public static IOperationHolder StartOperation()

in BASE/src/Microsoft.ApplicationInsights/TelemetryClientExtensions.cs [101:231]
95 lines of code
16 McCabe index (conditional complexity)

        public static IOperationHolder<T> StartOperation<T>(this TelemetryClient telemetryClient, T operationTelemetry) where T : OperationTelemetry
        {
            if (telemetryClient == null)
            {
                throw new ArgumentNullException(nameof(telemetryClient));
            }

            if (operationTelemetry == null)
            {
                throw new ArgumentNullException(nameof(operationTelemetry));
            }

            var telemetryContext = operationTelemetry.Context.Operation;
            bool idsAssignedByUser = !string.IsNullOrEmpty(telemetryContext.Id);

            // We initialize telemetry here AND in Track method because of RichPayloadEventSource.
            // It sends Start and Stop events for OperationTelemetry. During Start event telemetry
            // has to contain essential telemetry properties such as correlations ids and ikey.
            // Also, examples in our documentation rely on the fact that correlation Ids are set
            // after StartOperation call and before operation is stopped.
            // Before removing this call (for optimization), make sure:
            // 1) correlation ids are set before method leaves
            // 2) RichPayloadEventSource is re-factored to work without ikey in Start event (or ikey is set)
            //    and does not require other properties in telemetry
            telemetryClient.Initialize(operationTelemetry);

            // Initialize operation id if it wasn't initialized by telemetry initializers
            if (string.IsNullOrEmpty(operationTelemetry.Id))
            {
                operationTelemetry.GenerateOperationId();
            }

            // If the operation is not executing in the context of any other operation
            // set its name as a context (root) operation name.
            if (string.IsNullOrEmpty(telemetryContext.Name))
            {
                telemetryContext.Name = operationTelemetry.Name;
            }

            OperationHolder<T> operationHolder = null;

            var isActivityAvailable = ActivityExtensions.TryRun(() =>
            {
                var parentActivity = Activity.Current;
                var operationActivity = new Activity(ChildActivityName);

                string operationName = telemetryContext.Name;
                if (string.IsNullOrEmpty(operationName))
                {
                    operationName = parentActivity?.GetOperationName();
                }

                if (!string.IsNullOrEmpty(operationName))
                {
                    operationActivity.SetOperationName(operationName);
                }

                if (idsAssignedByUser)
                {
                    if (Activity.DefaultIdFormat == ActivityIdFormat.W3C)
                    {
                        if (W3CUtilities.IsCompatibleW3CTraceId(telemetryContext.Id))
                        {
                            // If the user provided operationId is W3C Compatible, use it.
                            operationActivity.SetParentId(ActivityTraceId.CreateFromString(telemetryContext.Id.AsSpan()),
                                default(ActivitySpanId), ActivityTraceFlags.None);
                        }
                        else
                        {
                            // If user provided operationId is not W3C compatible, generate a new one instead.
                            // and store supplied value inside custom property.
                            operationTelemetry.Properties.Add(W3CConstants.LegacyRootIdProperty, telemetryContext.Id);
                            telemetryContext.Id = null;
                        }
                    }
                    else
                    {
                        operationActivity.SetParentId(telemetryContext.Id);
                    }
                }

                operationActivity.Start();

                if (operationActivity.IdFormat == ActivityIdFormat.W3C)
                {
                    if (string.IsNullOrEmpty(telemetryContext.Id))
                    {
                        telemetryContext.Id = operationActivity.TraceId.ToHexString();
                    }

                    operationTelemetry.Id = operationActivity.SpanId.ToHexString();
                }
                else
                {
                    if (string.IsNullOrEmpty(telemetryContext.Id))
                    {
                        telemetryContext.Id = operationActivity.RootId;
                    }

                    operationTelemetry.Id = operationActivity.Id;
                }

                operationHolder = new OperationHolder<T>(telemetryClient, operationTelemetry, parentActivity == operationActivity.Parent ? null : parentActivity);
            });

            if (!isActivityAvailable)
            {
                // Parent context store is assigned to operation that is used to restore call context.
                operationHolder = new OperationHolder<T>(telemetryClient, operationTelemetry)
                {
                    ParentContext = CallContextHelpers.GetCurrentOperationContext(),
                };
                telemetryContext.Id = operationTelemetry.Id;
            }

            operationTelemetry.Start();

            if (!isActivityAvailable)
            {
                // Update the call context to store certain fields that can be used for subsequent operations.
                var operationContext = new OperationContextForCallContext
                {
                    ParentOperationId = operationTelemetry.Id,
                    RootOperationId = operationTelemetry.Context.Operation.Id,
                    RootOperationName = operationTelemetry.Context.Operation.Name,
                };
                CallContextHelpers.SaveOperationContext(operationContext);
            }

            return operationHolder;
        }