Code has error but runs correctly in IDE

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C#

WPF

In this code convert "string[]" to "string",and display on "TextBox". ,using "IValueConverter";I find the code have error,but IDE running correctly.

<src:StringArrayConverter x:Key="stringArrayConverter"/>
<!--"ConverterParameter" is "char"-->
<TextBlock Text="{Binding Path=Authors,
    Converter={StaticResource stringArrayConverter},
    ConverterParameter=','}"/>

class StringArrayConverter:IValueConverter
{
    public object Convert(object value,Type targetType,object parameter,CultureInfo cultureInfo)
    {
        string[] str = (string[])value;
        //how can convert "char" to "string"
        string separator = (string)parameter;
        return string.Join(separator, str);
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo cultureInfo)
    {
        string str = (string)value;
        char c = (char)parameter;
        //how can "Split" Method receive a char?
        return str.Split(c);
    }
}