best way to validate user input across multiple form fields in C#?

Question

I have a form with several textboxes, dropdowns, and checkboxes. I want to implement a clean and scalable way to validate all inputs before submission. Should I use custom validation classes, data annotations, or something else? Examples would be appreciated.

Sandhiya Priya · Answer

Best Way to Validate User Input Across Multiple Form Fields in C# When you have multiple input controls (textboxes, dropdowns, checkboxes) and want a clean, scalable approach , there are a few standard patterns in C# and .NET: 1. Data Annotations (Recommended for Simplicity) Use attributes on your model properties to define validation rules. Works well for both ASP.NET MVC / Razor Pages and WinForms/WPF with binding . Automatically integrates with validation helpers and client-side validation in MVC. Example: public class StudentForm { [Required(ErrorMessage = "Name is required")] [StringLength(50, ErrorMessage = "Name cannot exceed 50 characters")] public string Name { get; set; } [Range(18, 100, ErrorMessage = "Age must be between 18 and 100")] public int Age { get; set; } [Required(ErrorMessage = "Please select a course")] public string Course { get; set; } [EmailAddress(ErrorMessage = "Invalid email format")] public string Email { get; set; } } Validate in code: var student = new StudentForm { Name = "", Age = 17, Course = null }; var context = new ValidationContext(student, null, null); var results = new List (); bool isValid = Validator.TryValidateObject(student, context, results, true); foreach (var error in results) { Console.WriteLine(error.ErrorMessage); } ? Pros: Minimal code, declarative, easy to maintain. ? Cons: Less flexible for complex custom logic. 2. Custom Validation Classes Useful if your rules are dynamic or involve multiple fields together. Create a Validator class to centralize logic. Example: public class StudentValidator { public List Validate(StudentForm student) { var errors = new List (); if (string.IsNullOrWhiteSpace(student.Name)) errors.Add("Name is required."); if (student.Age 100) errors.Add("Age must be between 18 and 100."); if (string.IsNullOrEmpty(student.Course)) errors.Add("Course selection is required."); if (!string.IsNullOrEmpty(student.Email) && !student.Email.Contains("@")) errors.Add("Invalid email address."); return errors; } } Usage: var validator = new StudentValidator(); var errors = validator.Validate(student); if (errors.Any()) { foreach (var error in errors) Console.WriteLine(error); } else { Console.WriteLine("All inputs valid!"); } ? Pros: Full control, easy to extend for multiple forms. ? Cons: More code than data annotations. 3. FluentValidation Library (Modern, Clean, Scalable) Third-party library, widely used in .NET projects. Supports complex rules , cross-field validation , and reusable validators . Example: using FluentValidation; public class StudentValidator : AbstractValidator { public StudentValidator() { RuleFor(x => x.Name).NotEmpty().WithMessage("Name is required") .MaximumLength(50); RuleFor(x => x.Age).InclusiveBetween(18, 100); RuleFor(x => x.Course).NotEmpty(); RuleFor(x => x.Email).EmailAddress().When(x => !string.IsNullOrEmpty(x.Email)); } } // Usage var validator = new StudentValidator(); var results = validator.Validate(student); if (!results.IsValid) { foreach (var failure in results.Errors) Console.WriteLine(failure.ErrorMessage); } ? Pros: Clean, maintainable, reusable, supports complex scenarios. ? Cons: Requires NuGet package. Recommendation For simple forms: Use Data Annotations . For complex forms with cross-field logic: Use Custom Validation Class or FluentValidation . Always centralize validation logic to make it scalable and maintainable .

Prasad Raveendran · Answer

Small forms: Use Data Annotations . Quick, built-in, automatic client-side support. Medium-to-large projects: Use FluentValidation . Centralizes rules, highly readable, easily testable. Very dynamic/complex forms: Use custom validator services . You can choose the approach you prefer, and based on that, I will provide a sample code snippet.

best way to validate user input across multiple form fields in C#?

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