata in programming, metadata describes

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types used in the application.

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First, let's consider this simple person class.

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There is a lot of metadata describing this class.

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For example, the metadata contains the information that this class is named person.

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It is public, not static, non-zero, etc. It contains too good only public properties called name

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and year of birth.

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It has one public constructor taking two three parameters and one taking one parameter.

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The actual data stored in an instance of this class would be the string representing the name and int

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representing the year of birth.

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We can access all the classes metadata using reflection.

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What we learned about in the previous lecture.

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Sometimes we want to add extra metadata to a type or a method, and this is what attributes our form.

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Let's consider the following example You want to have a common way of validating some data in the application.

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No matter the type, we want to be able to specify that some members, for example, 3GHz, must have

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certain length.

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This is what they want.

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I want to create some validating grass that will be able to take objects off any grass and took it for

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any of their properties.

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This validation is required.

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If so, it should check if the values of those properties are valued.

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First, let's write how we would like to use the validator and then we'll implement it.

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All right.

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So what we want to do is to add some more metadata, the name properties in both person and dog types,

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we found there are minimal and maximal lengths.

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This is some extra metadata and to define it, we must use a custom attribute.

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This is how I would like to use it.

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To add an attribute to a member or type, we simply must write its name in the brackets above the type

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or member who wants to add it to.

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As you can see, this attribute will require two parameters.

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Minimal and maximal length.

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Now let's define the string length validate attribute graphs.

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All attributes must derive from the attribute the base class also its name should earn with attribute.

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As you see here, this postfix is omitted when we actually use the attribute.

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Well, so that's it.

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These attribute must only hold the metadata about the minimal and maximal length of this -- property.

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One more thing, though, we can also define what the attribute can be applied to.

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In our case, we want it to be applied to properties.

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To enforce that, we must actually use a built in attribute called attribute usage.

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All right.

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We added this metadata to the person and dog process.

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Now you must implement the validator across.

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This grass simply contains validated method, which can take an object for this object will look for

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its properties.

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Will the three grant validate attributes defined?

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Here we select those properties of this type for which this attribute is defined using the links were

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matured along with attribute is defined method.

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Now we can iterate those properties and for values, check their legs are correct.

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But first, we must make sure that the property is a string.

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If not, we want to throw an exception because it means that a developer others this attribute to a

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different type by mistake.

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Now, when we are sure that the value of the property is string, we can validate it.

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All right.

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Let's see what happens here.

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First of all, I've cast the property value to string.

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I know it's safe because I already checked it here.

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Then I needed to access the attribute object to do so, I called the Get Custom Attributes metric on

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the property.

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It takes the type of the attribute and a Boolean saying if you should also check the attributes that

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have been inherited from the base type.

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This returns a correction and they took the first element from it.

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Here happens the actual validation and based on it, I returned either false or true.

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All right, let's make sure it works.

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The result seems correct.

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The person is valid, but the dog isn't because the property name has only one letter.

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As you can see, attributes can be quite powerful.

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They are widely used in native grasses as well as external libraries.

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For example, if you ever used and you need, you must have used some of its attributes.

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Let's summarize attributes at metadata to a type.

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In other words, they are a way to have information about a type or some of its members to the metadata

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which describes it to add attributes to our member or type.

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We must simply write its name in the brackets above the thing we want to add it to.

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There are plenty of built in attributes in such a standard library, but we can also create attributes

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of our own.

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Simply by creating classes derived from the attribute based grass during the interview, you might be

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asked what this method they call metadata is data describing of our data.

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Like with databases, the metadata is the structure of tables and relations between them in programming.

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Metadata describes types used in the application.

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Who can access it by using reflection and the this way?

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Good information about some type, for example, what methods or what constructors it contains.

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How to define a custom attribute simply by defining a class derived from that attribute based class.

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All right, that's it for this lecture.

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Thanks for watching, and I'll see you in the next one.