Check if Property is List using Reflection in C#

I'm stuck at putting out the values of my objects at the moment. Some of them do have List<string>properties which causes trouble by using the ToString()Method. Here is the code I use in my base class to get the name and the value of the properties into a string.

public override string ToString()
    {
        string content = "";
        foreach (var prop in this.GetType().GetProperties())
        {
            if (prop.PropertyType is IList<string> && prop.GetType().IsGenericType && prop.GetType().GetGenericTypeDefinition().IsAssignableFrom(typeof(List<>)))
                content += prop.Name + " = " + PrintList((List<string>)prop.GetValue(this));
            else
            content += prop.Name + " = " + prop.GetValue(this) + "\r\n";
        }
        content += "\r\n";
        return content;
    }

    private string PrintList(List<string> list)
    {
        string content = "[";
        int i = 0;
        foreach (string element in list)
        {
            content += element;
            if (i == list.Count)
                content += "]";
            else
                content += ", ";
        }
        return content;
    }

Anyhow, the check if the propertie is a List does not work. This might be a dumb question and or a bad way to work with reflection but I'm kinda new to it and will appreciate any help to figure out what is going on.