How to Fix Validation Error in .NET

Validation Errors in .NET

.NET uses Data Annotations and FluentValidation for model validation. Validation errors surface when model binding fails or when explicit validation checks reject input data.

When It Happens

• Model binding fails on required properties
• Custom validators reject business rule violations
• ModelState.IsValid returns false

How to Fix

Combine Data Annotations with a validation filter:

public class CreateOrderRequest
{
    [Required(ErrorMessage = "Customer ID is required")]
    public int CustomerId { get; set; }

    [Range(1, 10000, ErrorMessage = "Quantity must be 1-10000")]
    public int Quantity { get; set; }

    [Required, StringLength(500)]
    public string Description { get; set; } = "";
}

// Automatic validation via filter
public class ValidationFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context)
    {
        if (!context.ModelState.IsValid)
        {
            var errors = context.ModelState
                .Where(x => x.Value?.Errors.Count > 0)
                .ToDictionary(
                    k => k.Key,
                    v => v.Value!.Errors.Select(e => e.ErrorMessage)
                );
            context.Result = new BadRequestObjectResult(errors);
        }
    }

    public void OnActionExecuted(ActionExecutedContext context) { }
}

Register the filter globally so every endpoint gets consistent validation behavior.

Bugsly for .NET

Bugsly logs validation failures with the model type, field errors, and request context — giving you a clear picture of which API consumers send invalid data most frequently.