How to validate if a string string already exists in asp.net?

I would like to know how I can do to validate in asp.net if a text string entered is equal to another. I am doing it using the ValidationResult method but it does not work, I leave the code.

public class ValidacionPlacaExistente
    {
        public static ValidationResult ValidarPlacaExistente(string ValidarPlacaExistente, ValidationContext validationContext)
        {
            Vehiculo vehiculo = (Vehiculo)validationContext.ObjectInstance;
            return ValidarPlacaExistente.Equals(vehiculo.numeroDePlaca.Length)
                ? new ValidationResult("El número de placa ya existe.")
                : ValidationResult.Success;
        }
    }