MVC ViewModel Validation

ValidationExample.cs

// Ensure that that ViewModel class inherits IValidatableObject, otherwise
// nought is gonna happen.

public class ViewModel : IValidatableObject
{
    public DateTime StartDate { get; set; }
    public DateTime EndDate { get; set; }
    
    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        var errors = new List<ValidationResult>();
        
        if (StartDate < EndDate)
            errors.Add("The Start Date cannot be earlier than the End Date", new[] { "StartDate" }));
            
        return errors;
    }
}