public virtual string ExecuteMethod( 
   string value 
)

using Dynamicweb;
using Dynamicweb.Rendering;
using Dynamicweb.Extensibility;

//AddInName is the name of the extension method - <!--@DwPageName.Substring(12)--> will give an AddInName of 'Substring'
[AddInName("Substring")]
public class TagExtensionMethodSample : TagExtensionMethod
{

    public override string ExecuteMethod(string value)
    {
        //This is the entire string passed to the Substring method - everything between the ()
        object fullPassedArgument = this.Argument;

        //This method requires at least one argument

        if (this.Arguments.Count > 0)
        {
            //First argument is the startIndex of the Substring method
            int startIndex = Input.FormatInteger(this.Arguments[0]);
            if (startIndex > value.Length - 1)
            {
                //Otherwise an exception would occur
                return value;
            }

            if (this.Arguments.Count > 1)
            {
                //If the method is passed 2 arguments, we use the Substring overload that takes a length.
                int length = Input.FormatInteger(this.Arguments[1]);
                if (length > value.Length - 1)
                {
                    //Otherwise an exception would occur
                    return value.Substring(startIndex);
                }
                else
                {
                    return value.Substring(startIndex, length);
                }
            }
            else
            {
                //There is only one argument, use the Substring method that applies.
                return value.Substring(startIndex);
            }
        }
        else
        {
            return value;
        }

    }

}