SDRSharp + CTCSS decoder plug-in
Visual Studio + dotPeek .NET decompiler

https://i.ibb.co/vJpv7Vd/sdr-ccir-dec.jpg

AuxWindows.cs
      this.lastToneLabel.TabIndex = 1;
      this.lastToneLabel.Text = "0000.0 Hz"; //modx 000.0 -> 0000.0
      this.lastToneLabel.TextAlign = ContentAlignment.TopRight;

      this.toneLabel.TabIndex = 3;
      this.toneLabel.Text = "0000,0 Hz"; //modx 000,0 -> 0000,0
      this.toneLabel.TextAlign = ContentAlignment.TopRight;

      this.ShowInTaskbar = false;
      this.Text = "Tone"; //modx CTCSS tone->Tone
      this.TopMost = true;

CTCSSDecoderPlugin.cs
  public class CTCSSDecoderPlugin : ISharpPlugin
  {
    private const string _displayName = "Tone decoder"; //modx CTCSS decoder->Tone decoder

    public string DisplayName
    {
      get
      {
        return "Tone decoder"; //modx CTCSS decoder->Tone decoder
      }
    }

Decoder.cs
  public class Decoder
  {
    private const int SubToneFilterOrder = 1000;
    private const int SubToneLow = 985; //modx 60->985
    private const int SubToneHigh = 2406; //modx 260->2406

    public Decoder(double sampleRate, float squelchTone)
    {
      if (this._squelchToneFilter != null)
        this._squelchToneFilter.Dispose();
      this._squelchToneFilter = new FirFilter(FilterBuilder.MakeBandPassKernel(sampleRate, 1000, (double) squelchTone - 2.0, (double) squelchTone + 2.0, WindowType.BlackmanHarris4), 1);
      if (this._subToneFilter != null)
        this._subToneFilter.Dispose();
      this._subToneFilter = new FirFilter(FilterBuilder.MakeBandPassKernel(sampleRate, 1000, 985.0, 2406.0, WindowType.BlackmanHarris4), 1); //modx 1000,60.0,260.0 -> 1000,985.0,2406.0
      this._squelchTone = squelchTone;
      this._samplerate = (float) sampleRate;
    }

CTCSSDecoderPanel.cs
  public class CTCSSDecoderPanel : UserControl
  {
    private readonly float[] _ctcssToneTable = new float[15] //modx 55->15
    {
      //67f,
      //69.3f,
      //71.9f,
      //74.4f,
      //77f,
      //79.7f,
      //82.5f,
      //85.4f,
      //88.5f,
      //91.5f,
      //94.8f,
      //97.4f,
      //100f,
      //103.5f,
      //107.2f,
      //110.9f,
      //114.8f,
      //118.8f,
      //123f,
      //127.3f,
      //131.8f,
      //136.5f,
      //141.3f,
      //146.2f,
      //150f,
      //151.4f,
      //156.7f,
      //159.8f,
      //162.2f,
      //165.5f,
      //167.9f,
      //171.3f,
      //173.8f,
      //177.3f,
      //179.9f,
      //183.5f,
      //186.2f,
      //189.9f,
      //192.8f,
      //196.6f,
      //199.5f,
      //203.5f,
      //206.5f,
      //210.7f,
      //213.8f,
      //218.1f,
      //221.3f,
      //225.7f,
      //229.1f,
      //233.6f,
      //237.1f,
      //241.8f,
      //245.5f,
      //250.3f,
      //254.1f,
      930f, //modx add these
      991f,
      1124f,
      1197f,
      1275f,
      1358f,
      1446f,
      1540f,
      1640f,
      1747f,
      1860f,
      1981f,
      2110f,
      2247f,
      2400f // end modx
    };
    private AudioProcessor _audioProcessor;

    public void Reset()
    {
      this._toneInText = "----.- Hz"; //modx ---.- -> ----.-
      this.lastToneLabel.Text = this._toneInText;
      this._auxWindow.LastTone = this._toneInText;
    }

    private void CustomPaint(object sender, CustomPaintEventArgs e)
    {
      SpectrumAnalyzer spectrumAnalyzer = (SpectrumAnalyzer) sender;
      using (SolidBrush solidBrush1 = new SolidBrush(Color.FromArgb(100, Color.Black)))
      {
        using (SolidBrush solidBrush2 = new SolidBrush(Color.FromArgb(200, Color.White)))
        {
          using (SolidBrush solidBrush3 = new SolidBrush(Color.FromArgb(200, Color.LightGreen)))
          {
            using (StringFormat format = new StringFormat(StringFormat.GenericTypographic))
            {
              using (Font font = new Font("Arial", 10f))
              {
                format.Alignment = StringAlignment.Near;
                Rectangle rect = new Rectangle();
                Point point = new Point();
                string str1 = string.Format("Tone {0,6:F1} Hz", (object) this._audioProcessor.DecodeTone); //modx CTCSS->Tone
                if (this.squelchEnableCheckBox.Checked)

      this.squelchLevelNumericUpDown.Location = new Point(96, 18);
      this.squelchLevelNumericUpDown.Maximum = new Decimal(new int[4]
      {
        2410, //modx 260->2410
        0,
        0,
        0
      });

      this.subToneLabel.TabIndex = 11;
      this.subToneLabel.Text = "0000,0 Hz"; //modx 000,0->0000,0
      this.subToneLabel.TextAlign = ContentAlignment.TopRight;

      this.lastToneLabel.TabIndex = 12;
      this.lastToneLabel.Text = "0000,0 Hz"; //modx 000,0->0000,0
      this.lastToneLabel.TextAlign = ContentAlignment.TopRight;

        //modx
        private float ConvertTone(float tone)
        {
            if ((double)tone < 988.0 || (double)tone > 2402.0)
                return tone;
            float num1 = 976f;
            for (int index = 0; index < _ctcssToneTable.Length; ++index)
            {
                float num2 = Math.Abs(tone - _ctcssToneTable[index]);
                if ((double)num2 > (double)num1)
                {
                    return _ctcssToneTable[index - 1];
                }
                num1 = num2;
            }
            return _ctcssToneTable[_ctcssToneTable.Length - 1];
        }
        //end modx