Hello René,

Thank you for your feedback, I agree that the MaxLength would be a nice feature to have. When we implement it I will make sure to pass your feedback to the dev team. 

Regards,
Svetoslav Dimitrov
Progress Telerik

Hello René,

I would like to use the occasion to ask for some feedback from you.

What would you expect to be the default behavior when the limit is reached - a warning message which mimics the standard validation messages, a red border around the component, or should it be disabled? 

Regarding the character counter (current/max), I have opened a separate Feature Request on your behalf, which you can see from this link. I have given your Vote for it and you can Follow it for email notifications on status updates.

Regards,
Svetoslav Dimitrov
Progress Telerik

Hello Rene,

There are two approaches which you could use to handle this:

Input Validation

If you are using the TextArea in an EditForm you could benefit from the DataAnnotation attribute MaxLength(int maxLength, string ErrorMessage). An example of this approach could be seen in our Input Validation Article.

Using the TextArea outside EditForm

If you are using the TextArea outside EditForm you could create your own validation and utilize the ValueChanged event the component exposes. Below, I have added an example of how this could be done. Also, in this example, you could see a demo counter representing currentLength / maxLength. 

<TelerikTextArea Value="@TheValue" ValueChanged="@( (string i) => ValueChangedHandler(i))"></TelerikTextArea>
<div>
    <span>@TheValue.Length</span><span>/@MaxLength</span>
</div>
@if (!String.IsNullOrEmpty(WarningMessage))
{
    <div class="text-danger">
        @WarningMessage
    </div>
}

@code {
    public string TheValue { get; set; } = String.Empty;

    public string WarningMessage { get; set; }
    public int MaxLength { get; set; } = 10;

    void ValueChangedHandler(string input)
    {
        TheValue = MaximumLength(input, MaxLength);
    }

    public string MaximumLength(string value, int maxLength)
    {
        if (value.Length <= maxLength)
        {
            return value;
        }
        else
        {
            WarningMessage = $"The maximum length of the string should be {maxLength}";
            throw new Exception("My exception");
        }
    }
}

Additionally, you could wrap our component in a custom component for your application and pass the MaxLength as a [Parameter].

Regards,
Svetoslav Dimitrov
Progress Telerik