Calculate Exponential Moving Average on a Queue in C#

How about with LINQ:

return Quotes.DefaultIfEmpty()
             .Aggregate((ema, nextQuote) => alpha * nextQuote + (1 - alpha) * ema);

I would point out that for real-time financial data, this is highly inefficient. A much better way would be to cache the previous EMA value and update it on a new quote with the above (constant-time) recurrence-formula.

Do do not need a queue for an Exponential Moving Average because you only need to keep track of the previous EMA.

public class ExponentialMovingAverageIndicator
{
    private bool _isInitialized;
    private readonly int _lookback;
    private readonly double _weightingMultiplier;
    private double _previousAverage;

    public double Average { get; private set; }
    public double Slope { get; private set; }

    public ExponentialMovingAverageIndicator(int lookback)
    {
        _lookback = lookback;
        _weightingMultiplier = 2.0/(lookback + 1);
    }

    public void AddDataPoint(double dataPoint)
    {
        if (!_isInitialized)
        {
            Average = dataPoint;
            Slope = 0;
            _previousAverage = Average;
            _isInitialized = true;
            return;
        }

        Average = ((dataPoint - _previousAverage)*_weightingMultiplier) + _previousAverage;
        Slope = Average - _previousAverage;

        //update previous average
        _previousAverage = Average;
    }
}

Here's a minimal version of @MattWolf's answer with a slightly different API, and using C# 7.

public sealed class FloatExponentialMovingAverageCalculator
{
    private readonly float _alpha;
    private float _lastAverage = float.NaN;

    public FloatExponentialMovingAverageCalculator(int lookBack) => _alpha = 2f / (lookBack + 1);

    public float NextValue(float value) => _lastAverage = float.IsNaN(_lastAverage)
        ? value
        : (value - _lastAverage)*_alpha + _lastAverage;
}