How to Restrict User to Put HTML Tag into TextBox with mvc5.

you can use the built-in validation features provided by the framework. One approach is to use the [AllowHtml] attribute to explicitly allow HTML input for specific properties in your model, while disallowing it for others.

Here's an example of how you can achieve this:

public class MyModel
{
    [DataType(DataType.MultilineText)]
    [DisallowHtml]
    public string Description { get; set; }
}

@model MyModel

@Html.TextAreaFor(m => m.Description)

1. In your model class, decorate the property that should not allow HTML tags with the [DataType(DataType.MultilineText)] attribute. This attribute is used to specify that the property is a multiline text input.
2. In your view, use the Html.TextAreaFor helper method to render the TextBox for the property.

By using the [DisallowHtml] attribute, any HTML tags entered by the user will be automatically stripped out when the form is submitted. This helps prevent potential security vulnerabilities such as cross-site scripting (XSS) attacks.

You can do html encoding of text or use reqular expression to check for html tag entries.

$.validator.addMethod("noHtml", function (value, element) {
    return !/<[^>]*>/g.test(value);
}, "HTML tags are not allowed.");

$(document).ready(function () {
    $("#YourTextBoxId").rules("add", {
        noHtml: true
    });
});

Replace YourTextBoxId with the actual ID of your TextBox.