Templating from Object

The idea here is to provide a really lightweight template engine.
The point here is to demonstrate how light such a code can be. In his current state the code suffers some defaults discussed at the end.
A template is a String comprising tokens of the form [ProperyName] or [ProperyName:format]. PropertyName can be any name of any property of an object or navigable property of the object. However IEnumerable are not handled.

public static String MergeObjectFromTemplate(String st, Object o) {
    if (String.IsNullOrEmpty(st)) throw new ArgumentNullException("st");
    if (o == null) return st;

    Int32 i = st.IndexOf("[");
    Int32 j = 0;
    String pName;
    Object lo;
    String[] a;
    while (i >= 0) {
        j = st.IndexOf("]");
        if (j == -1)
            return st;

        pName = st.Substring(i + 1, j - i - 1);
        a = pName.Split(':');
        lo = GetObjectByReflection(o, a[0]);
        if (lo == null) { 
            st = st.Replace("[" + pName + "]", "_" + pName + "_");
        } else {
            if (a.Count() == 1) {
                st = st.Replace("[" + pName + "]", lo.ToString().Trim());
            } else {
                st = st.Replace("[" + pName + "]", String.Format("{0:" + a[1] + "}", lo));
            }
        }

        i = st.IndexOf("[");
    }

    return st;
}

The key function here is: GetObjectByReflection.

public static Object GetObjectByReflection(Object src, String propertyName) {
    Type type;
    Object o = src;
    foreach (String fragment in propertyName.Split('.')) {
        type = o.GetType();
        PropertyInfo pi = type.GetProperty(fragment);
        if (pi == null) {
            o = null;
            break;
        }
        o = pi.GetValue(o, null);
        if (o == null) return String.Empty;
    }

    return o;
}

That is it!
Well, not totally. It remains some defaults, but for 50 lines, that's may be not so bad. Known defaults are:

• Bad format string are not handled.
This is blocking, if throwing an exception is blocking. Correcting it is easy once expected behavior is known. At the time the code was updated I was just needing to format DateTime in a relatively safe environment: templates are edited by me. It is another story if you want to allow user to produce templates.
• Excepted for a bad format String, the code always returns a String, even if a bad object is provided, or bad property name is used.