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If a requirement comes your way to programmatically update the validators that are present on your Episerver Forms element(s), the snippets below will help!

Validators are registered in a slightly peculiarly way – it’s a single string property, which contains a delimited list of the validator type names:

The delimiter does have a strongly-typed constant to represent it – which is found here:

EPiServer.Forms.Constants.RecordSeparator

In addition, validator error messages are represented by a list of Key/Value pairs – the key being the validator type name, and the value being the error message.

Method to update a form element’s validator

public void ApplyValidatorToFormElement<T>(InputElementBlockBase formElement, string errorMessage= "", string extra= "")
    where T : ElementValidatorBase
{
    var formElementWritable = formElement.CreateWritableClone() as InputElementBlockBase;
    var formElementValidators = formElementWritable.Validators ?? string.Empty;

    var validators = new string[]
    {
        typeof(T).FullName + extra,
        formElementValidators
    };

    if (!string.IsNullOrEmpty(errorMessage))
    {
        // Strip out any existing validator messages of this type first
        var newValidatorMessages = formElementWritable.ValidatorMessages.Where(x => x.Validator != typeof(T).FullName)
            .ToList();

        newValidatorMessages.Add(new ValidatorMessage
        {
            Validator = typeof(T).FullName,
            Message = errorMessage
        });

        formElementWritable.ValidatorMessages = newValidatorMessages;
    }

    formElementWritable.Validators = string.Join(EPiServer.Forms.Constants.RecordSeparator, validators);
    contentRepository.Save((IContent)formElementWritable, SaveAction.Publish, AccessLevel.NoAccess);
}

Example

ApplyValidatorToFormElement<RequiredValidator>(formElement, "This field is required");

Regular Expressions

These are slightly more funky – as the Regex pattern is embedded into the validator name with a delimiter. The below method can help achieve this:

public void ApplyRegularExpressionValidatorToFormElement(InputElementBlockBase formElement, Regex regularExpression, string errorMessage)
{
    var regexFormat = $"##{regularExpression.ToString()}###{regularExpression.ToString()}";
    ApplyValidatorToFormElement<RegularExpressionValidator>(formElement, errorMessage, regexFormat);
}

Example

ApplyRegularExpressionValidatorToFormElement(formElementBlock, new Regex("^[a-zA-Z0-9 '-]+$"), "Text fields can only consist of letters or numbers");

Happy coding!