Fix CS1056 Unexpected Character '$' in NinjaScript

using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Windows.Media;
using System.Xml.Serialization;
using NinjaTrader.NinjaScript;

namespace NinjaTrader.NinjaScript.Indicators
{
    public class SafeStringFormatIndicator : Indicator
    {
        protected override void OnStateChange()
        {
            if (State == State.SetDefaults)
            {
                Name = "SafeStringFormatIndicator";
                Description = "Demonstrates safe string formatting in NinjaScript without CS1056.";
                Calculate = Calculate.OnBarClose;
                IsOverlay = true;
                Period = 14;
                AddPlot(Brushes.DodgerBlue, "Average");
            }
        }

        protected override void OnBarUpdate()
        {
            if (CurrentBar < Period - 1)
                return;

            Value[0] = SMA(Period)[0];

            // Avoid string interpolation that causes CS1056 in C# 5 compiler contexts:
            // Print($"Bar: {CurrentBar}, Close: {Close[0]}, SMA: {Value[0]}");

            // Solution 1: Use string.Format
            Print(string.Format("Bar: {0}, Close: {1:F2}, SMA: {2:F2}", CurrentBar, Close[0], Value[0]));

            // Solution 2: Use string concatenation
            // Print("Bar: " + CurrentBar + ", Close: " + Close[0].ToString("F2"));
        }

        [NinjaScriptProperty]
        [Range(1, 200)]
        [Display(Name = "Period", GroupName = "Parameters", Order = 0)]
        public int Period { get; set; }

        [Browsable(false)]
        [XmlIgnore]
        public Series<double> Average { get { return Values[0]; } }
    }
}

The example uses string.Format and an explicit property getter to avoid those C# 6 constructs. Compile it in the target NinjaScript Editor; compatibility with every installation is not established.