Typical File Structure:

/Controllers
    ProductController.cs
    HomeController.cs
/Models
    Product.cs
    Order.cs
/Views
    /Product
        Index.cshtml
        Details.cshtml
    /Shared
        _Layout.cshtml

Simple Form Submission

// Controller
[HttpPost]
public ActionResult Register(User user)
{
    if(ModelState.IsValid)
    {
        _service.Register(user);
        return RedirectToAction("Success");
    }
    return View(user);
}

// View (Razor)
@model User
@using (Html.BeginForm())
{
    @Html.TextBoxFor(m => m.Username)
    @Html.ValidationMessageFor(m => m.Username)
    <input type="submit" value="Register" />
}

// Code-behind
protected void btnRegister_Click(object sender, EventArgs e)
{
    if(Page.IsValid)
    {
        var user = new User {
            Username = txtUsername.Text
        };
        _service.Register(user);
        Response.Redirect("Success.aspx");
    }
}

// ASPX Page
<asp:TextBox ID="txtUsername" runat="server" />
<asp:RequiredFieldValidator 
    ControlToValidate="txtUsername"
    runat="server" ErrorMessage="Required" />
<asp:Button ID="btnRegister" 
    runat="server" Text="Register"
    OnClick="btnRegister_Click" />

Example: Domain Model

public class Product
{
    public int Id { get; set; }
    
    [Required(ErrorMessage = "Name is required")]
    [StringLength(100)]
    public string Name { get; set; }
    
    [Range(0.01, 10000)]
    public decimal Price { get; set; }
    
    public bool IsInStock { get; set; }
    
    // Business logic method
    public bool IsExpensive() 
    {
        return Price > 1000;
    }
}

Example: View Model

public class ProductCreateViewModel
{
    [Display(Name = "Product Name")]
    public string Name { get; set; }
    
    [DataType(DataType.Currency)]
    public decimal Price { get; set; }
    
    // Additional view-specific properties
    public List<SelectListItem> Categories { get; set; }
    public int SelectedCategoryId { get; set; }
}

Model with Data Annotations

public class UserRegistrationModel
{
    [Required(ErrorMessage = "Email is required")]
    [EmailAddress(ErrorMessage = "Invalid email format")]
    [Display(Name = "Email Address")]
    public string Email { get; set; }

    [Required]
    [StringLength(100, MinimumLength = 8, 
        ErrorMessage = "Password must be 8-100 characters")]
    [DataType(DataType.Password)]
    public string Password { get; set; }

    [DataType(DataType.Password)]
    [Display(Name = "Confirm Password")]
    [Compare("Password", 
        ErrorMessage = "Passwords do not match")]
    public string ConfirmPassword { get; set; }

    [Range(typeof(bool), "true", "true", 
        ErrorMessage = "Must accept terms")]
    [Display(Name = "Accept Terms")]
    public bool AcceptTerms { get; set; }
}

Controller Action with Validation

[HttpPost]
public ActionResult Register(UserRegistrationModel model)
{
    if (ModelState.IsValid)
    {
        // Process valid data
        return RedirectToAction("Success");
    }
    
    // Return to form with validation errors
    return View(model);
}

View with Validation Helpers

@model UserRegistrationModel

@using (Html.BeginForm())
{
    <div class="form-group">
        @Html.LabelFor(m => m.Email)
        @Html.TextBoxFor(m => m.Email, new { @class = "form-control" })
        @Html.ValidationMessageFor(m => m.Email)
    </div>
    
    <div class="form-group">
        @Html.LabelFor(m => m.Password)
        @Html.PasswordFor(m => m.Password, new { @class = "form-control" })
        @Html.ValidationMessageFor(m => m.Password)
    </div>
    
    <!-- Other fields -->
    
    <input type="submit" value="Register" class="btn btn-primary" />
}

Creating Custom Validators

public class FutureDateAttribute : ValidationAttribute
{
    public override bool IsValid(object value)
    {
        if (value is DateTime date)
        {
            return date > DateTime.Now;
        }
        return false;
    }
}

// Usage:
[FutureDate(ErrorMessage = "Date must be in the future")]
public DateTime EventDate { get; set; }

Model-Level Validation

public class EventModel : IValidatableObject
{
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    
    public IEnumerable<ValidationResult> Validate(
        ValidationContext validationContext)
    {
        if (EndDate < StartDate)
        {
            yield return new ValidationResult(
                "End date must be after start date",
                new[] { nameof(EndDate) });
        }
    }
}