Generic extension method for traversing object graphs

public static IEnumerable Traverse(this T root, Func> getChildren)
{
    return Enumerable.Repeat(root, 1)
        .Concat((getChildren(root) ?? Enumerable.Empty())
            .SelectMany(child => Traverse(child, getChildren)));
}

Calculate the exponential moving average of a stream of numbers

public static IEnumerable<double> ExponentialMovingAverage(this IEnumerable<double> source, double alpha)
{
    double? last = null;
    return source.Select(
        value =>
        {
            var average = last != null ? alpha*value + (1 - alpha)*last.Value : value;
            last = average;
            return average;
        });
}

Create a fast compiled method invoker using LINQ

var method = new Func<double, double, double>(Math.Pow).Method;
var parameter = Expression.Parameter(typeof(object[]));
var args = method.GetParameters()
    .Select((param, i) => Expression.Convert(
        Expression.ArrayIndex(parameter, Expression.Constant(i)), param.ParameterType));
var expr = Expression.Lambda<Func<object[], object>>(
    Expression.Convert(Expression.Call(method, args), typeof(object)), parameter);
var invoker = expr.Compile();
var result = invoker(new object[] {Math.PI, 2.0});

Remove matching C# region directives using LINQ

var flag = false;
var modified = false;
var lines = File.ReadLines(fileName)
    .Where(line =>
        {
            if ((!flag && line.Contains("#region " + regionName)) ||
                (flag && line.Contains("#endregion")))
            {
                flag = !flag;
                modified = true;
                return false;
            }
            return true;
        })
    .ToArray();
if (modified)
{
    File.WriteAllLines(fileName, lines);
}