ASP.NET Core MVC interview questions covering razor views, forms, tag helpers & validation.
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Razor is the view syntax used by ASP.NET Core MVC to combine HTML markup with server-side C# code in .cshtml files. The @ character is used to transition from HTML into C# expressions, statements, directives, and code blocks.
The @model directive declares the type of object that a Razor view expects. Model is the runtime property through which the view accesses the actual instance supplied by the controller.
A Razor code block is written with @{ ... } and allows C# statements to execute while the view is being rendered. Local variables can then be used in the surrounding Razor markup.
Razor supports normal C# conditional statements such as if, else if, else, and switch. They allow the rendered HTML to change according to the ViewModel.
Razor supports standard C# loops such as foreach, for, while, and do-while. A foreach loop is commonly used to render a collection from a ViewModel.
Razor HTML-encodes normal string output by default. This means characters with special meaning in HTML are encoded before being written to the response, helping protect against cross-site scripting when displaying untrusted text.
Html.Raw tells Razor to render a string without HTML encoding. It is appropriate only when the content is known to be trusted or has been sanitized by a suitable HTML sanitization process.
Razor directives configure how a Razor file is compiled or behaves. Common directives include @model, @using, @inject, @section, @namespace, @implements, and @addTagHelper.
The @inject directive can inject a registered service directly into a Razor view. This can be useful for presentation-focused services such as localization or simple formatting helpers.
A Razor section defines named content in a child view that a layout can render at a designated location. Sections are commonly used for page-specific JavaScript or styles.
A layout renders a named section using RenderSectionAsync or RenderSection. Setting required to false allows views that do not define the section to render normally.
The Form Tag Helper generates an HTML form action using MVC routing information. Attributes such as asp-controller, asp-action, asp-route-* and asp-area can describe the target endpoint without hard-coding a URL.
The Input Tag Helper binds an input element to a model property using asp-for. It uses model metadata to generate attributes such as name, id, type, and validation-related metadata.
The Label Tag Helper generates a label associated with a model property. With asp-for, it uses model metadata and creates the correct for attribute that corresponds to the generated input id.
The Validation Message Tag Helper displays validation errors associated with one model property. It uses asp-validation-for to identify the property and reads errors from ModelState.
The Validation Summary Tag Helper displays a collection of validation errors from ModelState. The asp-validation-summary value controls which errors are shown, such as model-level errors or all validation errors.
Data annotations placed on an input model provide validation metadata that MVC can evaluate after model binding. Common attributes include Required, StringLength, MinLength, MaxLength, Range, EmailAddress, Compare, and RegularExpression.
IValidatableObject allows a model to perform custom validation that can involve multiple properties. Its Validate method returns ValidationResult objects after ordinary property validation.
The POST action must rebuild any supporting data needed to redisplay the form before returning the view. A common example is a list of SelectListItem values for a category dropdown.
The Select Tag Helper uses asp-for for the selected model property and asp-items for the available options. The option list is commonly exposed by the ViewModel as IEnumerable<SelectListItem>.
A checkbox can bind to a Boolean property through the Input Tag Helper. MVC's generated markup handles the fact that an unchecked HTML checkbox does not normally submit a value by including the supporting value needed for reliable Boolean binding.
An HTML form that uploads files must use multipart/form-data encoding. MVC can bind an uploaded file to IFormFile or multiple files to a collection of IFormFile.
An action can receive an IFormFile parameter whose name matches the submitted form field. Before storing the file, validate that it exists, enforce an application-defined size limit, validate the allowed content policy, and generate a safe storage name.
Anti-forgery protection helps defend browser-based state-changing requests against Cross-Site Request Forgery (CSRF). The server issues an anti-forgery token associated with the user's request context, and a protected form submission must provide the expected token.
Use strongly typed ViewModels, keep business and database logic out of Razor, and use Tag Helpers for route-aware links and model-aware form controls. Keep views focused on presentation, reuse layouts, partials, and View Components appropriately, and preserve Razor's default HTML encoding.