add data-annotations docs

Author: gregsdennis

.jekyll-metadata

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+---
+layout: page
+title: JSON Schema Generation with System.ComponentModel.DataAnnotations
+bookmark: Data Annotations
+permalink: /schema/schemagen/:title/
+icon: fas fa-tag
+order: "01.5.3"
+---
+The _System.ComponentModel.DataAnnotations_ namespace defines numerous attributes that are commonly used within ASP.Net and other areas to validate data models.
+
+The primary downside to using this approach is that the JSON has to be deserialized into the model before validation can occur.
+
+An alternative is to generate a JSON Schema from the model.  The _JsonSchema.Net.Generation.DataAnnotations_ library is an extension on _JsonSchema.Net.Generation_ to support the _System.ComponentModel.DataAnnotations_ attributes.
+
+> It's important to understand that the DataAnnotations attributes are not as flexible as the attributes defined in the main library.  For advanced schema features, using the main library attributes is imperative.
+{: .prompt-warning}
+
+## Usage
+
+In order to generate constraints from the DataAnnotations attributes, you'll need to first register the handlers.
+
+```c#
+DataAnnotationsSupport.AddDataAnnotations();
+```
+
+## Supported attributes
+
+The following attributes are supported:
+
+|Attribute|Description of support|
+|:-|:-|
+|`[AllowedValues()]`|Produces an `enum` keyword with the indicated values. (.Net 8 only)|
+|`[DeniedValues()]`|Produces a `not: { enum: []}` keyword construct with the indicated values. (.Net 8 only)|
+|`[Base64String]`|Produces a `format: base64` keyword.  Note that this format is defined by OpenAPI but not JSON Schema.|
+|`[EmailAddress]`|Produces a `format: email` keyword.|
+|`[EnumDataType()]`|Produces an `enum` keyword with the values of the C# enum type supplied in the attribute.|
+|`[Length()]`|Produces `minLength` and/or `maxLength` keywords.|
+|`[MaxLength()]`|Produces a `maxLength` keyword.|
+|`[MinLength()]`|Produces a `minLength` keyword.|
+|`[Range()]`|Produces `minimum` and/or `maximum` keywords.|
+|`[RegularExpression()]`|Produces a `pattern` keyword.|
+|`[StringLength()]`|Produces `minLength` and/or `maxLength` keywords.|
+|`[UrlAttribute]`|Produces a `format: uri` keyword.|